THE OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS
REPORT ON
INVESTIGATIONS MADE IN THE CHEMICAL LABORATORY
OF THE JOHNS HOPKINS UNIVERSITY
DURING THE YEARS 1899-1913
BY H. N. MORSE
Professor of Inorganic and Analytical Chemistry in the Johns Hopkins University
WASHINGTON, D. C.
PUBLISHED BY THE CAKNEGIE INSTITUTION OF WASHINGTON
1914
CARNEGIE INSTITUTION OF WASHINGTON
PUBLICATION No. 198
PRESS OF GIBSON BROTHERS, !NC WASHINGTON, D. C.
CONTENTS.
Chapter I. Page.
The cells and the manometer attachments 3
Treatment of the clays 6
First process 7
Second process 7
The formation of the cylinders 8
The cutting of the cells 11
The burning and glazing of the cells 12
The manometer attachments of the cells 17
Chapter II.
The manometers 27
Purification of the mercury 28
Calibration of the manometers 28
First method 29
Second method 30
The meniscus 33
The uncalibrated portions of the manometers 37
Capillary depression 39
The filling of the manometer 45
Determination of the volume of the nitrogen 48
Chapter III.
The regulation of temperature 51
Thermometer effects 51
The scheme for electrical regulation 57
The battery 59
The thermostat 59
The master relay 61
The minor relay 61
The bath for 0° 62
Baths for maintenance of temperature above zero 63
Type 1 65
Type II 66
Type III 68
Type IV 71
Chapter IV.
The membranes 77
The deposition of the membrane 82
Observations on the membrane 85
Temperature of deposition 85
Treatment of the cell while in use 86
The soaking of the cell 87
Activity of the membrane 88
Deterioration of the membrane 91
Effect of the electrolytes 92
Semipermeability of membranes 92
Removal of the membrane 93
Infection of the membrane 94
Chapter V.
The weight-normal system for solutions 97
Chapter VI.
Cane sugar Ill
Preliminary determinations of osmotic pressure Ill
Series 1 112
Series II 118
Series III 128
Series IV 132
Series V 135
Series VI 139
Series VII 140
Series VIII . . 142
IV CONTENTS.
Chapter VII. Page-
Glucose 151
Preliminary determinations of osmotic pressure 151
Series 1 151
Series II 154
Series III 156
Chapter VIII.
Cane sugar «»
Final determinations of osmotic pressure 159
Chapter IX.
Glucose 188
Final determinations of osmotic pressure 188
Chapter X.
Mannite 197
Determinations of osmotic pressure 197
Chapter XI.
Electrolytes 209
Experiment 1 211
Experiment 2 212
Determinations of the osmotic pressure of lithium chloride 214
Chapter XII.
Conclusion 221, 222
LIST OF ILLUSTRATIONS.
PLATES. Facing
Page. Plate 1. a, c, and e, thin sections taken from potters' cells, b, d, and/, thin sections taken
from cells made in the laboratory 16
Plate 2. View of "manometer house," cathetometer, arrangement for pressing clays, and
one style of rectangular bath 42
Plate 3. View of circular and rectangular laboratory baths in use 68
Plate 4. Rectangular bath, end view 70
Plate 5. Rectangular bath, side view 72
TEXT FIGURES.
Page.
1. Steel press for clays. Ball-bearing disk 8
2. Apparatus for pressing clays 10
3. Clay cylinder after pressing. Clay cell after shaping the cylinder on the lathe 11
4. Different views of special tool for cutting cell from cylinder 12
5. Electric kiln for baking cells. Inner and outer coverings for electric kiln 14
6. Electric kiln arranged as crucible furnace 14
7. First form of complete cell 19
8. "Fang" for introduction and removal of rubber stoppers 18
9. Second form of complete cell 19
10. Third form of complete cell 22
11. Fourth form of complete cell 22
12. Fifth form of complete cell; for use with substances which attack metals 23
13. Solid glass stopper for use with substances which attack metals 25
14. 15. Glass manometer attachments for cells with straight necks 25
16. First arrangement for calibrating manometers 30
17. Second arrangement for calibrating manometers 31
18. Simplest form of manometer 32
19. Manometer for high pressure 32
20. Manometer with glass cone for cells with taper necks 36
21. Manometer with glass connection for cells with straight necks 38
22. "Steel block" for determination of gas volumes in manometers, for comparison of
instruments, and for determination of capillary depression 40
23. Electric hammer for tapping manometers 41
24. "Manometer house" for calibration and comparison of instruments, etc 43
25. Improvement in cathetometers for the fine adjustment of the telescope, which also
serves as a substitute for the micrometer eye-piece 44
26. Arrangement for filling manometers with nitrogen 46
27. " Brass block " 49
28. Apparatus used in emptying, filling, and cleansing the uncalibrated portion of the
manometers 50
29. General scheme for the electric regulation of bath temperature 58
30. The thermostat 60
31. Interior ice-bath for measurement of osmotic pressure at 0° 62
32. 60-liter galvanized-iron bath for intermittent use 63
33. Coil of block-tin pipe for cooling or heating water before it enters the circulating system
within the bath 66
34. Rectangular bath for general laboratory use 67
35. 36. Lower and upper halves of rectangular bath for measuring osmotic pressure 69
37. Hot-water circulating system with end of bath removed 70
38, 39. Brass and copper bath for high temperature work 72
40. Brass-copper bath for high temperatures 73
41. Exterior view of bath for high temperatures 74
42. View between interior and exterior baths, i. e., of space filled with water 74
43. Automatic arrangements for maintaining temperature of upper door when open 75
44. Larger (elliptical) bath for high temperatures 75
45. Exterior view of larger bath for high temperatures 76
46. First bath employed for measurement of osmotic pressure 114
47. First bath in which water and air were circulated 119
48. Pumping arrangements on larger scale than in figure 47 120
49. Interior view of water compartment with covers partly removed 121
v o
THE OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS
BY H. N. MORSE
CHAPTER I. THE CELLS AND THE MANOMETER ATTACHMENTS.
Having found in the electrolytic method* an excellent means of depositing a considerable number of osmotically active membranes, it was imagined that the principal obstacle in the way of the measure- ment of osmotic pressure had been removed and that certain obvious mechanical difficulties connected with the preparation of a suitable porous vessel and the assembling of the various essential parts of the cell could be readily overcome. It was soon discovered, however, that the problem of preparing a satisfactory background for the membrane, i. e., the porous wall, was vastly more difficult than had been antici- pated.! It was necessary, in fact, to spend a large fraction of the first ten years of the investigation in experimental work in the manufacture of cells. The first four years (1901-1905) were devoted almost exclu- sively to the solution of that problem. At the close of the latter period (1905), only two porous vessels of faultless wall-structure had been pro- duced. These were the cells which were designated in the published records of the work by the letters "A" and "B."
The first experiments upon the activity of membranes deposited by the electrolytic method were made in such porous vessels as could be found about the laboratory, battery cups, etc. The earliest attempts at quantitative measurement were carried out with a portion of a lot of 100 small porous cups which were manufactured, in accordance with furnished specifications, at a pottery in a neighboring city.t In about one-fourth of these, considerable pressures were developed, but in no case the maximum pressure. All of them leaked, and most of them burst under pressures of less than 20 atmospheres. Only one of them survived a pressure of 30 atmospheres, and that for a short time only. It was not then doubted that the defects which had appeared in the first lot of cells from the pottery could be remedied by the potters themselves, provided the exact causes of the failure of their products could be correctly ascertained and explained to them. Accordingly, with that purpose in view, many thin sections were made of the cells with which quantitative measurements had been attempted, and these were examined microscopically and photographed. It soon appeared that most, if not all, of the conditions which determine the
*Amer. Chem. Journal, xxvr, 80 (1901); xxix, 173 (1903). •flbid., xxxn, 93 (1904); xxxiv, 1 (1905). ., xxvin, 1 (1902).
4 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
«d and bad behavior of the porous wall of the cell are susceptible of 'tear definition and of adequate explanation. All the essential facts which had been discovered in the laboratory were given to the potters, and they expressed their confidence in their ability to remedy the defects of the earlier consignment. In this expectation they were greatly mistaken; for, among the nearly 500 cells which were subse- quently made for us at various potteries, not one was found suitable for the measurement of osmotic pressure. In fact, in attempting to remedy rn tain defects, they generally aggravated others to such an extent as to render the later cells on the whole distinctly inferior to those of the first lot. Finally we ventured to offer certain suggestions involving methods of manufacture not in use among potters, but these were rejected on the ground that, to those familiar with the conduct of clays, they were obviously futile. As the potters declined to cooperate along lines of manufacture not approved by them, the problem of cell-making was taken out of their hands into the laboratory for solution.
The cells produced at the potteries were defective in various ways, but principally in the particulars enumerated below:
1. All were lacking in the strength necessary to withstand any con- siderable outward pressure. As mentioned above, only one of the few which proved at all serviceable survived a pressure of 30 atmospheres, while most of them cracked under pressures below 20 atmospheres.
2. All of them contained numerous "air blisters," which communi- cated with each other and with the interior surfaces of the porous wall in such ways as to give rise to the formation of a number of subsidiary interior membranes. Not unfrequently, when a cell was broken for examination, as many as four or five of these minor membranes, often nearly concentric over a considerable area, were found in several local- ities; and it frequently happened also that the last of them was near, or even at, the exterior surface of the cell.
3. The potters' cells also lacked uniformity in respect to porosity. The same cell would often exhibit the greatest diversity in this par- ticular. In some parts, the structure would be as close as in porcelain, while in others it might be so open that the membrane would form nearly midway between the interior and exterior surfaces of the cell wall.
A miscroscopic examination of thin sections of the cells in which membranes had been deposited revealed the fact that, excluding the peculiar and often fantastic effects of "air blisters,11 the distance of the iiH'mbrane from the interior surface of the cell wall is determined solely by the porosity of the latter. The more open the texture is, i. e., the larger the pores are, the more deeply within the wall will the deposition occur ; while witli a certain degree of closeness in this respect, the depo- it ion is just within the interior entrances of the pores, in effect, upon the inner surface of the cell, where it should be. Obviously the copper
CELLS AND MANOMETER ATTACHMENTS. 5
ferrocyanide membrane will always be located somewhat nearer the inner than the outer surface of the wall, however large the pores may be. It was evident that the hope of success in cell-making depended on the following conditions:
1. Great and uniform strength of wall.
2. The elimination of air-blisters.
3. An excessively fine and perfectly uniform texture of wall, a texture so fine, in fact, as to insure the meeting of the slower anion and the more mobile cation just within the interior mouths of the pores. In other words, the pores must be so small that the cation is able to pass through them, from the exterior to the interior of the wall, during the time consumed by the anion in just entering them from the interior.
The necessity of securing great strength of wall is obvious enough, as is also that of eliminating "air blisters," and the need of depositing the membrane at the interior surface of the wall will likewise become appar- ent if one considers the inequalities in the concentration of the solution which must result from its location elsewhere, i. e., within the wall. In the latter case, owing to the slowness of diffusion within the wall, the liquid in the neighborhood of the membrane will be permanently less concentrated than the main body of the solution. Moreover, since the wall is always necessarily filled with some liquid, it would be impossible to know exactly the final concentration of any solution which is intro- duced into the cell. On the other hand, if the discharge of the water entering the cell through the membranes is from a free surface, i. e., directly into the unencumbered solution, the conditions will be favor- able to its rapid distribution, and, therefore, to the maintenance of uniform concentration.
It was attempted to secure strength of wall by introducing into the clays the maximum allowable portion of cement ing material (feldspar)— that proportion, in fact, which is just insufficient to convert the baking cell into porcelain. It was hoped also, by thorough mixing of the con- stituents, to secure a more uniform texture of cell wall than had been found in the products of the potters.
Washed clays from several sources were mixed with varying quanti- ties of ground feldspar, and the mixtures were burned at different tem- peratures, either in a Seger experimental kiln with use of Seger cones, or in a calibrated electric furnace which was devised for the purpose. The products were altogether disappointing. They were, in reality, quite as uneven in respect to uniformity of strength and texture as the cells of the potters. The failure was evidently due to imperfect mixing, and it was hoped that better results might be obtained with finer mate- rials. Accordingly, both the clays and the feldspar were elutriated, and the wet mixtures of the finer materials thus obtained were passed repeatedly through silk bolting-cloth having 16,000 holes to the square inch. The bolting process was followed by a long-continued churning
6 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
of the mixtures with water, and, finally, by a most thorough kneading ,- 1 h<« -nutty " The results were still unsatisfactory in that the poros- itv of the baked samples lacked the high degree of uniformity which is indispensable in the measurement of osmotic pressure. It was evident, moreover, on comparing our products with those of the potters, that we had been trying exactly what they had attempted, except that, in every case but one, they had omitted the elutriation and bolting pro- cesses. It was concluded that the necessary binding material can not be successfully incorporated with the clays in the form of ground feldspar.
The final solution of the problem was easy and satisfactory. It occurred to us that perhaps sufficiently intimate admixtures could be obtained by bringing together two different clays, one of which is defi- cient in binding material, while the other is over rich in that constituent. This was the plan which was finally adopted, and with proper selection and manipulation of the materials, it has never failed to give products which are all that could be desired in respect to strength and uniformity of texture.
The pores were, however, still much too large, notwithstanding the fineness of the materials, and the air blisters were not eradicated by the usual method of forming such vessels. It was attempted to diminish the size of the pores by repeatedly burning the cells at high temper- atures, and in this way considerable but not sufficient improvement was effected. Two plans had been proposed to the potters for securing the required density of texture and for the simultaneous elimination of the air blisters. The first of these was to form the cell itself under high pressure, while the second was to form the wet clay into a cylinder under great pressure, and from this to turn out the cell upon the lathe. Both plans were declared to be impracticable by the potters. After many months of futile effort, we were forced to agree with them as to the first project, but the alternative plan — that of cutting the cell from a cylinder which had been formed under high pressure — was finally developed to a successful issue.
TREATMENT OF THE CLAYS.
A considerable number of clays, both American and foreign, were investigated with reference to their suitability for the manufacture of cells, and two were finally selected as being superior to any of the others for the purpose. These were a fire clay from Dorsey,* Maryland, and a so-called ball clay from Edgar, Florida.
The Florida clay had been washed before it came into our hands, while that from Maryland was in its original untreated condition. Two processes have been employed for the separation of the finer portions of the clays. Both give satisfactory products, but the earlier process has been abandoned because the later one is more economical of material.
•Erroneously stated to have been from Mount Savage, Maryland.
CELLS AND MANOMETER ATTACHMENTS. 7
FIRST PROCESS.
The dry and pulverized clays are sifted for the purpose of removing the coarsest parts. Three empty alcohol barrels, each with a spigot in the bung hole, are placed one above another, each of the upper two being set a little back of the one below it. The uppermost barrel is nearly filled with water, and into this is stirred about 3 kilograms of the sifted clay. After standing quietly for 3 minutes, the spigot is opened and the contents of the upper half of the barrel are allowed to flow into the barrel below. The residue is removed and the barrel is recharged and again partially emptied, precisely as in the first instance. When the intermediate barrel is nearly full, its contents are likewise stirred and then allowed to settle for 3 minutes, after which the spigot is opened to allow the contents of the upper half to flow into the lowest recep- tacle. The material which collects in the lowest barrel is bolted (wet) successively through Nos. 10, 14, and 16 silk bolting-cloth, having respectively 11,236, 19,600, and 24,336 holes to the square inch. The proportion of the clay which is thus acquired is not very large. In one instance where the original and final weights were recorded, 500 pounds of the fire clay yielded 180 pounds of the bolted material. In another case, 200 pounds of the Edgar clay gave 75 pounds of the final product.
SECOND PROCESS.
A wooden trough, 6 meters in length, with flat bottom and high sides, is divided into several compartments by means of transverse dams. The trough is given an inclined position, and in the highest compart- ment the sifted clay is stirred up with water. The water with its sus- pended matter is pushed from time to time over the dam into the next compartment. By repeating the operation in the successive divisions, the finer constituents of the clay can be quickly and quite completely separated from the coarser. The material which collects in the last compartment, or is allowed to overflow from that into other receptacles, is bolted in the manner described above.
The bolted clay is allowed to subside and the nearly clear water above it is drawn off by means of a siphon, but there still remains a large quan- tity of water in the clay which must be removed by evaporation, or filtration, or by other means. Its removal by either of the methods mentioned, however, isexceedingly slow and in many ways disagreeable. A much better and more rapid method is that which was suggested to us by our process for removing air from the porous walls of osmotic cells, i. e., the method of " electrical endosmose." A largeporous pot (usually a flower pot) is placed in a larger, water-tight vessel of any suitable mate- rial. The clay (generally in the form of a thick porridge) is poured into the former. Twro electrodes are inserted, the anode into the con- tents of the porous pot, and the cathode into the water which quickly
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
can
introduced.
8
collects in the outer vessel. As the level of ^the contents of the pot recedes, more clay is added until no more In the meantime, the level of the water m the outer receptacle is kept, by means of an automatic siphon, just high enough to permit the complete submersion of the cathode. When the clay becomes so far dried that the mass begins to crack at the top, it is packed down with a heavy pestle. In this way the excess of water can be separated from the clay much more rapidly and even more com- pletely than by nitration under diminished pressure. The method can also be applied with advantage to the separation of water from, and even to the washing of, other solids which, like clay, are filtered with difficulty. The voltage employed is 110 or 120.
THE FORMATION OF THE CYLINDERS.
A section of one of the steel presses in which the clay is formed into cylinders is shown in Figure 1 A. The barrel (1) is slightly tapered internally, the diameter at the bottom being 0.005 inch greater than at the top, in order to insure the ready release of the clay cylinder when it is to be pushed out of the lower end of the press. It is threaded at both ends to receive the caps (2 and 3). The cap at the lower end (2) is bored to permit the escape of the water which is squeezed out of the clay. The cap at the upper end (3) is bored and threaded internally to receive the hollow plug (4). The steel disks (5 and 6) are also bored to facilitate the escape of water. The disks (7 and 8) are of porous hard-burned clay or of asbestus.
The upper steel disk (6) is less simple than it appears in the figure. In reality it consists <>f two grooved disks separated by hardened steel balls (bicycle balls), as shown in Figure
B, The upper half turns readily with the plug (4), while the lower half remains sta- tionary, thus preventing any twisting of the '•lay Ix-ncath. If a single disk is used, the of day. clay i- twisted in a direction the reverse of that of the screw, and
B
FIG. 1.
A. Steel press for clays. (1) Bar- rel; (2) lower cap; (3) upper cap; (4) plunger ; (5) and (6) steel disk ; (7) and (8) porous clay disk.
B. Ball-bearing disk, used in place of (6) to prevent twisting
CELLS AND MANOMETER ATTACHMENTS. 9
the cell, when burned, exhibits upon its exterior surface a series of spirally arranged elevations or depressions, as if the shrinkage of the clay in baking had not been entirely uniform. Considerable difficulty was experienced at first in securing the correct temper for the grooved disks, which, of course, should be equal to, but not much higher than, that of the steel balls which separate them. If the disks are insuffi- ciently tempered, they are badly lacerated by the balls. On the other hand, if they are made too hard, they frequently crack under the great pressure to which the clay is subjected.
In order that the diameter of the clay cylinders may be varied, the barrel of the press (Figure I A) is made quite wide (2.5 inches internally) and is provided with a series of steel "sleeves" of various smaller bores — which may be inserted. Each sleeve requires, of course, its own set of disks (Figure 1 A 5, 6, 7, and 8, and Figure 1 B). The length of the cyl- inder is regulated by the number and thickness of the disks (5), which are placed in the bottom of the press before introducing the clay.
The two clays, prepared as previously described, mingle readily in all proportions, giving products which, when baked, are uniform in respect to texture and strength. It was found that all the requirements of the situation are best met by mixing them in about equal proportions by weight. The process of mixing is as follows: (1) Equal weights of the air-dried and pulverized clays are mingled and repeatedly sifted; (2) the mixture is churned with water for several hours, after which (3) it is bolted — without unnecessary interruption of the churning process — through Nos. 14 and 16 bolting-cloth; (4) the material is allowed to subside, and the supernatant water is removed by means of a siphon; (5) the major portion of the large excess of water still remaining with the clay is removed by draining upon a filter of bolting-cloth resting upon one of paper, or by the "endosmose" method already described ; (6) finally the material is extensively kneaded and mixed upon a plate- glass surface until, through evaporation of the water, the "putty" has attained the consistency which experience has shown to be best suited to pressing.
The putty, which must never be touched without first covering the hands with rubber gloves, is "tamped" down in the press with a steel plunger which has been cleansed with ether.
The device for compressing the clay is shown in Figure 2, without the framework which holds the various parts in their places. The press (1) containing the clay is secured between two flat bars of steel, a por- tion (2) of one of which is seen in the figure. The lower end of the vertical shaft (3) is square in form, like the upper end of the plunger of the press (Figure 1-4.), and the collar (4), which joins the two, has a square hole of the same diameter passing through it. A portion of two of the timbers of the framework is shown in the figure (5 and 6). Through these, the shaft (3) slides freely up and down, except so far as its motion is limited by the set collar (7). The large wooden drum (8)
10
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
i> firmly attached to the shaft, and around it is wound the steel-wire cable (9). The loose iron pulleys (10, 11, and 12) serve to guide the cable. The large pulley (12) is situated in the attic of the laboratory. The cable, after leaving the horizontal pulley (11), ascends vertically through the ceiling of the room and passes over the attic pulley (12). The descending end of the cable is attached to a heavy iron rod (13), upon which may be loaded any required number of cast-iron weights (14 and 15). At the floor, the weight (consisting of 13, 14, and 15) enters a vertical shaft more than 50 feet in depth. The1 detachable wrench (16), which is provided with extensions, is employed in raising the weight and coiling the cable about the drum.
Fio. 2. — Apparatus for pressing clays.
(1) Steel press (see Fig. 1 A); (2) steel frame for holding press; (3) movable steel shaft; (4) collar
1) and (3); (5) and (6) parts of wooden framework; (7) set collar to limit vertical
"ft; (8) drum for coiling cable; (9) steel cable; (10) vertical loose guide pulley for
) horizontal loose guide pulley for cable; (12) loose cable pulley in attic; (13)
saddle for weights; (14) and (15) cast-iron weights.
Aft<T filling the press, and before placing it in the position shown in
it is.put into a vise and considerable of the water is forced out
by use of t lie wrench (16), which is given a turn from time to time as the
CELLS AND MANOMETER ATTACHMENTS.
11
escape of water makes further compression of the clay possible. When no more water can be forced out, the press is transferred to its place in Figure 2. The amount of weight to be applied and the duration of the period of pressing are judged entirely by previous observations on the fitness of the products for cutting purposes. In general, the weight employed is that which will give a calculated pressure of 20 tons upon each square inch of the surface of the clay cylinder after an allowance of one-third for loss by friction. The first descent of the weight (53 feet) is usually accomplished in about 2 hours and the second in about 16 hours. Ordinarily the pressing is discontinued at the end of the second excursion of the weight. Equivalent results can be secured by lighter weights and longer pressing or by heavier weights and shorter pressing. The object to be attained is, of course, that condition of the clay which will enable one to cut a perfect cell from the pressed cylinder, and for this purpose the clay must be neither too wet nor too dry.
When the cylinder is to be removed, the press is again placed in the vise, the upper cap (Figure 1 A) is removed and an additional disk is introduced. On replacing the upper cap and removing the lower one, and giving the plunger (Figure 1 A , 4) a slight turn, the cylinder is effectively released, and — owing to the tapered form of the barrel (Figure 1 A) — uninjured. The form of the cylinder is shown in Figure 3 A.
THE CUTTING OF THE CELLS.
One of the commoner forms of the finished cell as it is turned out of the cylinder (Figure 3 A) is shown in Figure 3 B. Other forms will be represented when the attachment of the manometer to the cell is discussed.
The cutting of the cells from the cyl- inders is an exceedingly critical opera- tion, which requires experience and well- developed mechanical instincts. Very few, even of those who have had mechanical training, ever succeed in the undertaking. The explanation of so many failures is very simple: The cell wall must not be weakened at any point by the pres- sure of the cutting tools; because, when the cell is baked, the shrinking material (the shrinkage is between 7 and 8 per cent) necessarily draws away from the regions of relative weakness toward those where the cohesion of the particles is stronger and cracks are developed. It is by no means necessary that the damage done by irregular or excessive pressure from the cutting tools should be apparent in the finished
A B
FIG. 3.
A. Clay cylinder after pressing.
B. Clay cell after shaping the cylinder
(Fig. 3 A) on the lathe.
12
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
product. In fact, it is rarely discovered until the cells are taken from the kiln. At first, the failures from the cause alluded to were over 90 per cent. At the present time, about 10 per cent of the cells develop cracks while in the kiln. The improvement has been due in a large measure to improvements in the cutting tools and to the increased atten- tion which has been given to keeping them in good order. It was found impossible to succeed with the usual lathe cutting tools, and others with new forms of cutting edge were designed. One of the more important of these is shown in Figure 4. It is the tool with which all the boring and nearly all the inside work are done. It will be seen that the tool cuts only in the longitudinal direc- tion of the cylinder, bringing no pressure upon the wall of the cell in a transverse direction. The same principle is employed in fashioning the tools with which the out- side work is done. But however good the design of the tool may be, failure is bound to attend its use in this work unless it is ground in accordance with correct princi- ples, i. e., with the proper "clearance," and is always maintained in a sharp condition.
A multitude of details relating to methods of mounting, speeds of cutting, etc., all of which are of importance to the operator, but of little interest to others, are omitted, and only one instance of the many precautions which it is necessary to observe will be men- tioned—the fact, namely, that, when the lathe has once been started, it must not be stopped until the cell is finished, owing to the danger of "sagging."
THE BURNING AND GLAZING OF THE CELLS.
The experimental work in the baking of
clays was done for several years either in a FlG- 4- — Different views of special Seger kiln or in an electric furnace. The electric furnace which was first employed was one in which the platinum wires were woven through holes in the walls and bottom of the furnace, so that the heat generated in the wires must penetrate a considerable thickness of clay before reaching the space to be heated. With such an arrangement a very long time is required to obtain, with a given current, the temperature which
tool for cutting cell (Fig. 35) from cylinder (Fig. 3 A).
CELLS AND MANOMETER ATTACHMENTS. 13
that current will eventually maintain in the furnace. In the case of the instrument here mentioned, from 7 to 9 hours were required for that purpose. A close regulation of the temperature was there- fore impossible. Another objection to this furnace was its waste- fulness. At 1250° the consumption of electrical energy was equivalent to 1200 watts. The furnace was improved to some extent by certain modifications which were introduced, but not sufficiently to justify its continued use. It was therefore abandoned for one of our own con- struction,* in which the wires were all exposed in the space to be heated. The saving in electricity thereby effected was over 50 per cent. The new furnace is shown in Figures 5 A, 5 B, 5 C, and 6. It will be seen to consist (Figure 5 A) of platinum wires threaded through three clay rings (a, b, and c), which are held apart by three platinum rods. The rods expand in the same degree as the wires, and thus keep the latter taut, whatever may be the temperature of the furnace. Otherwise the wires would "buckle" and short circuit at high temperatures. The wires are in two pieces of equal length, so that they may be placed in series or in parallel, according to the amount of current which it is desired to use. Figure 5 B shows the furnace in place in the innermost (d) of the clay cylinders which surround it when in use. The cover (e), the bottom (/), and the truncated cones (q) on which the furnace rests are also represented in the figure. Figure 5 C represents the outer clay cylinder and its various accessories. In Figure 6 all the parts, lettered as in Figures 5 A , 5 B, and 5 C, are assembled as a crucible furnace. The outer covering (m) is a sheet-iron cylinder, which is covered, internally and externally, with asbestus paper. The purpose of the remaining parts (n, o, p, q, r, and s) is obvious without explanation.
The electric kilns (of which three were usually in operation) were all calibrated by means of a Le Chatelier pyrometer. They thus became, in themselves, resistance pyrometers, the temperature of which could be easily ascertained at all times. The electric kilns answered well the purpose for which they were constructed up to about 1200°, i. e., to a temperature at which platinum begins sensibly to volatilize in an atmosphere containing oxygen. At higher temperatures, the loss of platinum was sufficient to make an occasional recalibration necessary.
The best results were obtained at about 1300°, i. e., between the melting-points of Seger cones Nos. 8 and 9. Having ascertained the most advantageous temperature for burning the cells, there was no longer any good reason for baking them in the laboratory rather than at the pottery. Fortunately, at the opportune time, we were offered the free use of the kilns of the Chesapeake Pottery Company by the late president of that concern, Mr. D. F. Haynes. A similar courtesy was also extended to us by the Bennett Pottery Company. At the
*Amer. Chem. Journal, xxxu, 93.
14
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
FIG. 5.
A Electric kiln for baking cells, (a), (6), and (c) perforated clay rings, held m place by three platinum rods which prevent the platinum wires from " buckling" when hot.
B. Inner covering for electric kiln, (d) Clay cylinder; (e) cover; (/) bottom; (0) trund
C. Outer covering for electric kiln, (h) Clay cylinder; (;) bottom; (fc) truncated clay cones.
FIG. G. — Electric kiln arranged as crucible furnace.
(a) to (fc) the same as in Figs. 5 A, 5 B, 5 C; (V) Le Chatelier pyrometer; (m) sheet-iron cylinder covered with asbestus; (n), (o), and (p) parts of base; (q), (r), and (s) parts of elec- trical connections; (0 rest for crucible.
CELLS AND MANOMETER ATTACHMENTS. 15
potteries, we could neither control nor know with certainty the tem- perature of any part of the kilns, but the places in them where the best results are most frequently obtained were easily found, and since then all of the cells have been burned at the potteries.
It has been stated elsewhere that, in the endeavor to produce the cor- rect texture of cell wall, we made a study of thin sections, both of the potters' cells and of our own. In the course of this work, a considerable number of photographs were accumulated, 6 of which are here repro- duced. Three of them (Plate 1, a, c, and e) are from potters' cells, and three (6, d, and/) are from the first cells made by us which proved them- selves well suited to the measurement of osmotic pressure. It will be noted that the texture of the cells made in the laboratory is incom- parably finer than that of the potters' products. But we were con- vinced, after nearly five years of laborious investigation, that just this excessive fineness of texture is absolutely indispensable to the correct measurement of osmotic pressure. It is necessary, in the first place, in order that the membrane may be deposited exclusively upon the inner surface of the cell wall. It is not meant by this statement that no part of the membrane is to be found within the pores. On the contrary, all good membranes are found, on microscopic examination, to be firmly rooted in the mouths of the pores which open behind it. It is this feat- ure, in fact, which makes membranes produced by the electrolytic method so much superior to those which were made by the older pro- cess. Fineness of texture is also necessary in order to give the mem- brane a backing which will enable it to withstand pressure. If it is more open than that shown in Plate 1, 6, d, and /, the membrane is deposited, at least partially, within the cell wall, and it breaks under moderate pressure.
It is desirable to explain the numerous black specks seen in Plate 1, b, d, and /. They are particles of the emery used in grinding the sec- tions, and no part of what the photographs are intended to show.
The exact extent to which the sections here represented were magni- fied can not now be stated, the original records having been mislaid or lost, but it is believed to have been 125 diameters.
The question naturally arises, whether it is possible to make the text- ure of a cell wall too close, provided, of course, it still remains porous to some extent. The effective area of a membrane is equal to the aggre- gate area of the pore-openings upon the interior surface of the cell wall, and it has been found quite possible, by hard burning, so to diminish this area of membrane as to make the passage of solvent into or out of the cell intolerably slow. Some evidence has also been gathered to show that the reduction in the size of the pores may be carried to such an extent that the membrane no longer roots itself firmly into them. This is the explanation given to the formation of the detachable membranes which are sometimes deposited in very hard-burned cells. It is imagined that,
16 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
i 1 1 such cases, the ferrocyanogen ions do not get far enough into the pores before meeting those of copper coming from the opposite direction — in other words, that the membrane is formed at or without rather than within the mouths of the pores.
After baking the cells and before glazing them, they are mounted on the lathe and ground under the shoulder with a high-speed carborundum wheel, to fit the brass rings with which the manometers are fastened in their places. The necks are also ground to the exact taper of the cones upon the ends of the manometers.
The finding of a suitable glaze for the upper half of the cells was a matter of considerable difficulty. As might have been expected, the expansion coefficient of products made as these cells are is very different from that of any of the potters' wares. Hence none of the glazes which are used by the potters would meet the requirements of the situation. All such glazes were found to "craze" badly upon the biscuit. An attempt was made to glaze with feldspar, but with poor success. A wholly suitable glazing material was finally obtained by adding silica and feldspar to one of the glazes which are used by the potters upon the better grades of their white tableware. The earlier experiments in glazing were carried out in a Seger gas kiln, but at the present time the glazing, as well as the baking of the cells, is done at the potteries.
There is one objection to glazing the cells to which attention should be called. They are glazed, inside and outside, from the middle upward, leaving the lower half of the cells porous. The whole interior of the cell is therefore protected at all times, either by the glaze or the membrane, so that no material in solution can diffuse into the wall from the inside. On the outside, the case is different. There it is quite possible for the dissolved substances to diffuse upward and accumulate between the inner and outer glazed surfaces. If these were allowed to remain and should afterwards diffuse downward and distribute them- selves about the membrane, the pressure measured would not be that of the solution within the cell, but rather the difference between the pres- sures of the solutions on the opposite sides of the membrane. It is not believed that the results to be reported in later chapters have been at all vitiated by this possible source of error; because it has always been necessary, in order to maintain unimpaired the colloidal state of the membrane, to soak the cell for considerable intervals in pure water iHween any two successive experiments. Nevertheless, it seemed de- Me to produce a cell, the upper half of which has the non-permeable : actor of porcelain, while the lower half remains porous. The diffi- culty is, of course, to prepare clay mixtures for the two parts of the cell \\liirh shall maintain identical expansion coefficients throughout the rhole of the baking and cooling periods — at least at all points of union Keen them. Otherwise cracks or a condition of weakness must develop at the junction of the two clays.
MORSE
PLATE 1
FIGS, a, c, and e, thin sections taken from potter's cells.
FIGS, b, d, and/ thin sections taken from cells made in the laboratory
CELLS AND MANOMETER ATTACHMENTS. 17
Occasional experiments with a view to producing a half-porcelain, half -porous cell have been carried out along two lines: first, by so mix- ing the two kinds of clays that for a certain distance from the center, upward and downward, each kind would disappear gradually; second, by mixing some of the glazing material with the clay which was to form the upper half of the cell. The results have been encouraging, though up to the present time not wholly satisfactory.
THE MANOMETER ATTACHMENTS OF THE CELLS.
Great difficulty has been experienced in devising suitable arrange- ments for attaching the manometers to the cells. The problem is less simple than it might appear to be at first sight. Three things must be provided for in any workable device for closing the cell: (1) a junction which will not leak at high pressure; (2) means of adjusting, at will, the pressure in the cell (this is especially necessary when manometers of large capacity are used) ; and (3) an arrangement so simple in manipu- lation that the cell can be filled and closed and the proper initial pres- sure established in a fraction of a minute. Several schemes have been employed for joining the cell to the manometer, all of which, with two exceptions, are still in use. Some of the arrangements which worked satisfactorily at moderate temperatures failed utterly at high temper- atures.
The first crude experiments* were made with cells into which rubber stoppers — carrying manometers — were thrust and fastened in place as well as might be with wire. The highest pressure obtained by such means was only 4.5 atmospheres. The manometers were pushed out of the cells and, owing to the tendency of rubber to flow into regions of less pressure, the stoppers were badly distorted. The earlier experiments, however, were only qualitative. They were made in order to test the membrane rather than with a view to measuring osmotic pressure. Other qualitative experiments were carried out laterf with somewhat improved apparatus, but the earliest successful attemptsf to measure osmotic pressure were made in the apparatus shown in Figure 7.
The porous cell (A), which is unglazed, is ground out internally to a distance from the open end which is a little over one-third its depth, until the shoulder formed at the bottom of the ground part extends entirely around the cell and is of sufficient width to afford an ample sup- port for the soapstone ring (b) . Afterwards two channels, one of which is designated in the figure by the letter a, are cut into the wall to pre- vent the dislodgment of the cement under pressure. The glass tube (B), which connects the cell with the manometer, is enlarged in two places (c and d) to prevent its displacement, and is contracted at the top to give it a better grip upon the rubber stopper (e). The soapstone ring (6) is accurately fitted to its place in the cell and also to the glass
*Amer. Chem. Journal, xxvi, 80. \Ibid., xxvm, 1. \Ibid., xxxiv, 1.
18 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
tube (£), the end of the latter having been ground to a perfectly cir- cular form. The lower end of the glass tube is beveled inward to pre- vent the lodgment of air. The purposes of the brass parts (g, h, and o) are obvious without explanation. The tube (B) is set in the brass piece (o) and in the cell (A) with litharge-glycerine cement. But before proceeding to the latter operation, the glass tube, with the soap- stone ring in place, is inverted, and any space which is left between them is filled with molten shellac. The tube and ring are then heated in an air bath until the shellac remains solid at 100°. The cement employed to fix the tube and the ring (B and 6) in their places in the cell (A), and also the shellac used to join b to B, must be effectually protected from any contact with the solution in the cell or the water outside of it. For this purpose, the lower end of the glass tube, the soapstone ring, and the whole of the ground surface within the cell are repeatedly painted with a dilute solution of rubber. When a covering of sufficient thickness has been obtained, the soapstone ring — which is now firmly attached to the glass tube — is crowded into its place on the "shoulder." The operation is liable to lacerate more or less the rubber covering of the cell wall. To repair any damage of this kind, and also to insure a tight joint between the clay wall and the soapstone ring, the whole cavity above the latter is again painted with the rubber solution. The apparatus is then placed in an air-bath and maintained at 100° until the rubber becomes quite hard but not brittle. Finally the space between the glass tube and the cell wall is filled with the usual mixture of litharge and glycerine. The lower end of the manometer is enlarged (f) to prevent its being pushed upward through the stopper (k). The purposes of the cork (1) and of the bottle (m) do not require explanation. A special instrument, which came to be known as the "fang," is required both to close and to open the cell. It is shown in Figure 8. It consists of a round, slender, and tapered piece of steel, one end of which has been furrowed out upon one side and bent into the curved form seen in the figure. It was usually made from a small round file from which the temper had been drawn. The "fang" is inserted between the rubber and the glass tube at e, to permit the escape, through the furrow, of the excess of liquid when the cell is closed, and again to provide for the entrance of air when the cell is opened. It is likewise of great assist- ance, when manipulated as a lever, in introducing and removing the stopper through the narrow mouth of the tube. The stopper from e upward is tightly wound with shoemakers' waxed thread to prevent the
•The "fang" for the introduction and removal of the rubber stoppers (k, Fig. 7).
CELLS AND MANOMETER ATTACHMENTS.
19
Fia. 7. — First form of complete cell. (A) Porous cell; (£) glass tube; (C) manometer; (o) groove cut in cell; (6) soapstone ring: (c) and (d) enlargements in glass tube, to pre- vent slipping; (e) contraction at upper end of glass tube; (/) and (n) litharge-glycerine cement; (0) brass collar; (h) brass nut; (i) concave brass piece; (;') enlargement on end of manometer; (/c) rubber stopper; (I) cork; (w) glass bottle; (o) brass piece.
Fia. 9. — Second form of complete cell. (1) Brass collar; (2) brass nut; (3) lead washer; (4) orasscone; (5) manometer; (6) hollow needle; (7) fusible metal; (8) brass piece to which the needle is brazed; (9) steel screw threaded into (8); (10) packing; (11) fusible metal covering exposed part of needle; (12) rubber tubing; (13) and (14) windings with twisted shoemakers' thread.
20 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
rubber from oozing out of the glass tube. The initial pressure in the cell is adjusted by means of the nut (h) and the collar (g).
The arrangement described above was employed for the measurement of osmotic pressure from 1905, when the first good cells were obtained, until 1908, when, for reasons which will be stated, but not fully discussed until later, it was abandoned for the apparatus which is shown in Figure 9.
The principal objections to the first apparatus employed for quanti- tative purposes (stated in the order of their importance) were :
1. The length of time required to close and open the cell. During both periods, the contents of the cell, being necessarily under less than maximum pressure, became diluted by the water which entered through the membrane.
2. The difficulty of the manipulation required properly to introduce and remove the rubber stopper without injury to the manometer.
3. The frequent bursting of the glass tube (B), which was usually attended by the total loss of the cell (A) ; since, as a rule, the membrane was ruined by the measures taken to replace a broken tube.
The apparatus represented in Figure 9* is a decided improvement on that shown in Figure 7. In it the difficulties enumerated above are obviated, though, as will be seen later, it has certain defects of its own. The function of the brass collar (1) and of the brass nut (2) will be readily understood without explanation. The form of these pieces has varied but little from the beginning. The lead ring (3) separates the shoulder of the cell from the flange of the brass collar and serves to pro- tect the glaze upon the former. A ring of softer material, e. g., leather, can not be used for the purpose, since any upward movement of the collar, due to diminishing thickness of the ring under pressure, leads to an increase in the capacity of the cell and a dilution of the solution. In other words, the ring (3) must be of fairly rigid material. The brass cone (4) has two holes passing entirely through it, one for the mano- meter tube (5) and the other for the hollow needle (6), both of which (the manometer and the needle) are securely fastened in the cone by some fusible metal (Wood's, Rose's, or Babbit's). The holes through the cone are bored slightly larger than the tubes which are to occupy them, in order that the molten metal may flow down and completely fill the space between the latter and the walls of the former. In this way the tubes are more firmly fixed in their places and all danger of leakage upward through the cone is avoided. The hollow tube (6)— the needle— is nickel-plated and is brazed into the brass piece (8), which is bored out and threaded internally at the upper end to fit the closing plug (9). The upper end of 8 and the lower end of the larger portion of ) are made concave in form, and between them is placed the packing 10). The concave form of these two surfaces is essential, since it pre-
*Amcr. Chem. Journal, XL, 260; XLV, 91.
CELLS AND MANOMETER ATTACHMENTS. 21
vents any outward lateral movement of the packing and causes the latter to close up tightly on the thread of the screw. After fixing the needle and the manometer tube (or rather the tube (5) which is to be fused to the manometer) in their places, the cone (4) is extended by means of the fusible metal (11) in order to protect the lower end of the needle. Over the cone, thus extended, is slipped the rubber tube (12) which is tightly wound at the lower and upper ends (13 and 14) with twisted shoemakers' thread. Owing to the ease with which rubber moves in the direction of smaller pressure, the whole space (14) between the shoulder of the brass cone and the top of the cell must be covered and rigidly supported by the thread. In practice, the winding of the upper end of the rubber tube is carried so far down that one or more turns of the thread are forced into the tapered neck of the cell.
A manometer, on whose calibration, capillary depression, and final verification weeks and perhaps months of labor have been bestowed, is too precious an instrument to be unnecessarily exposed to danger. Hence the cones are not attached in the first instance to the manometers, but always to short pieces of tubing of the same kind, which are afterwards fused to the manometers or cut off from them, as the occasion may arise.
At low and moderate temperatures, the arrangement just described renders very satisfactory service, and between 0° and 60° it is still in use. At higher temperatures, it develops certain defects which are so serious as to render its use quite impracticable. Leaks appear, due to increasing difference between the expansion coefficients of brass, glass, and the fusible metal; the alloy attacks the brazing or solder used in attaching the hollow needle to the brass piece (Figure 9) ; the glass of the manometers becoming brittle after continued use at high temperatures, it is difficult to fuse them on the glass tubes which pass through the cones ; finally, at high temperatures, the rubber, used between the brass cone and the neck of the cell, also becomes brittle and liable to crack.
The deterioration of rubber at moderately elevated temperatures — apparently due, in our case, to a resumption and continuation of the vulcanizing process in the baths — has given much trouble, but we have not been able wholly to dispense with its use. We are able to make tight joints without it, but not, as yet, any satisfactory device for adjusting pressure in the cell.
The remainder of the manometer attachments which are here described were devised for use at the higher temperatures or with elec- trolytes, though they render equally good service at moderate and low temperatures, and, of course, also with non-electrolytes.
In Figure 10, the cone (a), which closes the neck of the cell (A), is turned on the lower end of the brass tube (B). At b there is a vent for the escape of air and any excess of solution. The usual collar and nut for fixing the manometer in the cell and for adjusting the pressure are seen at c and d. The manometer (C) is held tightly in its place in the
22
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
FIG. 10. — Third form of complete cell. (A) Porous cell— upper half glazed; (B) brass piece; (C) manometer; (a) conical end of (B);
(b) vent for solution; (c) brass collar; (d) brass nut resting on ledge of (£); (e) and (e) brass rings between which packing is placed; (/) brass collar screwing down upon (B) and com- pressing the packing between the rings (e)t (e); (g) brass ring.
FIG. 11. — Fourth form of complete cell.
(o), (a) Brass or porcelain rings for compressing the packing and displacing it laterally; (6) brass tube around which all metallic parts are assembled, upper end the same as in Fig. 10;
(c) brass piece employed in compressing packing and in adjusting initial pressure; (e) brass collar; (/) brass ring; (g) brass nut, threaded internally, which is employed in adjusting initial pressure by moving the tube (6).
CELLS AND MANOMETER ATTACHMENTS.
23
tube (B) by packing which is compressed between the rings (e, e) .
form of the rings is such as to force the material
of the packing in both of the lateral directions — on
the one side toward the manometer, and on the
other toward the wall of the brass tube. The com-
pression is effected by means of the hollow nut (/) .
The brass ring (g), which serves as a "follower,"
is made of any required length. A rubber tube
is slipped over the cone (a) and tied above and be-
low the neck of the cell in exactly the same manner
as in the apparatus shown in Figure 9. The reason
for the concave form of the surfaces between which
the packing of the vent (6) is compressed has al-
ready been explained. The packing between the
rings (e, e) usually consists of alternate
disks of leather and thin rubber, one
of the rubber disks being placed below
the lower ring, i. e., between it and its
' 'seat . ' ' The seat and the under side of
the lower ring are grooved to prevent
too much lateral movement on the part
of the rubber between them in the
direction of the manometer; otherwise
the greater part of the material of this
lowest disk would be crowded into the
cavity below the ring. All brass sur-
faces which are exposed to the liquid con-
tents of the cell are plated with nickel, silver,
or gold, according to the character of the
solutions whose pressure is to be determined.
The arrangement shown in Figure 10 has two great advantages over that presented in Figure 9. The use of fusible metal is avoided, and, if the right kind of packing is used, it is not necessary at any time to sepa- rate the calibrated end of the manometer from the end entering the cell, since the manometer is always sufficiently released by unscrewing the nut (/) to permit of its easy withdrawal from the tube (B). It also does away with the plated steel needle (Figure 9,6), which may be corroded if the solution contains an electrolyte.
In the apparatus seen in Figure 11 the cone (Figure 10, a, a,) is dispensed
The
(a) Enlarged end of manometer; (6) brass piece fastened to manometer by litharge- glycerine cement; (c) hollow brass nut resting upon (6) ; (d) brass collar; (e) vent for solution; (/) brass cap with packing in bottom.
24 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
with, and the union between the manometer attachment and the cell is effected by means of the brass rings (a, a, a, a) and the packing which is placed between them. The packing is compressed in the ver- tical direction and made to expand horizontally against the cell on the one side and the brass tube (6, b) on the other by the sliding piece (c, c). The collar (e, e) and the nut (/, /) do not differ essentially from the corre- sponding pieces seen in Figures 9 and 10. The vent and the arrange- ments for fixing the manometer are the same in Figure 1 1 as in Figure 10. The adjustment of pressure within the cell is effected by means of the nut (g, g). Turned to the right, it drives the tube (6, b), and with it the manometer, into the cell, increasing the pressure. If it is turned to the left, the tube and manometer are raised and the pressure diminished. The principal advantages of the arrangement seen in Figure 1 1 over that shown in Figure 10 are in the better means of adjusting the pressure and in the substitution of packing for rubber tubing in making the joint with the cell.
In measuring the osmotic pressure of electrolytes, it is desirable to avoid, as far as possible, any contact of the solutions with metallic sur- faces, even though the same are protected by plating with the more resistant metals. The covering is often imperfect in spots, notwith- standing the care which is taken in the plating. Accordingly, a number of schemes have been devised for joining the manometer and the cell, in which the solution comes in contact only with glass and rubber.
In Figure 12, the hollow glass cone (a) serves the same purpose as the brass cones seen in Figures 9 and 10. It is set in the brass piece (6) with litharge-glycerine cement. Its use, in connection with the usual collar (d) and nut (c), is apparent. The cone, like those in Figures 9 and 10, is covered with rubber tubing, which is wound and tied at the upper and lower ends with twisted shoemakers' thread. The side tube (e), which serves as a vent for the escape of surplus solution, is embedded with cement in a brass tube, which is threaded externally to receive the cap (/). The packing in the bottom of the cap closes the vent. It will be seen that the means of adjusting pressure within the cell is the same in all three of the instruments represented in Figures 9, 10, and 12.
Another form of glass cone which has rendered good service is seen in Figure 13. The cone, which is made of a solid piece of glass, is bored excentrically for the manometer (a) and the vent (b). The vent is closed at the lower end by the rubber disk (e), which is attached to and controlled by a platinum rod running through the hole in the stopper. The upper end of the rod is threaded and provided with a nut, as seen in the figure. To prevent the solution which escapes through the vent from coming into direct contact with the cement, all exposed parts of the latter are painted with a solution of rubber. The solid glass cone has some advantages over the hollow one, as will appear when the manipu- lation connected with filling and closing the cells is explained.
CELLS AND MANOMETER ATTACHMENTS.
25
15
FIG. 13. — Solid glass stopper for use with substances which attack metals, (a) Manometer tube; (6) vent for solution, closed by valve at lower end of stopper.
FIG. 14. — Glass manometer attachment for cells with straight necks.
(a) Manometer with straight tube fused to lower end; (6) space between manometer and glass tube; (c) brass ring; (d) and (e) porcelain rings for compressing packing; (/) brass collar; (0). W» (t), and (j), brass pieces with which to close the cell, and also to adjust initial pressure; (k) vent for solution.
FIG. 15. — Glass manometer attachment for cells with straight neck.
Like that shown in Fig. 14, except that the glass tube is left open at the top, and then closed with a brass cap and litharge-glycerine cement.
26 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
Still another glass device for attaching the manometer to the cell is seen in Figure 14. It was designed for use with the form of cell which is seen in Figure 11. The manometer (a) passes entirely through the closed tube (6), whose outside diameter is only a little less than the interior diameter of the cell. The means for compressing the packing (c, d, e, /, and g) and fixing the large tube (6) in the cell do not differ essentially from the analogous parts seen in Figure 11, except that the lower ring (e) is made of porcelain, in order that the solution in the cell may nowhere come in contact with metal. The adjustment of pressure within the cell is effected by means of the brass pieces (h, which is fixed in its place with cement) , (i), and (j) . The vent (k) is not absolutely necessary, though it is sometimes a convenience. Instead of opening the vent when it is desired to lessen the pressure, the nut (j) may be turned slightly to the left.
The device shown in Figure 15 is a substitute for that seen in Figure 14. The two differ only in that the large tube (6) is closed at both ends in Figure 14, while it is open at the top in Figure 15. The latter is easier to make, and is in no way inferior to the closed form.
CHAPTER II. THE MANOMETERS.
The possibility of correctly determining osmotic pressure depends upon four fundamental conditions, no one of which can be said to exceed another in importance. They are (1) a suitable cell, i. e., a cell which is able to support the membrane under high pressure and in which the membrane is always deposited upon the interior surface of the porous wall; (2) a truly semi-permeable membrane, i. e., a mem- brane which does not leak the solute; (3) a perfectly automatic and exact regulation of temperature ; and (4) an accurate calibration of the manometers. If any one of these conditions is unfulfilled, all efforts to measure the force must lead to erroneous results, which are not only futile but positively mischievous — mischievous because they furnish the opportunity for an indulgence of the propensity of the over-hasty and unwary to erect elaborate speculative structures upon foundations of what may be justly called tainted facts.
The manometers which are used for the measurement of osmotic pressure have an external diameter of about 6 millimeters. The length of the calibrated portion varies from 400 to 500 millimeters. The diameter of the bore ranges from 0.45 to 0.72 millimeter.
The reasons for using tubes of very small bore are:
1. It is necessary to fill the upper ends of the manometers with short columns of mercury, because in closing the instruments, after the intro- duction of the gas, the caliber of the tubes in that region is affected to an unknown extent. If the internal diameter is large, e. g., 1.0 millimeter or more, the mercury is often dislodged by the severe tapping to which the manometers are subjected at certain times.
2. The compression of the small volume of gas which they contain involves but little dilution of the cell contents.
3. Relatively small volumes of mercury are required by manometers of small bore. The importance of this fact will be better understood when the subject of ''thermometer effects" is discussed.
The disadvantages of using manometers of small bore are :
1. It is more difficult to deal satisfactorily with the meniscus in a narrow tube.
2. The capillary depression is large in small tubes and it varies greatly with slight irregularities of bore.
3. The movements of the mercury in narrow tubes are strongly influenced by the presence of minute quantities of impurities, whether the same are dissolved in the metal itself or are attached to the surface of the glass.
28 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
PURIFICATION OF THE MERCURY.
The material which ordinarily passes for pure mercury in the labora- tory is by no means suitable for manometric work, and to obtain it in adequately pure condition for this purpose requires unusually thorough treatment. The mercury which is used in our manometers — and also that which is now used in the bath thermostats — is cleansed in the following manner:
1. The commercial material is first filtered through paper filled with pin holes to free it from dirt. It is then heated for four hours to the boiling-point in a glass retort, to the neck of which a long glass tube has been fused for the condensation and return of the vapors; and during this time a current of air is forced through the boiling metal. On cooling, it is again filtered to remove the scum of oxides which usually forms in considerable quantity.
2. It is distilled in a vacuum.
3. The distillate is washed by the method of Lothar Meyer, but with water containing 2 per cent of nitric acid and 2 per cent of mer- curous nitrate instead of ferric chloride. The apparatus in which the washing is done consists of a wide tube two meters in length, to the lower end of which has been fused a quite narrow tube of the usual double U form, the proportions of the descending and ascending limbs being so selected that the mercury which supports the cleansing liquid shall lie wholly within the smaller tube. To admit the mercury at the top and to regulate its flow, a separating funnel is employed. The lower end of the funnel, instead of being drawn out to a fine point, as in the apparatus of Meyer, is widened out into the form of an inverted funnel, according to the suggestion of Hillebrand, and over this are tied two or three thicknesses of the finest silk bolting-cloth. The material to be purified is thus made to enter the cleansing liquid in hundreds and perhaps thousands of excessively fine streams. It is passed 1,000 times through the solution of nitric acid and mercurous nitrate, and is then thoroughly washed with water and dried.
4. After treating the mercury as described under 1, 2, and 3, it is again distilled in a vacuum, but not in the still (2) which is used for the first distillation.
The mercury which has thus been cleansed retains its brilliant luster in the air, and its movements in narrow tubes are highly satisfactory. We have also prepared mercury from the purest oxide which we could make, but have not found it superior in any way to the product obtained by the means desribed above.
CALIBRATION OF THE MANOMETERS.
The tubes which are used in making the manometers are the most nearly perfect for the purpose which it is practicable to obtain. The essential requirements are that any tube shall be of very nearly uni-
THE MANOMETERS. 29
form bore throughout, and that the form of the bore in every part shall be circular. Very few, if any, tubes conform perfectly to both require- ments. The material from which selections are to be made is imported in lots of several kilograms each, and the purveyors are urged to spare neither pains nor expense in procuring tubes of the highest possible excellence. In each lot of selected material thus obtained, there are usually found — though not always — a few tubes which answer all rea- sonable requirements.
The first step in making a manometer is to etch upon the tube two fine lines extending completely around the instrument. These are usually referred to as the "upper scratch" and "lower scratch," one being near the upper and the other near the lower limit of the calibrated portion of the manometer. These lines are made no coarser than is absolutely necessary in order that they may be distinctly seen through the telescope, since in small tubes a meniscus behind any line, however fine, is apt to give the observer trouble. No other graduation appears upon the manometers. All readings on the instruments are referred to one or the other of the two "scratches.'7 That is, a reading consists always in determining the distance between the meniscus of a mercury column and either one of the lines in question. Since the distance between them is accurately known, readings referred to one line can readily be transferred to the other. The distance between the lines depends upon the length which the manometer is to have — ultimately, of course, upon the height of the available space in the baths. Above the upper and below the lower scratch, a considerable length of tube is left to provide for subsequent operations.
Two methods of calibration have been employed, both of which will be briefly explained, though the earlier one is not now much in use except for preliminary explorations of the tubes.
FIRST METHOD.
Figure 16 represents the instrument which is employed to move and adjust the calibrating thread. A steel screw (a), with a long lever, is threaded through a cap of hard rubber (&), in which the glass tube (d)— enlarged at c — is set with litharge-glycerine cement. In order to make a mercury-tight joint, the upper end of the steel screw is slightly lubri- cated, and around the portion which extends into the glass tube some of the cement is allowed to solidify. The rubber stopper (/), carrying the manometer (g), is inserted (with the aid of the "fang," Figure 8) in the glass tube (d), which is sharply contracted at the upper end (e).
The manometer is drawn out at the upper end to a fine tube which is bent into the form of an inverted U. With the apparatus — including the manometer — nearly full of mercury, the screw is turned to the right until the column enters and reaches the highest part of the inverted U.
30
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
The outlet is then immersed in a globule of pure mercury and the screw is reversed. This manipulation brings a short calibrating thread (h) into the manometer, which is separated from the main body of the mercury by a cushion of air. The thread can now be made to take any desired position in the tube by simply turning the screw and simulta- neously tapping the manometer. The electric "hammer" employed for the latter purpose will be described in connection with the process for the determination of capillary depression.
The process of calibration consists in bringing the lower end of the thread to the point at which it is desired to begin, and then setting it exactly end to end up the tube, determining each time the length of the detached column. When the cali- bration has been carried as far up as is desired, the thread is run out and weighed. Subsequently a long thread of mercury, one filling nearly the whole length of the calibrated portion of the tube, is drawn in and measured, and then run out and weighed. It is evident that the weight of the short thread, multiplied by the number of settings, will
. , ., ,, . -i , ., i /*!!• ,1 FIG. 16. — First arrangement
be less than that of the long thread filling the same for calibrating manometers. length of tube, by the weight of the mercury re- (a) screw for setting caii- quired to fill the double meniscus spaces. By testing thread; (6) hard- means of this relation, the correction for the vol- ume of the double meniscus is readily calculated.
SECOND METHOD.
rubber cup; (c) enlarge- ment in glass tube (i) ; (d) litharge-glycerine ce- ment; (e) contracted end of glass tube; (/) rubber stopper ; (g) manometer ; (h) calibrating thread sep- arated from main column of mercury by air ; (i) glass tube filled with mercury.
The later procedure differs from the earlier one in manner rather than in principle. After etching upon the glass the two lines previously men- tioned, a small bulb is blown near each end of the tube outside the portion to be calibrated. These serve to catch and preserve the cali- brating thread in case of accident. For calibration, the tube is placed in the horizontal position, over a ruled mirror, on the dividing engine, the screw of which has been carefully compared with the graduated meter scales employed in the measurement of osmotic pressure.
The device employed for shifting the thread from one position to another is shown in Figure 17. A is the manometer with its two bulbs (a, a). The two lines of reference previously referred to as the "scratches" are seen at b, b. The shifting arrangement (B) for the cali- brating thread (c) consists of a steel ball (d), a large bicycle ball, which is located in the center of a rubber tube (e). A and B are connected through the glass tubes (/,/) and the rubber tubes (g, g).
THE MANOMETERS. 31
If it is desired to move the thread to the right, the rubber tube (e), to the left of the ball (d), is compressed between the thumb and fore- finger of the left hand until the meniscus has taken the right position under the miscroscope, when, without releasing the tube, the rubber over the ball (d) is pinched between the thumb and forefinger of the right hand until a passage for air is opened. The portion of the rubber tube which is held in the left hand may then be released, since any difference in atmospheric pressure at the two ends of the thread is quickly equal- ized through the passage which has been opened over the ball (d), and without disturbing the thread. If the thread is to be moved to the left, the rubber tube to the right of d is compressed between the fingers of the right hand, and the passage for air over the ball is made with the left hand. After a little experience, the exact adjustment of the cali- brating thread becomes easy and nearly automatic.
TETl
FIG. 17. — Second arrangement for calibrating manometers.
(A} Tube to be calibrated (bore from 0.45 to 0.65 millimeter); (B) rubber tube; (a, a) bulbs blown on each end of tube to prevent loss of calibrating thread; (6, 6) lines of reference etched on tube; (e) calibrating thread; (d) steel ball for setting calibrating thread; (e) rubber tubing ; (/, /) glass tubes ; (g, g) rubber tubes.
The calibration is commenced somewhat below the lower scratch — the etched line to the left — and consists, as when the tube is calibrated in the vertical position, in setting the thread exactly end to end and determining its length until the thread has passed the upper scratch. It is then run out of the tube and weighed. Afterwards the whole of the calibrated portion of the tube is filled with mercury, which is also run out and weighed.
From the length and weight of the long thread, the mean diameter of the bore is calculated; and from the observations on the length of the short thread in the different parts of the tube, a mean calibration unit is derived, and a curve of corrections constructed, exactly as in the calibration of a eudiometer. Finally, a mean value for the double meniscus is obtained from the length and weight relations of the long
32
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
and short threads. If we multiply the weight of the short thread by the number of times its length is contained in that of the long thread, i. e., by the number of times it was set end to end, and subtract the
Fio. 18. — Simplest form of manometer.
(1) and (2) bulbs with traps in the bottom to prevent liquids from working their way into the calibrated portion of the instrument; (3) nitrogen reservoir to prevent loss of gas under diminished pressure; (4) mercury filling the portion of tube whose caliber may have been altered in closing the instrument.
FIG. 19. — Manometer for high pressure.
Differs from that in Fig. 18 only in having a nitrogen reservoir within the calibrated portion of
the instrument.
product from the weight of the long thread, the difference is the weight of the mercury which would be required to fill all the meniscus spaces which were left vacant in setting the short thread end to end along the tube. Converting this difference in weight into volume, and divid-
THE MANOMETERS. 33
ing by the number of settings less one, we obtain a mean correction for a double meniscus, which is the meniscus correction to be applied in all measurements of pressure, since the nitrogen in the manometers is always included between two mercury columns.
The method which is explained above suffices for the simple form of manometer seen in Figure 18, but some modifications are necessary when a manometer of the form seen in Figure 19 is to be calibrated. The peculiarity of the latter instrument is the large reservoir for gas which lies between the two lines of reference and within the calibrated area. The narrower portions — below, from some point under the lower scratch to the bottom of the enlargement, and above, from the top of the wide part to the end of the tube — are calibrated in the man- ner already described. The meniscus correction also is derived from the weight and length relations of short and long threads. So far the procedure is without change. It remains, however, to ascertain the capacity of the wider part as a whole, and eventually in terms of the calibration unit. To do this, the wider part is slightly more than filled with mercury, so that both the upper and lower meniscus are well within calibrated portions of the narrow ends. From the weight of this mercury — with proper correction for overlapping in the narrower calibrated parts — the total capacity of the wider part of the tube is calculated. Two verifications of the correctness of the previous work are now undertaken. It will be noticed, on referring to Figure 19, that the upper line of reference is not very far above the upper end of the wider portion of the manometer. The first step in the verifica- tion is to fill the space between the two scratches with mercury — the upper meniscus may lie somewhat above the upper scratch. The volume of this mercury should, of course, be equal to the sum of the previously found capacities of all of the parts which were filled by it. The final step in the verification is to apply the same test to the whole tube by filling it with mercury from the lower scratch to the upper limit of the calibration.
THE MENISCUS.
In narrow tubes, owing to the small volume of the gas which they contain, the meniscus correction is of considerable importance, since it may amount — especially at high pressures — to an appreciable frac- tion of the volume of the gas.
The significance of the meniscus correction, when translated into pressure, increases with increasing concentration of the solutions with a rapidity which might well astonish one who has not clearly in mind the fact that, though in the first instance it is simply a space of fixed volume, its importance depends, not only on the pressure upon the gas which fills it, but also upon the volume of all the gas in the manom- eter. The effect of this relation in practice is illustrated by means of
34
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
the following tabulation of data taken from the record of a single manometer (No. 9). The meniscus correction (double) in this instru- ment is 0.17 calibration unit, and the volume of the nitrogen under standard conditions of temperature and pressure is 454.14 calibration units. The temperature in all cases is 25°. Column I in Table 1 gives the weight-normal concentration of the solutions; II gives the observed pressure in atmospheres; III shows the volumes of the compressed nitrogen reduced to standard temperature; IV, the corrections in fractions of an atmosphere for the double meniscus; V, -the relative osmotic pressures, the pressure of the 0.1 normal solution being taken as unity; and VI, the relative corrections for meniscus, the correction for the 0.1 normal solution serving as the unit.
TABLE 1.
|
I. |
II. |
III. |
IV. |
V. |
VI. |
|
Concen- tration. |
Osmotic pressure, atmospheres. |
Vol. N2 cal. units. |
Meniscus correction, atmosphere. |
Relative osmotic pressure. |
Relative meniscus correction. |
|
0.1 |
2.635 |
141 . 15 |
0.00317 |
1.0000 |
1.000 |
|
0.2 |
5.139 |
80.69 |
0.01083 |
1.9503 |
3.4164 |
|
0.3 |
7.738 |
55.59 |
0.02366 |
2.9366 |
7.4637 |
|
0.4 |
10.295 |
42.41 |
0.04126 |
3.9070 |
13.0158 |
|
0.5 |
12.947 |
34.01 |
0.06972 |
4.9135 |
21.9937 |
|
0.6 |
15.620 |
28.37 |
0.09360 |
5.9275 |
29.5268 |
|
0.7 |
18.436 |
24.11 |
0.12999 |
6.9928 |
41.0063 |
|
0.8 |
21.258 |
20.97 |
0.17233 |
8.1055 |
54.3628 |
|
0.9 |
24.126 |
18.53 |
0.22133 |
9.1558 |
69.8202 |
|
1.0 |
27.076 |
16.54 |
0.27834 |
10.2755 |
87.8044 |
Particular attention is called to columns V and VI, where it will be seen that, while the osmotic pressure increased a little over ten-fold, the value of the meniscus correction increased nearly 88-fold. Ex- pressed in heights of a mercury column, the correction for meniscus in the case cited in the table increases from a value of 2.4 millimeters to one of 211.5 millimeters.
The method of obtaining the meniscus correction which is given above is believed to be entirely correct in principle. Nevertheless it has been found, in applying it, that the calculated volume of the meniscus is always less than it would have been if the form of the meniscus were truly spherical, as it is generally assumed to be. The experimental correction is usually just about three-fourths that calcu- lated from the supposed spherical form of the meniscus. The difference may be due to unavoidable errors in reading the length of the short calibrating threads. If these are always read (ftoo short," the obvious result would be a too small correction for the meniscus. However, the error, if error it is, is not of a cumulative character. Moreover, if, in calibration, one reads habitually "too short," he will repeat the offense in reading pressures. For these reasons, it is believed to be
THE MANOMETERS. 35
safer to employ the experimental correction rather than that calculated from the known diameter of the tube and the supposed spherical form of the meniscus.
One great advantage of the practice of deriving the meniscus correc- tion from the calibration data is the excellent means which it affords of detecting faulty calibration. It is known that the best work in calibration leads uniformly to an approximately fixed value for the men- iscus, hence it is to be inferred, when another value is obtained, that the calibration which gave it is erroneous.
The inverse relation of the importance of the meniscus correction to the volume of the gas which is measured makes it desirable to increase the quantity of nitrogen in the manometers as far as may be done with- out creating other difficulties of a serious nature. This has been accom- plished by the form of manometer seen in Figures 19, 20, etc., in which the volume of nitrogen is relatively very large. In the manometers of this kind which are in actual use, a length of 1 millimeter in the wider part is about equal in capacity to a length of 16 millimeters in the nar- rower portion of the tube. The column of mercury which occupies the closed end of the manometer, being in the narrow portion of the ma- nometer, is not easily dislodged by tapping. In this respect, the instru- ment seen in Figure 19, etc., is not inferior to the earlier form seen in Figure 18. During a measurement of pressure, the whole of the nitrogen is compressed into the upper and narrower portion of the tube, hence the column of the gas is much longer under any given pressure in the latter than in the former instrument, and the errors due to faulty deter- minations of the value of the meniscus and of the amount of capillary depression are correspondingly less serious in their effects upon the accuracy of the measurement.
Manometers like that shown in Figure 19 are designed more espe- cially for the measurement of the pressure of concentrated solutions where errors of meniscus tell heavily on the results, unless large vol- umes of gas are used. In the case of dilute solutions, large gas volumes are obviously less necessary as a means of minimizing such errors.
The length of the wider portion of the second form of manometer is varied according to the range of pressure which it is desired to measure with the instrument; e. g., if the pressures in question lie between 4 and 6 atmospheres, the wide and narrow portions are so related that the mercury meniscus will appear in the latter at some pressure slightly below 4 atmospheres. In instruments designed for use with normal solutions, on the other hand, the nitrogen is not all compressed into the narrower portion of the tube until a pressure of more than 20 atmos- pheres has been reached.
No considerable dilution of the solution results from the larger vol- ume of gas in such manometers, because, at the time of closing the cell, a mechanical pressure — the so-called initial pressure — is brought to bear
36
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
on the contents, which is nearly equal to the osmotic pressure. Hence the subsequent diminution in the volume of the gas is small. The only disadvantage experienced in the later form of manometer is due to the larger volumes of mercury which must be stored up in them. The "thermometer effects," resulting from slight fluctuations in the temper- ature of the baths, are therefore more pronounced in them than in the other form of instrument.
FIG. 20. — Manometer with glass cone for cells with taper necks (see Fig. 12).
(1) Reservoir with trap; (2) reservoir for expansion of nitrogen under diminished pressure; (3) vent for solutions.
THE MANOMETERS. 37
THE UNCALIBRATED PORTIONS OF THE MANOMETERS.
It will be noticed (Figures 18, 19, 20, and 21) that the uncalibrated portion of all manometers is provided with two or three bulbs, or their equivalents in the form of inserted short pieces of tubing of larger diameter. The bulb nearest the calibrated end (Figures 18 and 19, 3; 20 and 21, 2) serves as a reservoir in which the nitrogen, when under diminished pressure, may expand without danger of escaping from the instrument. Its capacity is regulated by the volume of the gas to be accommodated, i. e., by its original or usual volume, and the maximum probable amount of diminished pressure to which it will ever be subjected. The bulbs nearest the cell (Figures 18 and 19, i and 2; 20 and 21, i) serve as reservoirs for the mercury which is to be driven forward in compressing the nitrogen, and their total capacity is, there- fore, to be regulated by the volume of the gas under ordinary conditions and the maximum pressures to be measured.
For reasons which will appear later, none of the bulbs should be made unnecessarily large. The requirements of the situation may be reduced to the simple rule that some mercury must be left in the bulb nearest the manometer proper under the lowest pressure, and some in the bulb nearest the cell under the highest pressure. It will be noticed that bulbs 1 and 2 in Figures 18 and 19, and their equivalents (1 in Figures 20 and 21) in other manometers, are provided with traps. By means of these, the mercury is made to enter the narrow tubes below at points somewhat above the bottom of the bulbs. The purpose of the arrangement will be understood from the following explanation : When the solution in the cell is under pressure, it drives the mercury before it and enters to some extent the upper end of the nearest bulb. When the pressure is afterwards removed, and the mercury which had been expelled returns, it is apt to entangle minute drops of the solution be- tween itself and the wall of the bulb. Occasionally, during the subse- quent movements of the mercury in the tube, one or more of these drops will persistently work its way forward toward the calibrated end of the manometer, making it necessary, sooner or later, to open, cleanse, and refill the instrument. The "traps" are an effectual prevention of such calamities. Before their introduction, it was frequently necessary to inspect the manometers for the presence of these migrating particles of liquid, and it happened at times that, notwithstanding the greatest vigilance, they escaped detection until it was discovered that the ma- nometers were no longer measuring correctly. Straight tubes, because of their greater strength (Figures 20 and 21, i and 2), are used for mercury reservoirs instead of bulbs (Figures 18 and 19, i, 2, and 3) when high pressures are to be measured.
The manometers shown in Figures 18 and 19 have no vents. They are suitable for use in the arrangements seen in Figures 9, 10, and 11,
38
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
in which the vents are provided for in the metallic parts of the appa- ratus. When all contact of the solutions with metals is to be avoided, as in the case of electrolytes, the vent is of glass and is made a part of
Fio. 21.
Manometer with glass connec- tion for cells with straight necks (see Fig. 14).
the manometer, as in Figures 20 and 21, 3. It has been given a variety of positions on the manometer (see Figures 12, 13, 14, and 15), but on the whole that seen in Figures 20 and 21 is preferred
THE MANOMETERS.
39
CAPILLARY DEPRESSION.
Before joining the calibrated to the uncalibrated portion of the ma- nometer, the former must be subjected to a thoroughgoing investigation of its capillary depression. The mean diameter of the bore of the whole tube is known, that having been calculated from the length and weight of the long thread of mercury which is used in the calibra- tion; also the mean diameters of a considerable number of short spaces, these having been calculated in the same manner from the weight of the short thread and its length in different parts of the tube. But, though such data are useful as a means of judging the excellence of the tube for manometric purposes, they can not be relied upon for the derivation of the capillary depression.
The mean capillary depression of the mercury in the manometer of smallest bore amounts to 18 millimeters, i. e., to more than 0.023 atmosphere. In the remaining instruments, the average depression is about 15 millimeters, or 0.02 atmosphere. The real difficulty with the capillary depression is due to the fact that in most tubes it varies frequently and largely within short distances. In addition to these sharp local fluctuations, there is nearly always a gradual increase or diminution of the depression due to a corresponding general change in the diameter of the bore, the diameter at one end of the tube being usually larger than at the other.
Owing to the large changes which may occur within short distances, it is necessary to determine the amount of the capillary depression at a great many points in a tube. By way of illustrating the importance of doing so, the following partial record of the capillary depressions which were found at different places in one manometer is given. In one column of the table, there are recorded the distances above the lower " scratch" at which observations were made; and in the other, the depressions which were found at these points.
TABLE 2.
|
Distance above scratch. |
Capillary depression. |
Distance above scratch. |
Capillary depression |
|
8.65 |
7.92 |
117.43 |
11.42 |
|
22.70 |
10.85 |
224 . 12 |
11.18 |
|
47.35 |
9.87 |
280 . 30 |
11.74 |
|
71.38 |
10.04 |
361.10 |
11.80 |
|
114.28 |
10.42 |
414.10 |
12.14 |
A difference of 1 millimeter in the capillary depression is equivalent to about one calibration unit in determining the volume of the nitrogen in the manometer. Suppose now the capillary depression of this tube had been determined only at nine points, beginning with the second one, 22.7 millimeters above the scratch. The mean of the values
40
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
is 11.05, which number might have been accepted as the mean capillary depression of the manometer. But suppose when the volume of the nitrogen in the manometer is determined, the meniscus stands 8.65 millimeters above the scratch, where the depression is in reality only 7.92 millimeters. The error, if the mean number 11.05 is used in correcting for capillary depression, would be about 11.05-7.92 = 3.13 calibration units. The whole of the nitrogen in this manometer amounts to only 400 calibration units. The error made in determining the volume would therefore be 0.78 per cent. This example of what might happen if the condition of the tube at 8.65 millimeters above the scratch had escaped detection will serve to convince one of the necessity of a detailed investigation of the capillary depression in tubes of small bore; also of the advisability of using manometers of large capacity, like those seen in Figures 19-21, in order to minimize errors of capillary depression as well as those of meniscus.
FIG. 22. — "Steel block" for the determination of gas volumes in manometers, for the comparison of instruments, and for the determination of capillary depression.
(1) Mercury reservoir; (2) plunger for coarse adjustment of pressure; (3) plunger for fine adjust- ments; (4), (5), and (6) manometers; (7), (8), (9), (10), and (11) packing; (12), (13), (14), (15), and (16) nuts for compression of packing.
Capillary depression appears twice as an important factor in the measurement of osmotic pressure: (1) in determining the volume of the nitrogen under standard conditions of temperature and pressure; and (2) in correcting its volume under an unknown pressure, which (i. e., the osmotic pressure) is a quotient of the two volumes.
An instrument much used in the determination of capillary depres- sion, and also in the comparison of manometers, is the " steel block" seen in Figure 22. It contains a reservoir for mercury (1) and two plungers, one of which (2) is large, and the other (3) small. The larger one is employed for the coarser, and the smaller one for the finer, adjustments of pressure in tubes 4, 5, and 6. The packing (7, 8, 9, 10, and 11), which may be of leather or rubber, or partly of both, is com- pressed in each case between the concave surfaces of two steel disks and the required pressure is brought upon these by means of the
THE MANOMETERS.
41
threaded plugs 12, 13, 14, 15, and 16. The instrument has been tested and found to be mercury-tight up to 350 atmospheres.
Pure mercury only is put into the block, but it can not be presumed, under the prevailing conditions, to maintain its purity unimpaired; hence some precautions are necessary to prevent contamination of the mercury in the instruments under investigation or a fouling of the glass walls of the tubes. The usual precaution is to fuse the calibrated portion of the manometer to one end of a glass tube of nearly equal bore, which has been bent to a double U form. In the intermediate limb a bulb is blown that serves as a reservoir of pure mercury for use in the manometer proper. Having filled the instru- ment with pure mercury, it is fastened in place in the steel block. The arrangement for adjusting the height of the mercury in the tube under examination and for determining capillary depression by differ- ence of level consists of a glass tube having an internal diameter of 35 millimeters, which is connected, by means of a rubber tube, with a second glass tube occupying one of the holes in the steel block. In order to render the rubber tube sufficiently rigid, and thereby to avoid unnecessary oscillations of the mercury meniscus, it is tightly wound with several thick= nesses of insulating tape. The remaining hole in the block is usually occupied by a tube whose capillary depression has been investigated in great detail.
Formerly it was attempted to determine capillary depression by means of comparisons with a standard, i. e., by dispensing with the
wide tube mentioned above and inserting in one of the holes of the steel block a tube whose capillary depression in every part was known. This is a much more convenient method, but it was abandoned because it was found that the errors of the standard add themselves to those of the other instrument. The same difficulty makes itself felt when it is attempted to compare one manometer with another. In such cases it is impossible to tell to what extent the observed discrepancy is due to the incorrectness of the values assigned to the capillary depression of each instrument. It is as likely to be the sum as the difference of the two. In any event, it is, of course, their algebraic sum. Another important instrument in connection with the investigation of manometers is the " tapper " seen in Figure 23. In the measurement
FIG. 23.
Electric hammer for tapping manometers.
42 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
of osmotic pressure, the mercury has ample time to adjust itself, or the adjustment is aided by means to be described hereafter; but in operations connected with the determination of capillary depression, and with the comparison and verification of these instruments, the "lag" of the mercury must be overcome by jarring the tubes, and frequently the tapping to which it is necessary to subject tubes of small bore is severe and prolonged. This is notably the case in manometers in which the glass is not perfectly clean or has been slightly roughened by the reagents employed in cleansing it.
The construction of the "tapper" is so obvious that it is only neces- sary to notice two or three of its features. It is strongly inclined, when in operation, to move away from the tube which the "hammer" is striking. Hence the base is made of lead, for the sake of greater weight, and is mounted upon three very sharp-pointed pegs, which sink somewhat into the wood on which the instrument rests. The hammer is covered with rubber or leather to prevent the possible shattering effect of its blows. The tapper is connected, by means of a flexible wire cord, through the battery, with a portable push-button which is held in the hand of the observer behind the cathetometer, who can therefore at any time hammer the tube without removing his eye from the telescope.
During the determination of capillary depression and other operations which are connected with the preparation of manometers for use, the instruments must be kept at constant temperature. Otherwise the all- important meniscus is continually changing its form, to the great con- fusion of the observer. The first effective device for the maintenance of temperature was the so-called "manometer house," which is seen— stripped of its coverings — in Figure 24 and Plate II. It was in this that, for several years, all experiments on manometers, except calibra- tion in the horizontal position, were carried out. The "house" con- tains the "steel block," the "brass block"— to be described later— the "tapper," a meter scale, a thermostat for the regulation of temperature, electric heaters (lamps), and a fan motor, all of which will be recognized in the figures. The shelf (Figure 24), on which rest the various instru- ments, is supported by heavy steel brackets (not shown in the figure), which are bolted to the heavy masonry wall behind, and afford a satis- factory degree of stability. At each end of the shelf, a space 5 centi- meters wide is left for the passage of air. Lamps are employed as the source of heat, for the reason that they heat up and cool down more quickly than other electric heating appliances. They are under the control of the thermostat seen in the upper part of the house. The fan s stationed before a hole of equal diameter in the partition 2. By means of it, the air, heated by the lamps, is kept in continuous circu- lation over all the instruments. The temperature which is maintained in the compartment is always higher by a few degrees than the highest temperature of the room in which it is located.
MORSE
THE MANOMETERS.
43
The remaining features are better seen in the photograph (Plate 2), where the manometer house is represented with the plate-glass front removed. The end to the right and the top of the house are also of
FIG. 24. "Manometer house" for the calibration and comparison of instruments, etc.
glass, though the latter is usually covered with a thick woolen pad and the former with a flannel curtain. The front is also provided with a flannel curtain (not seen in the figure), which may be parted at con-
44
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
venient places for observation. The frame for the glass at the top is removable to provide for the extension of the house upward when very long tubes are to be accommodated. The various windows and doors are made to close tightly against rubber cushions, or the cracks between them and the frame- work are covered with surgeons' tape. The tubes through which the wires enter the house are, however, left more or less open to provide for equalization of atmospheric pressure.
During the past year or two, the more exacting parts of the in- vestigation of manometers have been carried out in the bath seen in Figure 45. This bath is am- ple enough to accommodate the steel block, the tapper, and all other accessories required for a determination of capillary de- pression, or of nitrogen volume, and for the comparison of mano- meters; and in it temperatures can be maintained for long per- iods which are constant to 0.01°.
Plate 2 shows the type of cathetometer used, and under it a specimen of the devices by means of which the requisite degree of steadiness for all the instruments is secured, notwith- standing their location in the third story of the laboratory. The foundation for the catheto- meter consists of two heavy
WOoden brackets. One end Of FIG. 25.-Improvemcnt in cathetometers for Define the horizontal timbers is buried adjustment of the telescope, which also serves as a
in the thick brick wall behind Sub9titute for the mi<*°meter eye-piece. the house, while the descending timbers pass through the floor and enter the same wall in the room below. There is nowhere contact with a floor or with a parti- tion wall. Two such brackets are required for a cathetometer and three for a bath.
In Figure 25 is shown an improved arrangement for fine adjustment of the height of the telescope, and for reading fractional parts of a milli-
(a), and graduated in hundredths; (c) ring attached to sleeve carrying telescope, and resting on (6) .
THE MANOMETERS. 45
meter on the graduated scale. It consists of a sliding collar (a), on the upper and heavier end of which has been cut a thread of 1 millimeter pitch. Over this runs the internally threaded collar (6) ; and upon b rests the sleeve (c) on which is mounted the telescope. The collar (b) is graduated in 100 equal parts, while c has engraved upon it a vertical zero line. One entire revolution of b corresponds therefore to a rise or descent of 1 millimeter in the telescope, and its movements up and down can be read directly to hundredths, and estimated to thousandths of a millimeter. The device is a substitute for the usual micrometer eye-piece on the telescope, and has the advantage over the latter that it is not necessary to have a precisely fixed distance between the eye- piece and the graduated scale. A second advantage, considered as a means of elevating and lowering the telescope, is that the whole weight of the telescope and its balanced carriage is uniformly distributed upon the top of the collar (6) and ultimately upon the upper side of the thread. Hence, when the collar is turned, there is neither any of that "lurching" of the telescope which is so offensive in the older arrangements, nor any "back lash" on the thread.
THE FILLING OF THE MANOMETER.
When the manometer has been calibrated and the value of the menis- cus correction ascertained, and the extent of the capillary depression has been determined at a great many points, it is joined to the uncalibrated portion of the instrument and filled with nitrogen.
Originally the manometers were filled with purified and dried air, but it was found that, however pure the mercury in them might be, the volume of the included air slowly diminished. At first it was suspected that this diminution in the volume might be only apparent; in other words, that the capacity of the manometers was increasing under the pressures to which the gas was subjected. To test this suspicion, long columns of mercury were placed in calibrated tubes, like those used for manometers, between columns of air; and these were then subjected to pressures equal to the highest osmotic pressures which were being measured. The purpose was to discover whether the columns of mer- cury, under such treatment, diminished sensibly in length — either temporarily or permanently. The results were wholly negative. It was therefore concluded that the observed decrease in the volume of the imprisoned air must be due to the action of the oxygen on the mer- cury, though no fouling of the glass, such as would be expected from the presence of oxides, had been noticed. A third possible explanation, namely, that in the course of the movements of the mercury back and forth some of the gas had been "rubbed out" of the tubes, was not seri- ously considered. If the loss in volume of gas was due to the disap- pearance of oxygen, the obvious remedy was to fill the manometer with nitrogen. The remedy was so complete that, after years of use, no change in the volume of that gas in the manometers has been observed.
46
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
The nitrogen used in the manometers is obtained by passing air first through an alkaline solution of pyrogallol, and then, in the order named, over heated copper oxide, heated copper, heated copper oxide, cal- cium chloride, fused potassium hydroxide, and resublimed phos- phorus pentoxide. The glass tubes containing the dry reagents are all connected with each other and with the receptacle for the nitrogen by fusing the ends to- gether.
The arrangement of apparatus for filling the manometers is shown in Figure 26. The method of filling, because of its complexity and the difficulty of some of its parts, will be described in considerable detail.
A is the reservoir in which the purified nitrogen is stored up, and from which the manometers are filled. The unlettered stop- cock at the top is that through which the gas, after purification, enters the reservoir. B is the calibrated and thoroughly cleansed manometer which is to be filled and closed, and C is an arrangement for filling and emptying the manometer. B is joined to A, at d, by fusing together the ends of the glass tubes; and to C, at E, by means of rubber tubing. The mercury in C is sep- arated from that in the manometer by the air which nearly fills the wide tube below E. In this way, the mercury in C, which may be impure from its contact with rubber tub- ing, is prevented from entering the manom- eter and contaminating the very pure mer- cury with which that instrument is filled. This ah* also plays an important role when the manometer is closed.
Before joining the manometer B to A and C, its lower end is immersed in pure mercury and, with the instrument in an inclined position, gentle suction is applied until the two bulbs are filled as nearly as may be with mercuiy. Owing to the presence of one or more traps, some air will be left in the bulbs, and this must be expelled by bringing the instrument into the vertical position and forcing the
FIG. 26. — Arrangement for filling manometers with nitrogen.
(^1) Nitrogen reservoir; (B) cali- brated manometer; (C) air cham- ber to separate mercury in (i) from pure mercury in manometer ; (E) connector between (B) and (C); (Z) and (t) mercury reser- voirs; (fc) and (K) two-way stop- cocks; (/) and (g) vents.
THE MANOMETERS. 47
mercury in the other direction. When the bulbs and more or less of the tube B have been filled, and the junctions at d and E have been made, the manometer is repeatedly washed out with air which has been dried by resublimed phosphorus pentoxide. For this pur- pose, by lowering the reservoir (i), the dried air is admitted through the stopcock at the top, and likewise through /, which is also provided with a drying tube. By raising i, it is again expelled, mostly through g, but partly through /.
The next step, after drying the manometer, is to fill it with nitrogen from the reservoir (A). The reservoir (i) is raised and the air in the manometer is expelled through g until the mercury column reaches j, when the stopcock (h) is closed, and a small quantity of mercury is driven into the side tube (/). This is the mercury which is afterwards to occupy the upper end of the closed manometer. The rubber tube connecting/ with its drying tube is tightly closed and the air remaining between j and the stopcock (h) is expelled through h, care being taken not to allow the mercury quite to reach the stopcock, lest it should be contaminated by some of the lubricant on the latter. Some of the nitrogen in A is repeatedly wasted through the stopcock at the top and through g in order to remove any air still remaining in the upper part of the apparatus. Then by lowering i or raising I, with stopcock k open, the manometer is filled with nitrogen. This is wasted through g, and the manometer is again filled from A , and the operation of filling and empty- ing it is repeated as many times as may be thought necessary.
When the manometer has been filled with nitrogen for the last time, the reservoir (i) is adjusted to the right level, and the gas is placed under a slight over pressure by raising I. The stopcock (h) is opened and then quickly closed. This leaves the nitrogen in the manometer under a pressure equal to that of the atmosphere.
By gently pinching the rubber tube which closes/, a little mercury is forced out of the side tube into the vertical one between j and d. If it breaks into globules at j, they are reunited at d by tapping the tube. The mercury thus transferred does not enter the manometer, because of its small bore.
The reservoir (i) is now lowered until all the mercury collected at d has been drawn into the manometer to some convenient distance below that point, when the glass at d is softened in the blowpipe flame and the manometer is detached, but so as to leave both tubes sealed.
The glass at the detached end of the manometer is again softened in the flame and then drawn out to an exceedingly fine capillary tube, which is afterwards filled with mercury by raising i. Finally the capil- lary is closed in the flame, and the walls are thickened under slightly diminished pressure. Care must be taken, in closing the manometer, not to convert any considerable amount of the mercury into vapor, and to heat the glass so uniformly that the vapor which is necessarily
48 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
formed can not recondense until the operation of closing is finished. Otherwise the violent agitation of the mercury, due to rapid vaporiza- tion and condensation, is apt to shatter the tube. When closed, no bubble of air should be discernible at the top of the short mercury col- umn. First attempts at closing usually fail in this respect, but after a little practice, one is able to perform the operation with perfect success.
The short column of mercury in the upper end of the manometer has a twofold purpose. It prevents, during the closing of the instrument, any contamination of the nitrogen with air or with the combustion products of the flame; and it fills up all that portion of the instrument whose caliber may have been altered to an unknown extent by heating. DETERMINATION OF THE VOLUME OF THE NITROGEN.
For this purpose, the manometer is placed in the steel block within the bath (Figure 45), and the pressure upon the gas is regulated by the device used in the determination of capillary depression, i. e., a glass tube having an internal diameter of 35 millimeters, which is connected with the steel block by means of a flexible tube. Formerly it was attempted to use a stationary "side" tube. This consisted of a short piece cut from the same tube as the manometer itself and, like the ma- nometer, it was fixed rigidly in the block, the pressure being regulated by the plungers. The practice was, however, based on the mistaken assump- tion that in any given, fairly good tube the capillary depression is nearly uniform throughout. It was discontinued wrhen it was discovered that the best tubes we could obtain were very uneven in this respect.
The volume of the nitrogen is determined under a number of different pressures, all of them, of course, quite near that of the atmosphere. To determine it under high pressures, it is necessary to employ another closed manometer — a so-called "standard manometer." There is, how- ever, the same objection to the employment of standard manometers as to the use of narrow side tubes in the determination of capillary depres- sion and of gas volumes, the objection, namely, that all the errors of both tubes — principally of capillary depression — are charged to the tube under investigation.
Sometimes, in order to increase the quantity of the gas in the ma- nometer, more than the calibrated portion of the tube has been filled with nitrogen. This was frequently done before the introduction of manometers with large reservoirs of known capacity (Figure 19, etc.). In such cases the use of a standard manometer could not be avoided.
For the comparison of one manometer with another, the steel block and also the "brass block" seen in Figure 27 are used. The latter does not differ in construction from the former, except in the means for fixing the tubes in their places. The arrangements employed for that purpose are identical with those used in joining the cells and the manometers. Some other liquid than mercury— either water or a solution — is used in the brass block.
THE MANOMETERS.
49
When any operation is to be performed with a manometer which might endanger the calibrated portion, or contaminate the mercury in it, or foul the walls, the instrument is cut into two parts, the point of severance being usually between the bulbs, when that is practicable. If necessary, another piece of suitable form is then attached to the manometer, e. g., as when the instrument is to be placed in the steel block. Afterwards the detached portion is restored to its place.
FIG. 27. — "Brass block."
Construction like that of "steel block" (see Figure 22), except the manometer attachments, which are like those used with the cells.
In Figure 28 is seen an instrument much used in the manipulation of the manometers. The manner of its use will be best illustrated by describing a few of the operations in which it is most frequently employed.
1. Suppose the whole instrument (Figure 18 or 19), except the space occupied by the nitrogen, is filled with mercury, and it is necessary to cut the tube between the bulbs 1 and 2, either for the purpose of replacing the detached piece by another of similar form, or by a simpler piece of glass tubing. The rubber-covered cone which is usually upon the open end of the manometer, or a sharply sloping stopper through which the end has been passed, is placed in the cup 1 . The air is then exhausted through a rubber tube attached to the stem at 2. When a sufficient quantity of mercury has been drawn into the cup, the whole arrangement is tipped backwards until the end of the manometer is exposed. Air is then cautiously readmitted to fill the space in the instrument which was previously occupied by the mercury removed.
50 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
2. Suppose the open end of the manometer, e. g., to the middle of bulb No. 1, is filled with air and it is desired to replace it with mercury. A quantity of mercury is poured into the cup, the manometer is inserted and, with the instrument tipped so as to expose the open end, the air is exhausted until the mercury begins to run out. On bringing the manometer again to the upright position, so as to immerse the open end, and readmitting air, the mercury flows into the tube to replace the air which has been withdrawn.
FIG. 28.
Apparatus used in emptying, filling, and cleansing the uncalibrated portion of the manometers.
(1) Reservoir for solutions or mercury; (2) place for at- taching tubes containing absorbents.
3. Suppose, again, the manometer has been used in a measurement of pressure, and the open end — perhaps also a small portion of bulb No. 1 — is filled with the solution. Before the instrument can be used for another experiment, this must be removed and replaced, either by mercury or by some of the solution whose pressure is to be determined. The necessary manipulation is as follows: (1) the old solution is removed and replaced by air; (2) the air is replaced by the new solu- tion, and this, in turn, is replaced in succession by other portions of the same solution, until there is no danger that the concentration of the new solution will be affected by the older one. If mercury is to be substituted for a solution, the tube must be washed and dried before introducing it. In this case, portions of the wash liquids — water, alcohol, and redistilled ether — are introduced and removed in exactly the same manner as when one solution is to be substituted by another. The final drying of the manometer is accomplished by attaching a tube containing drying agents or absorbents to the stem (2) and alter- nately exhausting and readmitting air. When manometers of the forms seen in Figures 20 and 21 are to be dealt with, the instrument (Figure 28) is attached to the vent. The manipulation is then more complex but not less effective.
The time consumed in preparing a manometer for the measurement of osmotic pressure is usually about one month.
CHAPTER III.
THE REGULATION OF TEMPERATURE. THERMOMETER EFFECTS.
Because of certain obvious analogies between a closed osmotic cell and a sensitive thermometer, the name "thermometer effects" has been given to a large group of exceedingly troublesome manifestations which follow even slight fluctuations in bath temperature. The name is appropriate only in a very restricted sense. The phenomena thus classified are complex and often they are difficult to analyze satisfac- torily. To understand them, one needs to keep constantly in mind three fundamental facts: (1) That the capacity of the closed osmotic cell is a nearly fixed quantity; (2) that every change in the volume of its contents — due to rise or fall of temperature — is followed by a dis- charge or intake of solvent through the membrane, both of which acts also modify the volume and the osmotic pressure of the solutions ; and (3) that the passage of the solvent through the membrane, in either direction, is usually a much slower process than the changes in the volume of the cell contents which result from fluctuations of tempera- ture. The first and second of the enumerated facts are obviously true, but the third, which is responsible in largest measure for the complex and often perplexing results, can be learned only by experience.
The four elementary fluctuations of temperature and their conse- quences will be considered :
(1) The temperature of the bath (previously constant) rises and
becomes again constant at the higher level.
(2) After rising, it falls again to the original level.
(3) The temperature of the bath (previously constant) falls and
remains constant at the lower level.
(4) After falling, it rises again to the original level.
The question to be answered is, what changes in cell pressure will the observer at the telescope see in consequence of the temperature fluc- tuations enumerated above? For convenience, all positive pressure in the cell which is not osmotic will be called mechanical, and the sum of the two will be spoken of as the total pressure.
1. The conditions which are supposed to prevail are as follows : The cell contains a solution of known concentration, the temperature is con- stant, and the solution is exhibiting its true osmotic pressure only. Subsequently the temperature rises and becomes constant again at the higher level. This is the simplest of the four cases previously men- tioned. The volume of the liquids in the cell — the mercury in the manometer and the solution — and the tension of the gas in the manom-
51
52 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
eter increase. The total pressure is now the sum of the osmotic pres- sure of the solution and a considerable mechanical pressure due to the expansion of the liquid contents of the cell. In consequence of this over-pressure — the difference between the total and the osmotic pres- sures — the gas in the manometer is compressed to a smaller volume, and the mercury meniscus is seen to rise and finally to attain to a maximum height. Simultaneously with the expansion of the liquids in the cell, there is, in consequence of the over-pressure, a very slow outward dis- charge of the solvent through the membrane, the effect of which is two- fold. First, there is a reduction in the volume of the solution which reduces the mechanical pressure; and, second, an increase in osmotic pressure due to the increasing concentration of the solution. The two effects are of a mutually compensatory character, but they are not equal in their opposite influences upon the magnitude of the pressure in the cell. Hence the meniscus does not remain at the highest point reached by it, but sinks again and becomes stationary at a lower level only when the mechanical pressure has wholly disappeared and the only pressure in the cell is the osmotic pressure of a solution more concentrated than the original one. To recapitulate, the meniscus, in the case under con- sideration, takes three positions in the manometer, which may be called, in the order of their relative heights, the lowest, the intermediate, and the highest. The first and the second of these correspond to the true osmotic pressures of two solutions of different concentration, while the third is temporary and corresponds to the sum of an unknown mechan- ical pressure and an osmotic pressure of which it can only be said that it is higher than the osmotic pressure of the more dilute and lower than that of the more concentrated solution.
It will be seen that the maximum height to which the meniscus will temporarily attain depends upon both the magnitude and the rate of the rise in temperature, while its final position is determined solely by the former. In other words, a rapid rise in temperature always pro- duces a larger thermometer effect than a slow one. It will be seen also that the magnitude of the thermometer effect in question, when trans- lated into pressure, depends in large measure upon the volume of the nitrogen in the manometer.
2. If, after a rise, the temperature, instead of becoming constant, again sinks to its original level, a more complicated series of changes in cell pressure is observed. The cause of the increased complexity of the situation is the falling temperature which may begin to operate before or after the meniscus has reached its greatest height. If it begins before, the meniscus will evidently not rise so high as it otherwise would. For present purposes, let it be supposed that the fall in tem- perature sets in immediately after the meniscus has reached the highest point in its ascent, i. e., when the greatest pressure has been developed in the cell. Up to this point, then, the conditions are identical with
THE REGULATION OF TEMPERATURE. 53
those in the preceding case. But there is now a falling instead of a stationary temperature. The elements of the situation are (a) an over or mechanical pressure in the cell due to a previous rise in temperature, and (6) a falling temperature. The consequences of (a) are :
(1) An increase in osmotic pressure, due to the concentration of the
solution which follows the expulsion of solvent.
(2) A decrease in mechanical pressure, due to the smaller volume
of the solution after expulsion of solvent. The consequences of (b) are:
(3) A decrease in mechanical pressure, due to the diminishing
volume of the cell contents.
(4) A decrease in osmotic pressure, due to lower temperature.
(5) A decrease in osmotic pressure, due to dilution of the solution
through intake of solvent.
(6) An increase of pressure within the cell, due to the increase in
the volume of the solution through intake of solvent. Of the effects enumerated above, (1) and (6) are positive, i. e., they tend toward the maintenance or increase of pressure in the cell. In the same sense, (2), (3), (4), and (5) are negative. The amount of over or under pressure in the cell at any given moment is, of course, the alge- braic sum of all these effects. By "over" and "under" pressure is meant the difference between the actual pressure in the cell at any time and the true osmotic pressure of the solution at the original temper- ature, i. e., before the rise and subsequent fall of temperature. One would expect, perhaps, that the sum of the "over" and "under" pres- sures would become zero when the bath had recovered its original tem- perature. In other words, that the meniscus would stop in its descent and become stationary, when the pressure in the cell is equal to the true osmotic pressure of the solution at the original and now constant tem- perature. But this is by no means the case. It continues to descend, and, before coming to a rest, may go far below the level which corre- sponds to the osmotic pressure of the original solution at the given temperature. Here again the reason for the apparently anomalous conduct of the cell is to be found in the fact that changes in volume, due to fluctuations of temperature, are accomplished more quickly than the migrations of solvent through the membrane which follow such fluctua- tions. Having reached the lowest point in its descent, the meniscus rises again and finally comes to rest at its original level, i. e., at the level which corresponds to the true osmotic pressure of the original solution at the original temperature. The upward movement of the meniscus in the final return to its first position is the resultant of two opposite effects of the intake of solvent: (1) the increasing volume and (2) the decreasing osmotic pressure of the solution. To recapitulate : If, after a rise, the bath recovers its original temperature, the meniscus first ascends to a point whose elevation above the original position depends
54 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
(1) upon the magnitude and rate of the rise in temperature, and (2) upon the time at which the fall in temperature sets in and the rate of the fall. The meniscus then descends to a point below its original level. The distance between the two positions depends (1) upon the magni- tude of the upward displacement and (2) the rate at which the bath recovers its original temperature. Finally, the meniscus ascends to its first place.
The third and fourth situations are not more simple than the first and second, but enough has already been said for the present purpose, which is merely to emphasize the complex nature of thermometer effects. Hence in the remaining cases the movements of the meniscus only will be stated.
3. The conditions are as follows: The cell contains a solution of known concentration which is exhibiting its true osmotic pressure at a given constant temperature when a fall in temperature occurs. Afterwards the temperature becomes constant at a lower level. The movements of the meniscus which are observed are : (1) A fall (usually quite rapid) to a point below the position which it will finally take, and
(2) a rise to some intermediate point at which it becomes stationary. The final position corresponds to the true osmotic pressure, at the given lower temperature, of a permanently diluted solution. The difference between the lowest and final positions of the meniscus will depend upon the magnitude of the fall in temperature and upon its rate as compared with that of intake of solvent.
The movements of the meniscus are sometimes less simple than stated above, since at times one observes an extra excursion of the meniscus, i. e., it falls to its lowest level and then rises to a point above the position which it finally takes.
4. If, after the fall, the temperature rises and becomes constant again at the original level, which is the most frequent case, the move- ments observed are as follows : The meniscus falls to its lowest position, then rises to one higher than it had originally, and finally sinks to the place from which it started. Here again an extra excursion of the meniscus is sometimes observed, namely, a second one to a point below its final position. When the meniscus has finally recovered the position from which it first started, we have again, of course, the true osmotic pressure of the original solution at the original temperature.
The " extra" excursions of the meniscus mentioned under 3 and 4, as well as certain other anomalies not mentioned, are probably due to temporary inequalities of concentration in the solution — to the fact, namely, that when solvent is expelled or taken in, the solution in immediate contact with the membrane is, for the time being, concen- trated or diluted to a greater extent than the main body of the solution. The final adjustment of the meniscus can not, of course, be reached until the whole solution has become homogeneous through diffusion
THE REGULATION OF TEMPERATURE. 55
of the solvent. Evidently the magnitude of the so-called extra excur- sion will depend very much upon the rate at which the solvent can pass through the membrane in either direction.
The magnitude, and therefore the importance, and the peculiarities of thermometer effects depend upon several conditions which will be briefly recapitulated. They are :
1. The relative volumes of the liquids (solution and mercury) and of the gas in the cell. Since the latter is always very small as compared with the former, slight disturbances of temperature must always pro- duce large thermometer effects.
2. The degree of the "lag" in the passage of solvent through the membrane, which, in turn, depends upon temperature and the area and age of the membrane.
3. The rapidity with which the changes in temperature are accom- plished. The relation of 3 to 2 is self-evident.
4. The "lag" in the distribution of solvent through the solution by diffusion, which produces temporary conditions of non-homogeneity in respect to concentration.
It is obvious that, in one sense, we could have no thermometer effects if the passage of solvent through the membrane and its subsequent uniform distribution by diffusion were instantaneous, since, in that case, there could never develop in the cell a condition of over or under pressure. In other words, we should then have at all times simply the osmotic pressure of a solution whose concentration varies with the temperature. The fact that diffusion does not quite keep pace with transferences of solvent through the membrane is not a source of serious trouble, but in the lag of such transferences behind fluctuations in temperature we have a most formidable obstacle in the way of the accurate measurement of osmotic pressure. The only remedy for this unfortunate situation is to be found in the most perfect means which can be devised for the automatic maintenance of constant temperature.
It will be gathered from what has already been said that the duration of thermometer effects also depends principally upon the rate at which the solvent is able to diffuse through the membrane. In practice it is found that, according to the age of the membranes, they may last from 12 hours to 4 days after the bath has recovered its normal tem- perature. As regards the minimum temperature change which will give a sensible thermometer effect, it may be said that a fluctuation of 0.01° produces a movement of the mercury meniscus which can be detected. A change in temperature amounting to 0.05° gives a large thermometer effect, even when the membrane is new. It has not been found practicable to regulate the temperature of the large baths which are in use to within less than 0.01°; accordingly the meniscus is con- stantly moving within narrow limits, with the result that two successive readings, several hours apart, are rarely quite identical. Fluctuations
56 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
of 0.02° in bath temperature, if they follow one another with regularity, are tolerable, because the thermometer effects due to rise in tempera- ture are then partially neutralized by those due to falling temperature. Change in concentration without leakage. — It has been seen that a solution in a cell may become permanently concentrated if the tem- perature rises and becomes constant at a higher level; also that it may be permanently diluted if the temperature falls and becomes constant at a lower level. There are also two cases in which the con- centration of the solution may be altered without any change in the temperature of the bath. Alterations of this kind occur when the cells are filled with solutions whose temperature differs from that of the bath. In such cases, concentration or dilution of the solution ensues, according as the temperature of the solutions is lower or higher than that of the bath. There is, however, no essential difference between the two modes of effecting concentration on the one hand and dilution on the other, since both depend on changes in volume due to changes in the temperature of the solutions.
Except for the maintenance of zero temperature, all the devices for regulation conform to one principle, which may be stated as follows :
// all the water or air in a bath is made to pass rapidly (1) over a continuously cooled surface which is capable of reducing the temperature slightly below that which it is desired to main- tain, then (2) over a heated surface which is more efficient than the cooled one but which is under the control of a thermostat, and (8) again over the cooled surface, etc., it should be practi- cable to maintain in the bath any temperature for which the thermostat is set, and the constancy of the temperature should depend only on the sensitiveness of the thermostat and the rate of flow of the water or air. The principle is a general one and provides for the maintenance of any temperature between zero and the boiling-point of water. Moreover, any desired tempera- ture can be maintained without regard to the temperature of the surrounding atmosphere, since the air about the bath must always aid in the work either of the cooling or the heating surface.
The "cooling" surface is usually furnished by a series of brass pipes
through which water— under a constant pressure— is circulated. If the
temperature to be maintained is a moderate one, i. e., not far from that
of the atmosphere but above that of the hydrant water, the latter is
lirectly through the circulating system, the rate of flow being
) regulated as to maintain, without the cooperation of the heating
ce, a temperature which is slightly too low. This margin between
emperature which the cooling surface, acting alone, will maintain
that which it is desired to keep should, for economical reasons, be
THE REGULATION OF TEMPERATURE. 57
made as small as is consistent with safety. If the temperature to be maintained in the bath is very near to or below that of the hydrant water, the latter, before entering the circulating system, is passed through coils of metallic pipes which are surrounded by ice. If the desired temperature is not much above that of the atmosphere, the cooling effect of the surrounding air upon the exterior of the bath may suffice, in which case the circulation of hydrant water is discontinued. Finally, if the temperature to be maintained is considerably above that of the atmosphere, the system within is connected with one on the out- side, thus forming a closed circulating system which is partly within and partly without the bath, and through this hot water is circulated by means of a pump. The pump may be situated anywhere in the system, i. e., either within or without the bath. At some point outside, provision is made for heating the water by gas as it passes through the system.
Thus far, provision for the cooling surface only has been made. Not- withstanding the application of heat, and sometimes a good deal of it, the circulating system mentioned above is, essentially, a cooling device, inasmuch as its purpose is to reduce the temperature of the bath below that which is to be maintained. In this system, for economical reasons, gas is used for heating rather than electricity, but care is taken so to regulate its flow that a "cooling margin" will be maintained, whatever may be the fluctuations in the pressure upon the gas. Since the "cool- ing surface" is not subject to exact regulation, it must never be allowed to become, in effect, a "heating surface, ' ' for in that case the thermostat becomes useless.
The "heating surface" usually consists of one or more copper cyl- inders, in which are inclosed ordinary electric lamps, which serve as stoves, whose purpose is to overcome the "cooling margin.'' If that margin is small — and it should be made as small as possible — the con- sumption of electricity is not large. Lamps are used rather than other forms of electric heating devices, because one can always select among them stoves whose capacity is suited to the work to be done, and because they heat up and cool down quickly, which is an important element in temperature regulation.
The circulation of water over the cooling and heating surfaces is effected by means of pumps, and it will be seen later that a single pump may be made to circulate the water over these and also through the cooling system. The air in the baths is circulated by means of rotating fans.
THE SCHEME FOR ELECTRICAL REGULATION.
The device by means of which the margin of under-temperature pro- duced by the so-called "cooling surface" is exactly and automatically overcome is shown in Figure 29. Everything not essential to an under- standing of its plan is omitted. It consists, in its simplest form, of (1)
58
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
Stove
Fio. 29. — General scheme for the electric regulation of bath temperature.
(CO, (Cj), and (C») condensers spanning spark-gaps of thermostat and relays; (h), (12), and (Is) lamps spanning the same spark-gaps as the condensers; (R) the master relay (250 ohms resistance), which is operated by battery and may control any number of stoves in parallel; (R\) stove relay operated by battery through the "local" of the master relay, (a with arrows) course of current through battery, thermostat and "main" of master relay, (b with arrows) course of current through battery, "local" of master relay and "main" of stove relay. Current of main line passes through "local" of stove relay and stove. "Stops" of all relays reversed.
THE REGULATION OF TEMPERATURE. 59
a battery of one cell; (2) a thermostat; (3) two relays; (4) three lamps (I) and three condensers (c) which span the spark gaps of the ther- mostat and the two relays; (5) two battery circuits which operate the relays and a third one of 120 volts which passes through the stove.
1. THE BATTERY.
For charging purposes, all the cells in use (about twenty in number) are placed in series upon a single circuit in which a lamp of appro- priate resistance is also inserted. The charging is continuous. The cells themselves, though in series on a single circuit for charging, are distributed, in numbers corresponding to the amount of work to be done, at points conveniently near to the various baths. A single cell suffices to operate the system at any point, hence each cell in a local battery consisting of more than one is at work only a part of the time, i. e., every third day, if the local battery consists of three cells.
2. THE THERMOSTAT.
No single element in a system of regulation is of greater importance than the thermostat. Its efficiency depends upon a considerable number of conditions, some of which are worthy of more than a passing mention. That the mercury must be of exceptional purity, and that its volume must be so related to the diameter of the capillary as to secure a large movement of the meniscus for a small change of tem- perature, are facts too obvious to require discussion. The feature to which too little attention is usually given is the mechanism for adjust- ing the contact point. This should be located directly over the center, i. e., the highest part of the meniscus, and the mechanism should be such that it can never take any other position with reference to the surface of the meniscus. The best form of thermostat which we have in use is shown in Figure 30. The platinum rod (a) is finished to a smooth point at the lower end, and just above the latter is a guide (6) of glass, which is designed to keep the point near the center of the tube, and therefore nearly over the highest part of the meniscus. At the upper end, the platinum rod (a) is firmly set in the threaded brass rod (c). The adjustment is made by means of the nut (e, e), which is so nicely fitted into its framework that it can move in a horizontal- circular direction only. The dotted circle indicates the apertures through which the adjusting nut (e, e) is grasped between the thumb and forefinger when the contact-point is to be lowered or raised. The guide (6), which must fit the tube rather loosely, does not suffice to compel the contact-point to keep exactly its proper position with reference to the meniscus. The rod (a) is never absolutely straight, hence the point, if the rod is allowed to turn, will describe a circle over the meniscus. For this reason, having once correctly adjusted the point, its motion must be limited to the vertical direction; in other words, the threaded rod (c) must not be allowed to turn with the nut
60
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
(e, e). The device by which any movement of (c) in a horizontal direc- tion is prevented is seen in the upper part of the figure. The two nuts— (/) and (g) — are threaded internally to fit c. The lower and wider one is bored at two oppo- site points for two rods, of which only one (h) is seen in the figure. Corresponding to the holes in g, two other holes are bored in the brass cap (i) . Having found the correct position for the con- tact-point, the rods h and hi (not seen), are in- serted, and the set nut (/) is turned down upon g. The rod (c), though still free to move in a vertical direction, can not now turn with the adjusting nut (e, e).
Sparking at the point of contact in the ther- mostat is effectively prevented by spanning the spark gap (Figure 29) with a lamp of high volt- age (li) and a condenser (d). Since removal of the condenser has not been found to induce visible sparking at the point of contact, it is doubted whether it serves any useful purpose. As a matter of fact, it is often omitted. The lamp (li) which is ordinarily employed is one of 16 candle-power at 250 volts.
The water in the baths is always in rapid motion, and a thermostat which is immersed in it without protection is subject to slight but constant jarring, which results in a phenom- enon which has come to be known under the name of "frosting." The air between the mer- cury in the thermostat and the glass wall col- lects at a multitude of points in minute bubbles, which give to the glass a frosted appearance. In the course of time, the bubbles of gas coalesce, forming aggregations so large that the true FIG. so.— The thermostat. nature of the phenomenon can be discovered by (a) Platinum rod, pointed at the naked eye. The first indication which one lower end; (6) glass guide to usually receives that "frosting" has commenced is a "chattering" of the relays.
"Frosting" can be prevented by protecting the thermostats from the shock of the moving water by surrounding them with metallic tubes, or by exhausting them before introducing the mercury. The boiling-out process employed for barometers has not been found practicable for thermostats of the form used by us.
keep (a) in center of tube; (c) threaded brass rod ; (e) in- closed nut for adjusting con- tact point; (/), (0), and (h) arrangements for preventing all movements of the contact point, except in a vertical di- rection. The figure to the left shows the glass parts of the thermostat.
THE REGULATION OF TEMPERATURE. 61
3. THE MASTER RELAY.
The master relay (R, Figure 29), so called because it controls all the relays connected directly with the stoves, has a resistance of 250 ohms, and is of the type commonly used in telegraph lines. High resistance in this relay is desirable in order to reduce to a minimum the current which must pass through the thermostat. The course of the only circuit which passes through the thermostat and the magnets of the master relay is indicated in Figure 29 by the letter a. The current in this circuit amounts to 8 milamperes plus, of course, what may pass through the high-resistance lamp (7i).
4. THE MINOR RELAY.
The " local " of the master relay is made the " line " circuit of the minor relay (Ri). The course of this circuit is indicated by the letter b. The spark-gap of the master relay, like that of the thermostat, is spanned by a lamp (72) and a condenser (c2), although the latter is often dispensed with. The minor relay has a resistance of only 20 ohms. Its spark-gap is also spanned by a lamp (73) and a condenser (c3). The stove circuit, as shown in the figure, passes through the "local" of the minor relay.
In all of the relays — both master and minor — the usual arrangement of the " stops" is reversed, so that the closing of the circuit through the thermostat opens the circuit through the " locals" of the relays. Obviously, what it really does is to cut down the current in these circuits by throwing into them the resistance of the lamps (12 and Z3). None of the three circuits employed in the system is ever fully broken. But the currents which pass continuously through the lamps (h and Z2) are insufficient to operate the relays, while that which passes continu- ously through ?3 does not overheat the bath.
By putting the minor relays in parallel, a single master relay is made to operate any number of stoves. In some of our baths, the " master" controls as many as eight stoves.
For convenience in use, the system of electrical control is divided into two units, and the apparatus belonging to each is permanently installed on a portable board which may be fixed in any suitable position with reference to a bath. All connections, except the perma- nent ones on the boards, are made by means of flexible leads, to the ends of which are attached insertion plugs. On the "master board" are placed and wired together the master relay, the lamp which spans the spark-gap of its "local," and plug attachments for the battery, the condenser, and for several stove boards. On each of the minor or stove boards are placed and wired together two minor relays in parallel for as many stoves, the lamps which span their spark-gaps, and plug attachments for the master board, the condensers, and the stoves. The "unit" boards of each kind are uniform in arrangement and size and are therefore interchangeable.
62
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
THE BATH FOR 0°.
As previously intimated, the bath which is employed for determina- tions at 0° does not conform to the general principle upon which all baths for higher temperatures are constructed. The essential difference between it and the others is that in the bath for 0° there is no forced circula- tion of water and air. An attempt was made to construct a bath in partial conformity with the general principle in question. In this, a large mass of ice was made the "cooling surface," and the exterior of the bath the "heating surf ace." Water was passed rapidly over the ice, and then over the compart- ments in which the cells were located. It was found, however, that the lowest temperature which it was practicable to maintain in this manner was always a little above 0° ; and that owing to imperfect cdntrol of the heating surface, the temperature was subject to con- siderable fluctuations which produced large thermometer effects.
The bath which was finally evolved for the determination of osmotic pressure at zero is seen in Figures 31, 31 L, and 31 M. The ap- paratus, which is made of heavy galvanized sheet iron, consists of three principal parts: First, a can (^4.), in which the cells are placed ; second, a much larger one (B), in which A is suspended by means of the arrangement seen in Figure 31 L; and third, the cylinder (C), which shuts down tightly upon B. There is an inclosed chamber (e) running
through the Whole length Of C and Open at FIG. 31. -Interior ice bath for meas- . ... urmg osmotic pressure at 0°.
both ends, in which are located the upper
ends of the manometers and the two ther-
mometers which are seen in the figure. The
thermometers and manometers are exposed
to view, when a reading is to be made, by
opening the felt-lined door (/). In order
that the door may be opened and closed
from the outside, the detachable rod (g) is
made to pass through the top of the larger
bath (to be described later) which surrounds
A, B, and (7. The bottoms of both A and B are perforated so that
no water can collect in the cans.
ing cells; (B) galvanized-iron ice-container surrounding (.4) ; (C) galvanized-iron ice-container which shuts down upon (B) ; (e) protected compartment for ma- nometers and thermometers; (/) padded door to (e) ; (g) arrange- ment for opening and closing the door from above the outer ice bath, which is not shown in the figure; (L) arrangement for suspending (A) in (By, (M) cover to (A).
THE REGULATION OF TEMPERATURE. 63
The "larger bath" referred to above is one of those ordinarily used for measurements of pressure at higher temperatures. To prepare it for use at 0°, it is stripped of all its interior accessories, including circulating pipes and pump, leaving the copper-lined rectangular tank entirely empty. On the bottom of this, in the center, is placed a staging about 5 centimeters high, on which rests the ice-filled arrangement consisting of A, B, and C. All the space in the tank which is not occupied by A, B, and C is filled with closely packed broken ice, and the water which collects upon the bottom is removed by means of an automatic siphon. All the space in the upper part of the bath — usually designated as the "air space" — which is not occupied by the upper part of C is filled with ice containers of such form that they surround C except directly in front of the door (/). One of these occupies the space between the upper end of C and the top of the outer bath, the upper end of the cham- ber (e) being covered to prevent the entrance of water.
All of the ice-containers in the air space above are open at the lower end, so that the broken ice moves constantly downwards as it melts away underneath, keeping the tank below and also the can (B) always full. A little over 150 kilograms of ice are required to fill the bath properly, and the amount of fresh ice which it is necessary to introduce daily is between 25 and 30 kilograms.
The container above C and C itself, after the ice in them has been picked out, can be lifted through the opened top of the outer bath when- ever cells are to be removed from A . If cells are to be introduced, all parts of the bath except C and the container above it are closely packed with ice, and, after waiting until the temperature in A has fallen to 0°, the cells are placed in position. C is brought down upon B and packed with ice. Finally, the container which belongs directly above C is placed in position and filled with ice.
The arrangement described above serves its purpose perfectly. The temperature in A does not deviate sensibly from 0°. There are, there- fore, no appreciable thermometer effects. The temperature of the manometer space (e) may be affected somewhat by the lamp used in reading, unless one interposes a screen for the purpose of cutting down the heating effect of the light. We have employed for this purpose a 4 per cent solution of nickel sulphate, which — as determined by means of a thermocouple — reduces the heating effect of the lamp nearly 99 per cent.
BATHS FOR MAINTENANCE OF TEMPERATURE ABOVE ZERO.
Descriptions of the earlier forms will be omitted. The baths which were first employed in an attempt to measure osmotic pressure were found to be incapable of maintaining sufficiently exact temperatures. In other words, the thermometer effects produced by their fluctuations of temperature were intolerably large. The baths which are now used and which will be described are the products of a persistent attempt to reduce these effects to harmless proportions. They all belong to cer-
64
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
To relay Fio. 32. — 60-liter galvanized-iron bath for intermittent use.
(a) and (6) wooden base; (c) hair padding; (d) water-tight inclosure for electric stove (lamp); (e) electric stove; (/) and (0) removable base for lamp; (h) frame for lamp; (i) supports for circulating system; (j) coil of block-tin pipe for the circulation of cold or hot water (shown better in Figure 32, B) ; (fc) and (I) pipes by which water enters and leaves the circulating system (j) ; (m) and (ri) cylinder open at top and resting on pegs between coils of 0') ; (o) holes for escape of gas expelled from water; (p) holes for escape of water into outer bath; (r) pro- peller for pumping water over heated chamber (d)\ (s) pulley; (0 oil cup; (u) thermostat; (») thermometer; (w) adjustable iron frame for fixing the propeller in place; (x) vertical pipe leading to pressure regulator.
A. Adjustable support for bottles, etc. B. Block-tin circulating system (j in Figure 32).
C. Constant-pressure arrangement, (ad) Stand-pipe; (bd) overflow; (cd) stopcock for the regu- lation of flow of water through circulating system in bath; (dd) entrance place of water-supply.
THE REGULATION OF TEMPERATURE. 65
tain fairly distinct types, though differing much among themselves in respect to details. An example of each type will be given.
TYPE I.
This bath (Figures 32 and 33), which was designed for general but intermittent use in connection with the work, consists of a cylindrical tank of galvanized iron, which holds from 60 to 80 liters. It is sur- rounded by a thick covering of hair felt (c, Figure 32) , and rests upon a wooden base (a, a) , which is raised above the table or floor by the blocks and rubber pieces (6, 6). Inverted over the hole in the center, and riveted and soldered to the bottom of the bath, is the cylinder (d, d, d), which serves as a receptacle for the lamp (e), or any other suitable kind of electrical heating device. The lamp is mounted, in the manner indicated in the figure, upon the removable block (/), which is held in its place by buttons screwed to the base (a, a) . Resting upon the frame- work i, i (Figure 32) and i, i, i (Figure 32 B) is the continuous block-tin pipe 0', j, j, j), through which the hydrant water circulates. The cyl- inder (d, d, d) constitutes the "heating" surface, and the pipe (j,j,j,j) the "cooling" surface. The running water enters the bath at k and leaves at I (Figures 32 and 32 B). The course of the water in the pipe, after entering the bath, is continuously horizontal or upward — never downward. This arrangement is necessary in order to prevent the lodgment of air in any part of the pipe. The successive coils of pipe (six in number) are separated by the pegs seen in Figures 32 and 32 B, and on these rests the galvanized iron disk (m, m). The hood (n, n), of the same material, shuts down tightly over a flange on the disk (m, m) and is adjusted and secured in its place by set screws directed towards d, d. The form of the hood will be clear from the figure, and it is neces- sary only to call attention to the small holes for the escape of air at o, o, and to the larger holes at p, p, through which much of the water raised by the propeller (r) escapes into the outer bath.
The purpose of the various parts which go with the propeller — the adjustable cross-bar (w, w), which is clamped to the sides of the bath, the oil cup (i), the pulley (s), etc. — is sufficiently obvious.
It is quite essential that the hydrant water which flows through the pipe shall be under constant pressure, otherwise much water and heat are necessarily wasted. The arrangement by which the constant pres- sure is secured is shown in Figure 32 C. It consists of a large standpipe (ad) with an overflow (bd) near the top. The water from the tap enters at the bottom (dd) and passes to the bath through cd, where the flow is controlled by a stopcock. The circulating water is thus brought under an invariable pressure, and it is possible to regulate the quantity pass- ing through the bath with considerable nicety and for any length of time. At the highest point in the waste pipe (2, Figure 32) is placed a vent through which any air carried along by the water may escape.
66
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
If the temperature of the hydrant water is above that at which the bath is to be maintained, the block-tin spiral pipe (Figure 33) is inserted between the tap and dd (Figure 32 (7). To cool the water which enters at a, before it passes through b, dd, and cd into the bath, the large and well-protected box in which the spiral is located is packed with ice. In this manner, it is practicable to maintain a quite low temperature in the warmest weather.
The bath just described is used principally for bringing solutions to temperature and for maintaining them at temperature, for the com- parison of thermometers and the adjustment of thermostats, and for other similar purposes. The various instruments and vessels are held in their places in the bath by means of adjustable supports or clamps, of which that for bottles is shown in Figure 32 A.
The maintenance of any temperature from a little above 0° to that of the room can be readily accomplished by means of the hydrant water, with or without ice. If, however, a temperature above that of the room is to be maintained, the flow of the hydrant water is cut off. The outer surface of the bath and the exposed surface of the water then become the "cooling" surface, and the bath works on precisely the same principle as before. If a temperature above 50° is to be maintained, the consumption of electric energy becomes expensive in large baths, and it is well to accomplish a portion of the heating by means of gas. This is done in various ways, but most simply by removing the wooden base and mounting the bath on a large iron tripod over a ring burner of suitable diameter, taking care, of course, so to regulate the quantity of burning gas that the stove alone can not raise the temperature of the bath to the required height.
TYPE II.
Figure 34 represents one of the more recent forms of bath, in which the membranes are deposited and in which the cells and the solutions are maintained at the temperature at which osmotic pressure is to be measured.
The "cooling" surface is furnished by the horizontal brass pipes L to 8) . The hydrant water, cooled by ice if necessary, enters by pipe 1 and, after circulating through all the six intervening pipes in the order in which they are numbered, it leaves the bath by pipe 8. For all tem- peratures below the highest temperature of the room, it is necessary to keep some water in circulation in this system of pipes. The amount to be sent through will, of course, depend on the difference between the
FIG. 33. — Coil of block- tin pipe for cooling or heating water before it enters the circulat- ing system within the bath.
(a) Entrance; (6) exit.
THE REGULATION OF TEMPERATURE.
67
temperature of the hydrant water and that which is to be maintained in the bath. If the hydrant water is to be cooled before entering the bath, as when a low temperature, e. g., 5°, is to be maintained in sum- mer, it is first passed through the coils of pipe seen in Figure 33, which are embedded in ice. The arrangement shown in Figure 32 C is also employed in this bath to secure a constant pressure upon the circulating water.
FIG. 34. — Rectangular bath for general laboratory use.
(1) to (8) Brass tubes for circulation of hydrant water; (9) and (10) copper cylinders, opening on opposite sides of the bath, for the lamps; (11) pump; (12) and (13) pipes through which water is drawn out of the bath and over the gas stoves seen at the end; (14) large pipe through which water heated by the gas stoves is drawn and delivered at (11).
A word of caution may be given regarding the valves to be used when a constant pressure on running water is to be maintained. Our first pressure arrangements were constructed in accordance with correct prin- ciples, so far as we knew, but it was found that they would not maintain constant pressures. The flow of water diminished continually, and very small streams ceased altogether after a time. After a long search, the difficulty was located in the valves. Those we were using — the so-called "gate-valves" — were found to be so constructed as to permit the accumulation of the gas which is expelled from water, when its tem- perature is raised, to such an extent as to impede the flow of the water, and to stop it altogether if only a little were passing through the valves. After replacing the "gate- valves" by others of the common lever variety, the difficulty disappeared.
The "heating" surface is furnished by the two copper cylinders (9 and 10), the latter of which is broken in order to show the location of the stoves. The large wooden box is lined with copper, and the two copper cylinders in question extend entirely through it from side to side, and are opened at both ends upon the outside of the bath. They are closed with caps, upon the inside of which are fastened the lamps. Provision is thus made for four lamps which are usually of 16 candle-power, though lamps of 8 candle-power often suffice at low temperatures. The lamps (stoves) are regulated according to the scheme already explained.
68 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
The circulation of the water in the bath over the cooling and heating surfaces is effected by means of the pump (11). It enters the pipes (12 and 13), which end just inside the rear end of the bath, and passes, in the direction of the arrows, into the large pipe (14), thence to the pump and out again into the open bath. It will be observed that the tendency is to draw the colder water upon the bottom of the bath very rapidly into the pipes (12 and 13), but that, as it enters these, it is neces- sarily mixed with water which has passed over the heating surfaces (9 and 10). Many positions for the heating surfaces have been tried, but that given in Figure 34 has been found most satisfactory.
The rate of pumping depends upon what is found to be necessary in order to secure identical temperatures at the two ends and the middle of the bath. Ample provision is made for any rate which may be required. A moderate rate for some of the larger baths is 400 liters per minute.
The purpose of extending the pipes (12, 13, and 14) outside of the bath, where, at their junction, the circulating water passes over a gas stove, is obviously to economize electricity. The rule here, as in all other baths, is to utilize gas for heating purposes to the utmost safe limit, leaving for the electrical appliances only so much as is indispen- sable for regulation.
Five baths of Type II are in use, varying in size and equipment, but all conforming in principle to that just described. Plate 3 presents their appearance, also that of the baths of Type I.
TYPE III.
An example of one kind of bath in which osmotic pressure is measured is shown in Figures 35 and 36. The first (Figure 35) represents the lower part, which is filled with water and in which are located circulat- ing systems similar to those described under Type II. In the second (Figure 36) is seen the upper part of the bath, the so-called "air space." Both divisions are lined with copper and are separated by a vapor-tight brass plate (1, Figure 35 or 36), which is divided diagonally across the bath into two parts which are reunited by the brass strip (2). The brass plate (1) is screwed down upon the upper edge of the outer wooden bath, but between the two, as also between 2 and 1, strips of sheet rubber are placed to prevent the passage of water vapor from the lower part of the bath into the "air space" above. The reason for keep- ing the latter as dry as possible will appear later. Six lead-weighted copper cans are suspended from the brass covering plate, the flange of each resting upon a rubber collar; they serve as receptacles for the cells. During a measurement of pressure, the space in the cans above and around the cells is filled with wool. In two of the three baths of Type III, the cans have been replaced by two long, narrow troughs, whose depth is equal to that of the cans. The troughs have covers which are divided into many readily removed sections. A bath so arranged will easily accommodate 24 cells instead of 6.
MORSE
THE REGULATION OF TEMPERATURE.
69
FIG. 35. — Lower half of rectangular bath for measuring osmotic pressure — the "water compartment." (1) and (2) Brass vapor-tight cover from which are suspended the copper cans which contain the cells; (3), (7), and (8) one-half of the brass tubes belonging to the circulating system for hydrant water; (9) and (10) copper tubes which open at both ends on the outside of the bath; (12) and (13) tubes through which the water is drawn from bath and over the gas stoves; (14) large pipe through which water heated by the gas stoves is again pumped into the bath.
FIG. 36. — Upper half of rectangular bath for measurement of osmotic pressure — "air" or
''manometer compartment."
(1) and (2) Vapor-tight cover to "water compartment;" (3), (4), (5), and (6) screened lamps; (7) brass pipes for circulation of hydrant water; (8) circulating system for hot water; (9) electric fan; (10) thermostat.
70
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
It will be seen (Figure 35) that the ar- rangements in the lower part of the bath are nearly identical with those of the bath de- scribed under Type II. There is the same system of brass pipes (3, 7, 8, etc.) for the circulation of hydrant water, and the same arrangement for pumping water out of the bath (through 12 and 13) to be heatedly a gas stove and returned through the * pipe (14). There is also in both baths the same pro vision for the "heat- ing surface," except that the copper cylin- ders (9 and 10), in which the lamps are located, are somewhat differently placed in the two cases.
The copper-lined upper part of the bath— the space" — (Figure 36) is elec- trically heated by means of the lamps (3, 4, 5, and 6), which are shaded for the pro- tection of the various instru- ments containing mercury. There are two systems of pipes in the air space. That seen in the top (7) is for the circulation of hydrant water. It serves the same purpose in the air space as the system of pipes (3, 7, 8, etc.) in the lower part of the bath. The system of pipes situated at the end of the bath (8) is for
,1 i ,. f i (1), (2), and (3) Gas lamps (outside of bath) for heating
the Circulation OI not Water. circulating water; (4) pump; (5) and (6) stopcocks which It may also be USed for COld are use(^ wnen hydrant water is to be circulated through
water. The air in the upper thepipes>
part of the bath is kept in circulation by means of the fan (9).
The heating and pumping arrangements for the hot water are situated on the outside of the bath. Their relation to what is seen on the inside is shown in Figure 37. The gas burners (1, 2, and 3) heat the water on its way to the pump (4), from which it is returned in the direction of the arrows. When the system is used for the circulation of hydrant water, the water enters through 5 and leaves through 6. In one of the baths
FIG. 37. — Hot- water circulating system with end of bath removed.
MORSE
PLATE 4
Rectangular bath, end view.
THE REGULATION OF TEMPERATURE. 71
of Type III, the interior portion of the system has been replaced by a single large pipe of ring form, which is so arranged on the inside that the hot water is returned to the upper part, while the colder water is con- stantly pumped out of the bottom.
The motor fan (9) is employed to keep the air in the inclosed space in circulation over the heated pipes and over the lamps; but it serves also to keep the manometers gently but constantly agitated, and thus to overcome the tendency of the mercury to lag in the tubes. This agita- tion is increased to any desired extent by attaching bits of stiff paper to the upper ends of the manometers.
The external appearance of the bath is seen in Plates 4 and 5. In the latter, the system of pipes for the circulation of hydrant water, which should be seen at the top of the interior, has been removed, as this bath is but little used for temperatures below that of the air.
The other baths of the same general type (two in number) were planned with reference to the measurement of osmotic pressure at low or very moderate temperatures. They differ from the bath described mainly in the care which has been taken to protect the interior from external temperature conditions. Their wooden walls are all double and the intervening space is filled with hair. Moreover, the small rooms in which they are located are made subject to temperature regu- lation by means of pipes covering the ceiling through which hydrant water is circulated when necessary. A further means of cooling these bath rooms consists of a chute opening upon the outside of the building, through which air is introduced into the room at any desired rate by means of a rotary fan. Formerly it was attempted — by means of a cir- culating system for hydrant water, by the introduction of a regulated quantity of air from the outside, and by means of gas stoves under the control of thermostats — to keep the bath room as nearly as possible at the temperature of the bath; but with the present improved facilities for the internal regulation of the baths, this is no longer necessary. The recent practice is, in general, to keep the temperature of the room 4° or 5° below that which is to be maintained in the bath. The flexibility of the system of temperature regulation, however, is such that differences of temperature amounting to 25° can be easily tolerated. At the highest temperatures at which osmotic pressures have been measured, differ- ences of 60° were not infrequent.
TYPE IV.
The baths previously described, which are made partly of wood, are not adapted to the measurement of osmotic pressure at high temper- atures. For this purpose, it was necessary to construct baths of differ- ent design and wholly of metals.
The baths for high temperatures, which are equally well adapted to work at low temperatures, are of two sizes and are made of heavy sheet brass and copper— mainly of the former. A (Figure 38) exhibits a
72
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
section of the inner compartment of one of the smaller baths. It is this compartment which is maintained at any desired constant temperature, and in which the cells are located during a measurement of pressure. It is circular in form in the smaller baths, 300 millimeters in diameter and 1 meter in height. Surrounding this is a large cylinder (B, B)
38
39
FIG. 38. — Brass and copper bath for high- temperature work. Vertical section.
(A) Inner bath; (B) outer bath; (L),(L), (L).and (L) lamps; (P) brass plate to prevent water rising directly from heated bottom of (B) to bottom of (A) ; (Z) caps for lamp compartments.
FIG. 39. — Brass and copper bath for high tem- peratures, second vertical section.
(A) Inner bath; (C) pumping tubes; (E) space into which water coming up (C), (C) is deliv- ered; (S), (S), and (Si) gas stoves.
which is twice as wide and much higher. In the space between the two brass cylinders, the water is heated and made to circulate rapidly over the exterior surface of the inner compartment (A). The circulating system and the arrangements for heating the water by gas are shown in Figure 39. C, C are the pumping tubes, 100 millimeters in diameter, which unite at the top in the short but wider tube (D) . At the bottom, they open directly over the gas stoves (S, S) . There is a third gas stove
MORSE
PLATE 5
THE REGULATION OF TEMPERATURE.
73
(SO which is not ordinarily in use. The water, heated by the stoves (S, S), is pumped up through the tubes (C, C) at the rate of about 500 liters per minute. At the top of D it is delivered into the space (E, E), whence it returns to the bottom of the outer cylinder to be reheated by the gas stoves and again pumped up through the tubes (C, C). In its downward course the water passes over the outer surface of A and also over the lamp compartments (L, L, L, and L, Figure 38). In the smaller baths there are 6 or 8 of these lamp compartments, and in the larger ones 8 or 12. They are distributed in pairs, one in each pair being located directly over the other. The circular openings, through which the lamps are introduced from the outside, are closed by means of caps, one of which (Z) is shown in Figure 38. Below the inner compartment (A , Figures 38 and 39) is the disk (P), which pre- vents the water which has been heated by any of the gas stoves from rising directly against the bottom of A . In Figure 39, there are also to be seen three tubes, indicated by dotted lines, which serve as passage ways between the exterior and interior for the intro- duction of thermometers, wires, etc. Figure 40 is a horizontal section of one of the smaller baths, in which the positions of the pumping tubes are indicated by (7, (7, and those of the lamps by L, L, L, and L. The entrance to the inner bath (A, 150 millimeters in width) is closed by the plate-glass door (/, Figure 40) and by the hollow metal door (M). There are two such doors, as will be seen in Figures 41 and 42. The lower one, which is opened only when it is necessary to introduce or remove the cells, is packed with hair. The upper door, on the other hand, must be opened whenever an observation is to be made. On this account, it is provided with an inde- pendent temperature-regulating device, portions of which can be seen in Figure 43. By means of this, the door is prevented from cooling down when open. The space between the inner glass door and the outer metal doors is about 40 millimeters in width. It is occupied by a brass frame, as large as the glass door, which is filled with minute doors, any one of which can be opened independently of the others. Between this frame (which is placed close to the glass door) and the outer metal doors is a hair- filled pad, which is so divided that any small portion of the frame of brass
FIG. 40. — Brass-copper bath for high tempera- tures. Horizontal section.
(A) Inner bath; (C) and (C) pumping tubes; (L) , (L), (L), and (L) lamp compartments; (/) plate- glass door; (M) lower hollow door filled with hair.
74
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
doors behind it may be exposed to view. With these arrangements, and with artificial illumination by screened lamps, it is practicable to observe objects in the bath with very little exposure of the interior.
Figure 41 shows the external appearance of the circular bath, and Figure 42 the appearance of the interior when a portion of the outside is removed.
FIG. 41. — Exterior view of bath for high temperatures.
FIG. 42. — View between interior and exterior baths, i. e., of space filled with water.
Figure 44 is a horizontal section of one of the larger baths of Type IV. Like the smaller ones, they are used for the measurement of osmotic pressure, and also for all of the purposes for which the so-called "man- ometer house" was formerly employed — such as the determination of capillary depression, the determination of nitrogen volumes, the com- parison of manometers, etc. It is elliptical in form, the longer axis being twice the diameter of the compartment (A) in the smaller baths. It is also higher than the circular baths by 100 millimeters, giving a height of 1.1 meters for A. It has three pumping tubes (C, C, and C) instead of two, and 8 or 12 lamp compartments (L, L, L, and L) instead
THE REGULATION OF TEMPERATURE.
75
of 6 or 8. The two smaller metal doors of the larger bath are like the corresponding doors in the smaller bath, except that they are inserted in a larger door, which serves as a frame. The relations of the three doors will be seen in Figures 44 and 45. The largest door and the lower small one are packed with hair and are opened only when it is necessary to introduce or remove apparatus, or to make some adjustment of the instruments within. The upper of the two smaller doors, which must be opened whenever an observation is to be made, is provided with a device (Figure 43) for the inde- pendent regulation of its temperature. As regards the disposition of the space between the glass and the metal doors,
there is no difference between the larger FlG> 43.— Automatic arrangements for main- baths and the Smaller Ones. taming temperature of upper door when open.
GLASS POOR
LARGE- // 2 5T1ALL DoOR5\\ DOOR
// in LARGE- \
FIG. 44. — Larger (elliptical) bath for high temperatures. Horizontal section. (C), (C), and (C) pumping tubes; (L), (L), (L) lamp compartments.
No lamps of more than 16 candle-power are used in baths of Type IV, even when they are maintaining temperatures near the boiling-point of
76 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
water. For most purposes, lamps of 8 or 10 candle-power suffice. If temperatures but little above that of the room are to be maintained, the electrical appliances only are employed to heat and regulate the baths. For low temperatures, a coil of block-tin pipe is placed in the space (E, E, Figure 39), and through it there is made to circulate, under constant pressure, a current of hydrant water, which has previously been cooled with ice if necessary.
FIG. 45. — Exterior view of larger bath for high temperatures.
In Figure 39 the thermostat is shown within the compartment (A), while in Figure 38 it is represented as being immersed in the water of the bath. The latter has been found to be the better of the two pos- sible positions for the instrument. The temperature of the water about the thermostat may not be precisely that of the interior compart- ment, and at other points it may vary slightly, but when the instrument has once been set for a given interior temperature, it is quite capable of maintaining it for any length of time to within 0.01°, which is a rather better regulation than can be secured in baths of Type III.
CHAPTER IV.
THE MEMBRANES.
Pfeffer was the first to give to artificial semi-permeable membranes the support of a rigid background, and to him, therefore, belongs the credit of having originated the only practicable method of measuring directly and correctly the osmotic pressure of solutions. But many years of persistent investigation were necessary in order to overcome the great difficulties which are inherent in the method and to reduce it to a workable form through the elimination of its sources of error.
The following is an accurate restatement, though not an entirely literal translation, of Pfeffer's description of his method of depositing membranes :
"The clay cells were first completely injected with water by means of repeated evacuations under the air pump. They were then filled with, and placed for several hours in, a 3 per cent solution of copper sulphate. After- wards, they were several times quickly rinsed (upon the inside only) with water, and well dried internally and as expeditiously as possible with strips of filter paper. Having been dried slightly upon the outside, they were left exposed in the air until the exterior surface was still just moist to the feel. The cells were then filled with a 3 per cent solution of potassium ferrocyanide, and returned to the solution of copper sulphate.
" After standing quietly from 24 to 48 hours, the cells were filled completely with a solution of ferrocyanide and closed. The contents now developed grad- ually a certain over-pressure due to the superior osmotic pressure of the interior solution. After another period of from 24 to 48 hours, the apparatus was opened and refilled with a solution containing 3 per cent of potassium ferro- cyanide and 1.5 per cent of saltpeter (by weight), which developed ordinarily an osmotic pressure of something more than 3 atmospheres. If the cells were to be used for higher pressures, they were tested with solutions containing more saltpeter."
Pfeffer found that a "slow increase in pressure and a certain period of low pressure" were essential to success in the preparation of his mem- branes. The explanation which he gives is that, at certain points, the membrane spans depressions in the cell wall, and is, therefore, more liable to rupture, if the pressure, which is to force the membrane against the wall in such unsupported places, is rapidly or suddenly increased. He also states that a somewhat prolonged period of deposition is neces- sary in order to give to the membrane the strength which will enable it to withstand pressure.
The attempts which were made in this laboratory many years ago to reproduce the membranes of Pfeffer with a view to measuring osmotic pressure, especially that of concentrated solutions, were failures, as have been all similar attempts on the part of other investigators. It was not impossible to make membranes which would yield impressive
77
78 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
osmotic phenomena, but when they were tested as to their sufficiency for quantitative purposes by the rule that a perfect membrane must be able to develop and maintain maximum pressures, without leakage of the solute at any time during an experiment, they were found to be wanting.
The failure of the membranes to justify the hopes which had been entertained of them could be ascribed to any one, or all, of several causes — to the want of perfect semi-permeability on the .part of the copper ferrocyanide; to the faulty character of the supporting porous wall; or to the imperfect attachment of the membrane to the wall; or to all three of these possible defects. It was strongly suspected that the third cause for failure existed; in other words, that the membranes made by the method of Pfeffer are not at all points firmly attached to the supporting wall.
The ideal membrane (as regards location) is obviously one consisting of innumerable plugs which are driven so firmly into the mouths of the pores which open on the interior surface of the cell that no pressure can rupture or dislodge them — that is, the membrane must be firmly embedded in the wall and not consist of a mere (more or less detached) cover for its interior surface. Looked at from this point of view, the practice of Pfeffer of carefully drying the interior of his cells with filter paper was a highly rational procedure. Subsequent observations have proved it to be the vital feature of his process. Pfeffer himself recog- nized this in a practical way, but he appears to have regarded these membranes as "aufgelagert" rather than embedded, though he gives no explanations; for he says (page 9), ((wdhrend ich anfangs mit grossen Schwierigkeiten zu kdmpfen hatte, und ehe ich zu partieller Abtrocknung meine Zuflucht nahm, uberhaupt keine aufgelagerte Membran zu Stande brachte." The obvious purpose of the "Abtrocknung" was to empty the mouths of the pores of the solution of copper sulphate, in order that they might be filled with the solution of potassium ferrocyanide and thus force the formation of an embedded membrane. It seemed probable, however, that the procedure of Pfeffer failed, in some degree, to accom- plish its purpose, i.e., that the embedding was emperfect; and the ques- tion arose whether it might not be possible to devise some method by means of which these "plugs," of which the membrane should exclu- sively consist, could be more firmly driven into and more securely fixed in their places than is the case when the diffusion method of Pfeffer is employed. It was in this connection that the electrolytic process for the deposition of the membranes occurred to the writer.
It should be stated, however, that the first suggestion which led up to the solution of the problem came accidentally. While the subject of measuring osmotic pressure was still uppermost in the mind of the writer, he was engaged in an attempt to procure pure aqueous solutions of permanganic acid by the electrolysis of potassium permanganate.
THE MEMBRANES. 79
The process consisted in placing two porous cups, partly filled with water, side by side in a solution of the salt, and electrolyzing with the anode in one cup and the cathode in the other. At times the pores of the anode cup became filled with a deposit of peroxide, and it was noticed that whenever this occurred the volume of the contents of that cell seemed to increase more rapidly than could be accounted for by any probable "electrical endosmose" of the solvent. It was imme- diately suspected that the deposit of peroxide in the porous wall was playing the part of a semi-permeable membrane and, upon further investigation, the suspicion was proved to be well founded. The pro- cess by which the semi-permeable peroxide was deposited in the pores of the cell and that by which osmotic membranes are now made differ radically ; but the mere fact of having formed one active membrane by electrolytic means sufficed to suggest the practicability of employing electricity for the production of all semi-permeable membranes.
The work of Pfeffer was unique and brilliant, and its consequences have been far-reaching and beneficent. The writer, after fourteen years of activity in the same difficult field, has more reason than any other to appreciate its great merit and less justification than any other for detracting from its value. This statement of attitude is made with a view to disarming any possible suspicion of careless criticism on the part of the writer, when he states, on the basis of his own experience, that none of the pressures recorded by Pfeffer could have been the maximum pressures of his solutions.
The final step in the preparation of Pfeffer's cells for quantitative measurements was by means of a solution containing 3 per cent of potassium ferrocyanide and 1.5 per cent of potassium nitrate. The cells were filled with this solution, closed, and placed in 3 per cent solu- tions of copper sulphate. Pfeffer states that the pressure subsequently developed was usually something more than 3 atmospheres. If we add the excess of the ferrocyanide's pressure (over that of the copper sulphate) to the true pressure of the nitrate, the total pressure which should have been developed is something more than 6.5 atmospheres. The unavoidable conclusion is that the membranes did not perfectly retain the solute, and that the subsequent measurements were under- taken with defective cells.
In order to show that the osmotic pressure of solutions obeys the law of Boyle for gases, van't Hoff cites the pressures of the 1, 2, 4, and 6 per cent solutions of cane sugar which were obtained by Pfeffer. These pressures are strikingly proportional to the concentration, which is the form of the law as applied to solutions. There was, at that time, and for many years thereafter, no reason known why this evidence should not be accepted at its face value, and its validity appears never to have been questioned; accordingly one finds it repeated and emphasized in every presentation of the subject of osmotic pressure from 1887 to the present
80
OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
time. The validity of the proof depends, however, upon the question whether the pressures obtained by Pfeffer were the full (maximum) pressures of his solutions. Pfeffer believed them to be so; for he men- tions having ascertained, by means of specific-gravity determinations, that the solutions did not lose appreciably in concentration while in the cells. It is questionable, however, if this method is sufficiently delicate, unless carried out with extraordinary precautions, to detect differences which, if discovered, might well have led to a verdict unfavorable to the applicability of Boyle's law. The writer and his associates have not yet exactly repeated the experiments of Pfeffer, though they expect to do so in the near future. They have, however, done enough work at temperatures and concentrations approximating to those of Pfeffer to enable them to predict with considerable confidence about what the pressures in question will be found to be. The following table gives the pressures found by Pfeffer which have been universally quoted as proof of the conformity of osmotic pressure to Boyle's law and those which the work of the writer and his associates enable them to predict:
TABLE 3.
|
1. |
2. |
3. |
4. |
5. |
|
Concentration. Grams of sugar per 1000 c.c. H,0. |
Pressures found by Pfeffer. |
Pressures required by Boyle's law at 15°. |
Pressures Pfeffer should have found. |
Differences between Pfeffer's and the author's results. |
|
10 |
0.71 atms. |
0.69 atms. |
0.75 atms. |
5.6 per cent. |
|
20 |
1.34 |
1.37 |
1.48 |
10.4 |
|
40 |
2.74 |
2.77 |
3.00 |
9.5 |
|
60 |
4.05 |
4.16 |
4.41 |
8.8 |
If we compare the pressures found by Pfeffer with those required by the law of Boyle at 15° — that is, columns 2 and 3 — the agreement is astonishing, and it is not surprising that these data have played an important part in nearly all discussions of osmotic pressure during the last twenty-five years. If, on the other hand, we compare the pres- sures of Pfeffer with those calculated for the same solutions from the results of the author and his associates — that is, columns 2 and 4 — it will be observed that the latter are considerably higher than the former. The differences (column 5) vary from 5.6 to 10.4 per cent. The data given in column 4, which are designated (somewhat presumptuously perhaps) as "the pressures which Pfeffer should have found," are all calculated from measurements made after the method of measuring had been brought to its highest state of perfection— that is, after the last vestige of leakage of solute had been eliminated. The inference to be drawn from the facts, as stated above, is that the smaller pressures of Pfeffer were due to some leakage of the solute. The solutions of Pfeffer which are cited in this discussion contained in 1,000 cubic meters of
THE MEMBRANES. 81
water 10, 20, 40, and 60 grams of sugar; while those from whose pres- sures the values given in column 4 were calculated contained in the same volume of solvent 8.49, 16.98, 33.96, and 67.92 grams. They were, in fact, 0.025, 0.050, 0.100, and 0.150 " weight-normal" solutions, whose pressures were 0.64, 1.28, 2.54, and 4.99 atmospheres respectively.
To one who has had long experience in the measurement of osmotic pressure, the conformity of Pfeffer's results to the requirement of Boyle's law — stated merely as proportionality of pressure to concentration — is not surprising. It is, in fact, almost a necessary consequence of using cells which do not quite perfectly retain the solute. The leakage of solute from any series of defective, but equally good, cells is propor- tional to the pressures of the solutions rather than to their supposed concentrations. In other words, the pressures of cane-sugar solutions, between 0° and 25°, are for some reason not proportional to their sup- posed concentration. If now a series of them of varying strength are placed in defective cells, the escape of solute will be proportional to the pressures, and not to the supposed relative concentration of the solu- tions. The necessary consequence is that the relative pressures will gradually approach proportionality to the (supposed) relative concen- trations of the solutions. In a later chapter the author will have occa- sion to show how membranes which do not leak may be the means of appearing to establish the applicability of Boyle's law to osmotic pres- sure upon evidence which is superficially convincing, but fundamentally unsound. It may be stated here that all solutions of cane sugar thus far investigated do obey the law of Gay-Lussac between 0° and 25°, while none of them appear to obey the law of Boyle until higher temper- atures are reached. That this failure, in the latter case, may be more apparent than real will be shown elsewhere.
It is fortunate that the validity of the generalizations of van't Hoff regarding solutions does not depend exclusively upon the correctness of Pfeffer's measurements; and, in one sense, it is also fortunate that Pfeffer obtained the results which he did, rather than the correct pres- sures of his solutions ; for, whatever may have been the relative impor- tance assigned to them by van't Hoff, they undoubtedly contributed more than any other one of his arguments to the immediate and general acceptance of his views.
The first announcement* regarding the electrolytic method of deposit- ing membranes was made in the following words:
"If a solution of a copper salt and one of potassium ferrocyanide are sepa- rated by a porous wall which is filled with water, and a current is passed from an electrode in the former to another electrode in the latter solution, the copper and the ferrocyanogen ions should meet within the wall and separate as copper ferrocyanide at all points of meeting, so that in the end there should be built up a continuous membrane well supported on either side by the material of the wall."
*Amer. Chem. Journal, xxvi, 81.
82 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
The electrolytic method of depositing membranes was soon success- fully applied to all the usual varieties of porous vessel which accumulate in a laboratory, and to several new forms of the same. It was found, in fact, that a highly active membrane could be produced without diffi- culty in every kind of porous wall. The method was also employed for the deposition in porous vessels of many other compounds than copper ferrocyanide, and a large number of these (about 25) proved to be osmotically active.
The success of the new method as a means of building up membranes was placed beyond question, but the possession of it did not enable us to proceed at once to the measurement of osmotic pressure. The resisting power of the membranes made by the electrolytic method was much greater than that of those produced by the process of Pfeffer, but even they were unable, in the porous vessels then available, to withstand high pressures without rupture. Our attention during the succeeding four years was given almost exclusively to the porous wall on, or within, which the membrane is deposited.
A brief account of this part of the investigation has been given in the first chapter; but a concise restatement of those structural character- istics of the cell wall, upon which the efficiency of the membrane was found to depend, will be useful in this place. They are :
1. A great and uniform strength of wall, which was secured by
employing mixtures of different clays.
2. Absence of "air blisters," which were eliminated by subjecting
the clays to great pressures.
3. An exceedingly fine and uniform porosity, which was secured by
the employment of the finest portions only of the clays, by subjecting the mixtures to high pressure, and by burning at high temperatures.
THE DEPOSITION OF THE MEMBRANE.
The first steps toward the formation of the membrane are the expul- sion of air from the pores of the cell and its replacement by water. This has been effected from the beginning by means of the considerable vol- ume of water transported by the cations whenever dilute aqueous solu- tions of salts are subjected to electrolysis. At first the salt employed was potassium sulphate, but the fact that the "atmosphere" of water which surrounds the lithium ion is much greater than that transported by the potassium ion suggested the use of lithium salts rather than those of potassium. A series of quantitative comparisons carried out by Frazer showed that the quantities of water drawn through the cell wall conform to the following rule:
The volumes of water carried through the porous wall of a cell, under identical conditions, are inversely proportional to the relative velocities of the various cations, divided by their respective valencies.
THE MEMBRANES. 83
The improvement in the method which followed the replacement of the potassium salt by lithium sulphate was very striking. A 0.005 nor- mal solution was employed. The cell is nearly filled with the solution and immersed in the same to the lower limit of the glazed portion. The electrodes are of platinum and the one within the cell is made the cathode. Provision is made for the automatic removal of the water which is drawn into the cell through the pores. At intervals the elec- trolysis is interrupted for the purpose of mixing the liquid which has been removed by the siphon with the solution in the outer vessel. When it is thought that all the air has been expelled from the pores, the cell is taken out, emptied, and rinsed with pure water; it is then soaked for a time in distilled water, which is frequently renewed; lastly, it is filled with, and partially immersed in, pure water, and the electrolysis is resumed. When the conductivity, after frequent renewals of the water, has fallen nearly to that which is normal for the distilled water, the cell is ready for the deposition of the membrane. If the membrane is not to be deposited immediately, the cell is placed, and kept until needed, in water in which a little thymol or formaldehyde has been dissolved. The reason for this precaution will appear later when the subject of the infec- tion of the membrane is taken up. It is also well to take any other pre- cautions against infection which suggest themselves, such as boiling all the water which comes in contact with the cells, covering the vessels in which they are kept, etc. Such precautions are by no means superfluous.
The arrangement for the deposition of the membrane is as follows: the anode, which consists of a cylinder of copper, nickel, or cobalt, according to the composition of the membrane to be deposited, is placed in an empty glass vessel, and within the cylinder is placed the cell, which is closed by a rubber stopper carrying (1) the cathode, a platinum cylinder; (2) a funnel with a stem nearly long enough to reach the bottom of the cell; and (3) an overflow tube. The circuit is closed, and, as nearly simultaneously as possible, the cell and the vessel outside of it are filled, each with its appropriate solution. The solutions, which in the majority of the experiments to be reported are potassium or lithium ferrocyanide, and copper or nickel sulphate, are made one- tenth normal. The voltage employed is 1 10. At first the resistance is very high, owing to the fact that the cell wall is filled with nearly pure water. Very soon, however, the current begins to increase, and within a short time it attains a maximum. It then drops steadily for two or three hours, and perhaps longer, when it reaches a minimum corresponding to the maxi- mum resistance of the membrane which it is possible to obtain at that "running." If the electrolysis is continued very long after the current has reached a minimum, the resistance begins to fall again, and the decline persists until the circuit is broken. This strange behavior of the membrane appears to have some connection with an accumulation of alkali in the cell and perhaps in the membrane itself; accordingly,
84 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS.
during the electrolysis, the ferrocyanide is renewed every 2 or 3 min- utes by pouring a fresh solution of it into the funnel. A temporary increase in resistance follows each renewal, but this may be due in part to a fall in the temperature of the contents of the cell. The final decline in resistance may be postponed by frequent renewals of the solution, but not indefinitely. For this reason, it is suspected that the phenomenon is possibly due to accumulation of alkali in the membrane. Having reached its maximum resistance, the membrane can not be further improved by electrolysis, without a thorough preliminary soaking in pure water— that is, water which is free from electrolytes. The cell is therefore placed in water in which a little thymol has been dissolved, and is allowed to soak, with frequent renewals of the water, for several days. The period of soaking is quite indefinite, but experience has shown that it should be not less than 3 days; and that, in general, the longer the soaking is the better will be the result of the succeeding electrolysis.
After soaking, not less than 3 days, the membrane-forming process is repeated. On this occasion the resistance usually rises much higher than before, but finally reaches a maximum beyond which it can not be driven, however frequently the solution of ferrocyanide may be renewed. If the electrolysis is continued beyond this point, a gradual fall in resist- ance sets in, which may eventually lead to the total ruin of the mem- brane. As in the first instance, when it is found that the resistance no longer increases, the electrolysis is interrupted and the cell is again placed in water for a period of 3 or more days. The further procedure is simply a repetition of the alternate "running" and "soaking" described above, which is persisted in until the resistance of the membrane can be forced no higher. The maximum resistance which is finally obtained varies greatly. Obviously, it varies with the effective area of the membrane; and this, in turn, varies from cell to cell, according to the porosity of the cell wall. In general, it is found that membranes in hard-burned cells have high final resistances. The temperature of depo- sition is also an important factor in determining resistance. In a given cell, a membrane deposited or repaired at 0° may have a resistance of more than 1,000,000 ohms, while at 80° the resistance can not be driven above 1,000 ohms. Again, the resistance of the membranes increases with age and repeated use.
Having developed the maximum resistance in the manner described, the membrane is subjected to a process of "seasoning" under pressure. For this purpose the cell is set up with a concentrated solution of cane sugar (not less than half normal) to which a small amount of ferro- cyanide has been added, an osmotically equivalent quantity of copper sulphate having been dissolved in the water in which the cell is to stand during the experiment. The initial mechanical pressure which is brought upon the contents of the cell at the time of closing may be
THE MEMBRANES. 85
above the known osmotic pressure of the solution or considerably below it. In the former case the mercury in the manometer falls, in the latter it rises. Eventually, in either case, the meniscus generally comes to rest at a point below the position it should have, showing that the solu- tion has been diluted. The irregular movements of the meniscus, which often precede the assumption of its final position, are all such as can be interpreted as being due to a breaking of the membrane under pressure, and a mending of the rents by the membrane-formers. What- ever may be the result of the first trial, which usually extends over sev- eral days before the pressure becomes constant, the cell is emptied and soaked from 3 days to a week in distilled water, when it is again sub- jected to the membrane-forming process. Afterwards, it is again set up with a solution of cane sugar in the same manner as for the first trial. On the second trial the pressure developed is usually higher than on the first. Sometimes, but not often, the full osmotic pressure of the un- diluted solution is obtained. The further procedure with the cell is simply a repetition of the steps already described. Sooner or later there is developed in this way a cell which gives maximum pressures on every occasion, and in which the solutions suffer no dilution. It is then ready for the measurement of osmotic pressure.
OBSERVATIONS ON THE MEMBRANE. 1. TEMPERATURE OF DEPOSITION.
As nearly as possible, the membrane is deposited and developed at the temperature at which it is afterwards to be employed for the measure- ment of pressure. The temperature rises somewhat during the deposi- tion, but is kept within limits by the frequent renewals of the ferro- cyanide solution. During the intervals of rest, i. e., while soaking, the cell is also maintained at the temperature at which its membrane was formed and at which it is to be used for the measurement of