T H E OSMOTIC ACTION O F SOLUTIONS OF CANE SUGAR, SILVER NITRATE, AND LITHIUM CHLORIDE I N PYRIDINE, WHEN SEPARATED FROM PYRIDIXE BY A RUBBER MEMBRANE BY ALFRED I$. KOENIG
I n Kahlenberg’sl paper, “On the Nature of the Process of Osmosis and Osmotic Pressure,” there are described a few quantitative measurements of the pressures obtained when solutions of cane sugar, silver nitrate, and lithium chloride in pyridine were separated from pyridine by means of a rubber membrane. Cohen and Commelin2 repeated some of these measurements with an apparatus of a slightly different design. Their results agree substantially with those of Kahlenberg. However, measurements carried on with the same solutions and under as nearly as possible identical conditions vary a great deal among themselves, as will be seen in connection with certain remarks in this paper. I n view of the fact that these results show considerable variation, it was thought worth while to obtain more satisfactory measurements of the osmotic pressures of the above named solutions. The results of this work have been set forth in this paper.
Materials Used The pyridine used was all of one lot of Merck’s best grade. After drying for several months over sticks of potassium hydroxide, it was boiled with potassium permanganate and freshly ignited barium oxide under a reflux condenser for eight to ten hours. After distillation, it was kept in glass-stoppered bottles. The outer and inner liquids, after measurements had been made, were poured into bottles containing sticks of potassium hydroxide and redistilled for further use. The pyridine boiled with a constant boiling point of I 14 t o I I j under 740 mm pressure. Jour. Phys. Chem., IO, 141 ( 1 9 ~ 6 ) Zeit. phys. Chem., 64, I (1908).
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Alfred E . Koenig
A fine grade of rock candy, which was found to contain no reducing sugars, was prepared for use in two ways. Some of it was ground in a mortar fine enough to go through an eighty mesh sieve. It was spread in thin layers on large watch crystals and dri'ed in a vacuum over calcium chloride at 90' for twelve to fifteen hours. Some of the sugar was made into a thick syrup with distilled water and thrown out as a very finely divided precipitate by means of alcohol, with constant stirring of the solution while the alcohol was added. The precipitated sugar was washed on a suction filter with absolute alcohol and dried in a vacuum over calcium chloride a t 90' as in the case of the other sample. The sugar, prepared by precipitation, dissolved more readily in pyridine, probably due to its finely divided condition, but otherwise gave no results that differed from those obtained with the sugar prepared in the other way. The dried sugar was kept in desiccators over fresh calcium chloride. The lithium chloride and silver nitrate, which were of Merck's make, were ground to a fine powder and dried in the same manner as the sugar. In the case of the silver nitrate the temperature at which it was dried was about zoo lower to avoid decomposition. The work with the lithium chloride was carried on during the cold dry weather of January and February, when it could be weighed without taking up an appreciable amount of moisture from the air. The solutions were prepared as follows: The required amount of solute was weighed, dissolved in pyridine, and diluted to one liter at 2 5 O , the temperature a t which all the measurements were made. The rubber for the membranes was a fine grade of dental rubber, obtained from the Goodrich Rubber Company of Akron, Ohio. It was very uniform in texture, light brown in color, and three to four-tenths of a millimeter in thickness when not stretched. Apparatus and Method of Making Measurements The arrangement of one of the cells is shown in Fig. I . The cell ( a ) was bell shaped, turned out of a piece of steel
Osmotic Action of Solutiom of Cane Sugar, Etc.
463
shafting. The removable bottom ( b ) was perforated with holes one millimeter in diameter. These holes were countersunk toward the interior of the cell so as to facilitate the removal of air bubbles when the solution was run into the cell. When this steel disk ( b ) was in place, it left a smooth surface
h
f
c
-
Fig.
I
on the outside of the cell, within the circle of eight holes for the screws (d). Upon this surface within the circle of screws, was laid the membrane consisting of a disk of rubber six and a half centimeters in diameter, the gaskets of rubber, and the muslin disks, the arrangement of which is described more fully in another place in this paper. The membrane was held in place by the steel disk (c), the central portion of which
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Ayred E . Koewig
was perforated with holes like those of the cell bottom ( b ) , although the holes of these two disks were not necessarily opposite each other. The holes in the disk (c) were countersunk toward the outside, so as to permit the outer liquid to come in contact with the membrane more readily. This disk (c) being about 3 mm thick was rigid and, when clamped on by eight screws which ran into the solid rim of thecell, held the membrane very firmly and evenly in place. The pyridine which bathed the outside of the membrane was put in the glass jar (e) which was set in the galvanized iron vessel (J). The stem of the cell was soldered into the hole in the center of the cover of this vessel (J) at the point (g). A heavy gasket of “rainbow” packing, well greased with vaseline, was laid between this and the flange of the vessel at ( h ) and the cover clamped on with six brass screw clamps like the one shown a t ( 0 ) . This kept the water of the thermostat from entering the iron vessel. On several occasions, a run covering a period of six to eight weeks was made and no water entered the vessel (j). The vertical tube, with the side arm for the manometer, and the manometer were made of glass. The joint between the glass and steel a t (m) was made in the following way: A piece of glass tubing with the same internal diameter as the opening in the stem of the cell, was cut off straight so as to fit evenly on the steel. The outer surface of this tube for about 5 cm from the end was roughened by rubbing with a paste of glycerine and emery dust. This surface was then thoroughly cleaned and painted with a solution containing about three percent of platinic chloride and two percent of stearic acid in ether and alcohol, about two parts of ether to one of alcohol. The ether and alcohol evaporated very quickly leaving a thin coating of platinic chloride and stearic acid on the roughened surface of the glass. The glass was then heated gently in the flame of a Bunsen burner, to reduce the platinic chloride and finally heated to redness to burn off all the carbon. Upon the strongly adhering film of platinum, a layer of gold was deposited electrolytically. The gold was plated with alayer of nickel about a tenth of a millimeter
Osmotic Action of Solutions of Cane Sugar, Etc.
465
thick. To stiffen the tube, a strip of zinc foil was wound about the nickel and soldered in place. Then the tube was soldered to the stem of the cell. The T tube, carrying the manometer on the side arm and giving a straight entrance to the cell through which a funnel was inserted for filling with the solutions, was fused to the upper end of the tube that was soldered to the cell. Whenever the cell was emptied, the manometer had to be broken off a t (w)and, on refilling, fused on again. The stirrer (sl) for the interior of the cell was made of iron. To the upper end of the wire handle, was soldered a thicker piece of iron wire, which extended into the part of the glass tube above the side arm for the manometer. This stirrer was actuated by means of a helix, which was slipped over the sealed end of the glass tube. Through this helix a current was sent a t intervals. Thus the contents of the cell were stirred as described in Kahlenberg’sl experiments. The pyridine on the outside of the cell in the glass vessel (e) was kept in motion by the stirrer (sz), the handle of which passed up through one of the copper tubes (i). The other tube was for the purpose of removing a sample of the outer liquid at any time without disturbing the progress of the experiment. These tubes extended about twelve centimeters above the water of the thermostat. The motion of this stirrer, as well as the interruptions of the current through the helix which operated the other, were brought about by gears drivenby an electric motor which also served to keep the water of the thermostat in circulation. This arrangement is not shown in the drawings. About 500 cc of pyridine were placed in the glass jar (e). The membrane having been put in place, the cell and the stirrer ( s z ) , with the cover (g) were adjusted and the latter clamped on with the six clamps like the one at ( 0 ) . Several vigorous strokes with the stirrer removed the air bubbles from the holes in the disk (c), thus wetting the membrane with pyridine. In experiments where the bottom of the cell could be observed, it was found that this procedure was effective Jour. Phys. Chem.,
IO,
141 (1906).
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Alfred E. Koexig
in removing the air bubbles. The manometer was sealed on a t (x). Its large arm was filled with mercury to about the level of the bend. The other arm ( k ) was a 0.25 mm capillary open to the air above. Since the pyridine on the outside of the cell and the mercury in the capillary were open to the atmospheric pressure, the barometric changes had no effect on the readings. The long drawn out stem of a thistle tube was inserted into the cell through the upright tube at the top and thesolution, whose osmotic pressure was to be obtained, was poured in to within 3 cm of the top of the tube. The capacity of the cell was about 150 cc. Suction was applied a t the upper endof the tube and suddenly released. Care had to be exercised not tosuck the mercury around the bend in the capillary, which would allow air to enter. This alternate suction and sudden release was repeated a number of times and served to remove any air bubbles that might adhere to the interior of the cell, especially those in the holes in the steel disk, that lead to the membrane. The upper end of the glass tube, above the solution was then heated and drawn out to a long fine point. This tip was then filled with solution by means of a thistle tube with a very finely drawn out stem. With some practice, it was possible to seal off this tip without enclosing a bubble of air or a t most about a cubic millimeter. The manometer was supported by clamps on an upright iron rod which was fastened by clamps to the vessel ( f ) . These supports are not shown in the drawing. The whole apparatus was then set in the thermostat and the necessary connections made for stirring the inner and outer liquids. The whole operation of filling and sealing a cell would require about thirty minutes. The temperature of the thermostat was regulated b y . means of the ordinary toluene regulator and was kept constant to within 0.05'. It was found by experience that it was best to seal the cells at a room temperature a degree or two below that of the thermostat and thus obtain an initial pressure due to the thermal expansion of the solution, when it was
Osmotic Actio.rz of Solutions o,f Cane Sugar, Etc.
467
raised to 2 5 ', the temperature of the thermostat, This initial pressure was attained very rapidly, then the increase would be at a slower rate till the maximum was reached in from 24 to 7 2 hours. This is best shown by the curves in Fig. 2 . When the initial pressure, due to thermal expansion happened to be higher than the maximum osmotic pressure of the solution, there was successively a rise of the mercury, a fairly rapid drop,
Fig.
2
a halt, and then a gradual falling off of the pressure. This is illustrated by curve No. 143 of Fig. 2 . For any solution, the halt in the decrease in pressure would correspond to the maximum pressure attained when the cell was sealed a t a temperature nearer to that of the thermostat, so that the pressure due t o the thermal expansion was lower than the osmotic pressure. The heights of the mercury in the capillary ( k ) and in the large arm ( I ) , above the bottom of the thermostat were measured by means of a meter stick. The difference between
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Aljred E . Koenig
these two heights gave the difference in the mercury levels. To this was added the capillary depression in the small tube. From the sum was subtracted the height of the solution from the level of the mercury in the wide arm to the top of the tube ( p ) , reduced to the equivalent centimeters of mercury. For this reduction, the density of pyridine at 25' was taken as being near enough to that of the solutions used. The resulting length of the mercury was reduced to the value for o o C, taking the average room temperature to be 2 3 ' . The room was steam heated and automatically regulated. Fluctuations of a few degrees would not change the results materially. The change of a column of mercury IGO cm long amounts to 0.017 cm per degree change in temperature. The following example shows how the readings were made and corrected. Height of the mercury in the capillary tube Height of the mercury in the wide arm
148.j cm 27.8 cm
Difference in level of the mercury 1 2 0 . 7 cm Capillary depression (added), I . j cm Height of solution above the level of the mercury in the wide arm, reduced to its equivalent in mercury (subtracted), I .8cm True differencein level of mercury (at room'temperature) 120.4 cm Correction to reduce to mercury at o o (subtracted), 0.48 cm -0. j Cm Corrected value for the pressure
119.9 cm
The Manometer Joint and the Adjustment of the Membranes The development and the stlccessful completion of this work centered around the arrangement of the membrane so as to make that part of the cell tight, and the production of a joint between the metal of the cell and the glass of the manometer so that this union would be absolutely tight. The solution of the latter problem was accomplished first, for without a perfect non-leaking joint with the manometer, the action of the membrane could not be successfully studied. The work described in this paper is the result of over four
-
Osmotic Action of Solutiom of Cawe Sugar, Etc.
469
years of experimentation of which nearly half was occupied in trying out various combinations of metals and methods of preparing the joint between the steel and the glass. The roughened part of the glass was not always uniformly coated by the reduction of the platinic chloride solution. On plating with copper, small bare or thin spots would often appear which would eventually plate over. The pyridine would gradually creep up between the copper and the glass. When gold was plated upon the film of platinum, the small bare spots became evident. The tube was then taken out of the plating solution, washed, the bare spots painted with the platinic chloride solution, and this coating reduced as previously described. If on further plating the gold did not form a continuous film, the above treatment was repeated. For some of the joints, copper was ‘plated upon the gold and this soldered to the steel. Nickel is not appreciably affected by pyridine, so it was substituted for copper. It was necessary to make a strong coating of some other metal over the gold as the hot solder dissolves the gold off the glass. This platinumgold-nickel combination was quite satisfactory, but it was further improved to overcome another difficulty. The weight of the manometers, which were two or three meters high, put a strain on the joint which sooner or later resulted in a leak, thus spoiling what would otherwise have been a good run. A strip of zinc, wrapped around the tube and soldered to the nickel all around, braced these joints so effectively that some of them lasted through over eight months of continuous use. The following scheme for discovering leaks in the joint was found very useful. The cell was set up, as for aregular run, with a tenth-molecular solution of sugar. It was left out of the thermostat. The leaks could be located by the tiny globules of sticky syrup which gathered where the solution oozed out. I n summing up his researches, Raoult stated that the measurement of osmotic pressures was chiefly concerned with the soft membranes and that work with these offered such experimental difficulties that he hesitated to continue
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Alfred E. Koelzig
further experiments of this nature. It is because of these difficulties that quantitative measurements of osmotic pressure have been practically limited to such membranes as can be prepared according to Pfeffer’s method of precipitation in the walls of a porous cup. I n fact, the best results available at the present time, those of Morse, Berkeley, and their coworkers have almost all been obtained with one membrane, copper ferrocyanide, and with water as the solvent. Because of these facts, Professor Kahlenberg urged upon me the importance of continuing the present research till it was perfectly certain that the membrane used was so placed in the cell that there was no leakage; and, further, that the soft flexible rubber was so firmly and uniformly held in place that there could be no wrinkling or warping which could affect the results obtained. The final arrangement of the rubber disks, rubber gaskets, and muslin disks was, therefore, the result of long and careful experimentation. Some experiments were also made with a thicker rubber both for membranes and for the gaskets. This sheet rubber was like that ordinarily used on foot-power bellows and was about a millimeter thick when not stretched. The action with this thicker rubber was much slower. It became very sticky and gave lower pressures. This fact is in harmony with the conclusions drawn later, namely, that the longer the contact of the rubber with the pyridine the lower the osmotic pressures obtained. Realizing the importance of the results of this work as exact quantitative measurements with a new membrane and a solvent other than water and that the apparatus described may be used in the investigation of other soft flexible membranes similar to rubber, the majn outlines of the development of the placing of the membranes in the cells have been included in this paper. At first a single disk of rubber dam was placed between the bottom of the cell ( b ) and the steel disk (c), Figs. I and 3. This rubber served as the membrane which separated the solution in the cell from the pyridine on the outside. With this arrangement, some pressure was developed but in no case
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was it very high, neither did the results for the same solutions agree. The pressures for a tenth-molecular sugar solution ranged from 30 crn to 90 cm. In fact, they resemble in magnitude and range those obtained by Cohen and Commelinl for the same solution, who give, for the tenth-molecular sugar solution, these pressures in centimeters of mercury : 82 -3, 75.6, 71.9, 71.7, 67.1, 64.4, 63.2, 54.2, 50.8, 50.1, 47.8, 46.4. The pressures that they obtained for the 0.050 and the 0.025 molecular solutions show even greater variations. These workers used a membrane consisting of a single sheet of rubber clamped to the open end of a hollow cylinder by means of a perforated steel plate and a ring. More will be said of this work a t another place. The next improvement made in the present work was this: Between the membrane, which was cut in the form of a disk 6.5 cm in diameter, and the bottom of the cell were put two gaskets cut from the same rubber dam as the membrane. Each of these gaskets had the same outer diameter as the membrane and an opening in the center 4.5 cm in diameter, this opening being about the same size as the perforated region of the bottom of the cell and of the plate that held the membrane in place. A disk of strong closely-woven muslin was placed between the membrane and the outer plate. This arrangement is shown in cross section in Fig. 3 . The pressures
L
J w Fig. 3
for a tenth-molecular solution of sugar now obtained varied from 130 cm to 160 cm of mercury. Finally, the circular Zeit. phys. Chem., 64,
I
(1908).
Alfred E. Koewig
472
space, marked (s) in Fig. 3, inside of the gaskets was snugly filled with two disks of muslin, just the size of theopenings in the rubber. The results obtained with this arrangement were fairly concordant and are those set forth in Table I of this paper. The rubber was used without any previous treatment with pyridine. Cohen and Commelinl state that in their work the rubber membrane was allowed to swell up in pyridine and, in this condition, was quickly put in place. Now this imbibed pyridine is given up very quickly by the fresh rubber so that the latter shrinks rapidly. Even though it may have taken only a fraction of a minute for them to put their membrane in place, their membranes must have shrunk somewhat due to loss of pyridine. Then when the pyridine solution came in contact with this membrane, the latter swelled and wrinkled, and there was no support on the inner side to prevent this. These wrinkles had to be pressed out by the pressure which was generated. This and the alteration of the rubber due to the action of the pyridine as explained later in this paper will no doubt account for the variations in the results obtained by Cohen and Commelin. The membranes used in the present work were so firmly clamped between muslin and steel plates that there was no chance of wrinkling or warping. Then, because of the fact that the experiments were all started with an initial pressure due to thermal expansion of the solution, there was a minimum entrance of solvent into the cell and consequently only a slight dilution of the solution whose osmotic pressure was to be determined. This was confirmed by experiment. The sugar solutions showed a decrease of only 0.03 to 0.08' in rotation in a tube two decimeters long, in experiments where osmosis had gone on for ten days to two weeks. The silver nitrate and lithium chloride solutions were found to be only slightly diminished in concentration after their osmotic pressure had been determined. Moreover, the outer liquid contained only traces of the solute. I n all cases, the diminution in the concentration 1
Zeit. phys. Chem., 64,
I
(1908).
Osmotic Action of Solations of Cane Sugar, Etc.
473
was much less than could have been detected by the accuracy with which the osmotic pressures could be measured. The general behavior of the cells during the course of an experiment has already been briefly described and is shown in several typical cases by the curves in Fig. 2 . In every experiment, when the cell had developed its maximum pressure, there was a gradual decrease in pressure, usually a t the rate of about a centimeter per day. A cell filled with pyridine gave a pressure due to thermal expansion when it was placed in the thermostat but this pressure diminished to almost nothing in the course of twenty-four hours. This shows that a pressure developed by thermal expansion cannot be maintained, for the pyridine under pressure flows out through the membrane till equilibrium is established. The following experiments throw some light on the cause of this decrease in the ‘pressure after the maximum has been reached : When a cell had developed its maximum pressure and this had begun to decrease, i t was emptied. The membrane was allowed to dry for several days. The rubber was found to be elastic and apparently in the same condition as when fresh, as far as could be determined by ordinary means and by examination with a microscope. This used membrane was again placed in a cell filled with a fresh portion of solution, and set into pyridine. Such a membrane never developed as high a pressure as one cut from unused rubber. For example, a tenth-molecular solution of lithium chloride in pyridine gave a pressure of 81.7 cm the first time the membrane was used. This pressure fell to 68.0 cm in the course of several days. This same membrane with a fresh tenth-molecular lithium chloride solution gave 61.1 cm pressure which decreased to 39.9 cm in a little over a week. When used a third time this membrane gave a pressure of only 33.0 cm. I n all these experiments, only traces of the solute had escaped through the membrane into the outer liquid. These amounts were entirely insufficient to account for the differences in the pressures developed. It would seem too that the change in the
Aljred E. Koewig
474
rubber, once started, keeps up even when not in contact with the pyridine, for it will be noted that the maximum attained in each of the above experiments is lower than the lowest pressure attained in the previous experiment. This may account in part for the variations in the results of Cohen and Commelin, previously referred to, for these workers soaked their rubber for some time before it was used. Thus it would seem that the different pressures developed by the same membrane when used more than once, might be caused by an alteration in the nature of the rubber due to its exposure to the pyridine. The following set of experiments also support this view. These experiments were made with a 0.075 molecular solution of sugar in pyridine. The pressures are the actual differences of the mercury levels of the manometer. Experiment I .-The maximum pressure attained was 120.7 cm. After three days the pressure had dropped to 119.4cm. The contents of the cell were removed. Experiment 2.-The cell was provided with a fresh rubber membrane and filled with the solution removed from the cell in Experiment I , and the same outer liquid was used. The maximum pressure attained was 120.0 cm and this had decreased to 116.4cm in five days after reaching the maximum. Experiment 3.-The cell was provided with a fresh membrane and filled with the sugar solution used in Experiments I and 2 and the same outer liquid used in these two experiments placed on the outside. This time the pressure developed was 119.1cm which decreased to 113.5 cm in the course of five days. The same cell was used for these three experiments and all conditions were, as nearly as possible, identical. After having been used three times, the outer liquid contained only a trace of sugar. Severa1,series of experiments of this kind were carried on with similar results. This would indicate that the decrease in pressure was due to an alteration in the nature of the membrane and not as Cohenl says, to a gradual accumulation of moisture in the outer liquid. 1
LOC.cit.
Osmotic Action o j Solutiom o j Cane Sugar, Etc.
T. p. in cm Cone. of mercury
--
l-
475
Pressures, expressed in cm of mercury, found for solutions of: Cane sugar
I
I
I
1 I
Silver nitrate 1
I
I
Lithium chloride
I
1
I
1 - - - - - 1'79.2173.5149.9 o . 1 j 0 2 7 8 . 8 8 2 5 3 . 0 2 5 1 . 6 2 5 1 . o 2 4 2 . o 2 4 2 . o 2 2 3 . 5117.1 116.6 0.200371.84
-
0 . 1 2 5 2 3 2 . 4 0 2 1 3 . 4 2 1 2 . 6 2 1 2 . 0 1 7 0 . 5 1 6 j . 2 164.0 -
-
0.100185.92 191.9186.5 185.1 138.5 137.4132.0 0.075 139.44119.9119.2 118.3 1 1 7 . 7 115.4108.5 - 0 . 0 5 0 92.96 6 0 . 3 59.0 j 8 . 9 - 0 . 0 2 5 46.48 34.5 2 5 . 8 24.3 -
8 0 . 9 80.8 75.8 46.9 45.7
82.9 76.7 58.5 17.4
-
-
-
Alfred E. Koevzig
476
184.6, 181.7, 179.6, 177.5, 176.3, 174.6, 173.5, 167.1, 162.7. Some of these results are also set forth in the curves in Fig. 4.
Discussion of the Results The osmotic pressures of the solutions of sugar in pyridine come the nearest to those demanded by the gas laws. I n fact, in the case of the tenth-molecular solution the pressures are a little higher than the theoretical. Solutions of lower and higher concentration than the tenth-molecular yielded pressures lower than required by the theory. Molecularweight determinations by Wilcoxl show that sugar dissolved in pyridine has nearly a normal molecular weight. Hence, it should have a normal osmotic pressure, provided the gas laws hold. The curve in Fig. 4 seems to have a break a t or near the tenth-molecular solution. The specific gravity, the specific rotation, and the molecular-weight determinations for these solutions show no such change at this concentration. 1
Jour. Phys. Chem., 5, 598 (1901).
Osmotic Action of Solutions of Cane Sugar, Etc.
477
The conductivity measurements for solutions of silver nitrate in pyridine have been made by Sakhanov, Anderson, and others. Sakhanov concludes that a tenth-molecular solution is about I 8 percent electrolytically dissociated. Walden and Centnerszwer3 found that solutions of this strength gave normal molecular weights for the silver nitrate by the boiling-point method. Schmuilow4from this molecular-weight determination concludes the salt is undissociated and un-ionized in pyridine. Sakhanov offers the usual explanation for the apparent I 8 percent dissociation and the normal molecular weight by saying that the silver nitrate is polymerized just enough to offset the effect of the electrolytic dissociation. The osmotic pressures found in the present investigation are much lower than those demanded by the theory even if the molecular weight were normal. Walden and Centnerszwer5 claim that the polymerization of the silver nitrate increases with the increase of concentration. Then the osmotic pressure ought to increase less rapidly as the concentration becomes greater. The reverse, however, seems to be the case for it increases more rapidly with the concentration. Lithium chloride according to Sakhanov6 and others, yields poorly conducting solutions in pyridine and apparently shows normal molecular weights by the boiling-point method. The osmotic pressures determined in this investigation are all much lower than the theoretical for an undissociated substance. The curves in Fig. 4 show the relation of the pressures found to the theoretical values. In plotting these curves, the highest values obtained for each concentration were used. Now, if a different membrane were used, it would seem to be possible to obtain an entirely different set of results for the osmotic pressures of these same solutions. Consequently Jour. Phys. Chem., 21, 188 (1917). Ibid., 19,753 (1915). Zeit. phys. Chem., 5 5 , 3 2 1 (1906). Zeit. anorg. Chem., 15, 18 (1897). LOC.cit. e Ibid.
,
Alfred E. Koenig
478
attempts like those of van I,aar,l to reconcile the results of Cohen to the gas laws by means of mathematical calculations, become quite meaningless. This possibility of obtaining different sets of results for the same solution when different membranes are used would be in accord with the chemical or selective theory of osmotic action as maintained by Kahlenberg in explanation of the researches he has carried out. Kahlenberg2 says: “Whether osmosis takes place in a given case or not depends on the specific nature of the septum and the liquids that bathe it, and if osmosis does occur, these factors also determine the direction of the main current and the magnitude of the pressure developed.”
Conclusions An improved cell for the measurement of osmotic pressures with flexible sheet membranes, such as dental rubber, has been described. 2 . The osmotic pressures, for solutions of sugar, silver nitrate, and lithium chloride in pyridine when separated from pyridine by rubber membranes, have been determined. The pressures for the sugar solutions were near those demanded by the gas laws but those for the silver nitrate and lithium chloride solutions were found to be much lower than those for the sugar solutions of corresponding molecular concentrations. 3. It was found that, after having reached a maximum, the pressure gradually decreased. This seemed to be due to an alteration in the nature of the rubber owing to its contact with the pyridine. Membranes used a number of times, with fresh solutions, did not give as high pressures as the unused rubber. The same solution used several times, each time with a fresh membrane, yielded practically the same pressure, This would lead to the conclusion, previously stated by Kahlenberg, that the magnitude of the pressure developed by a given I.
Zeit. phys. Chem., 64, 629 (1908). Jour. Phys. Chem., IO, 141 (1906).
Osmotic Actiovt of Salutiovts of Cane Sugar, Etc.
479
solution depends upon the nature of the semipermeable membrane used. The author wishes to take this opportunity to express his indebtedness to Professor Kahlenberg, a t whose suggestion this work was undertaken and whose interest and encouragement made its development possible. Laboratory of Physical Chemistry University of Wisconsin Madison, April, 1918
