PROPYL ALCOHOL SOLUTTONS ANT) SEMI-PERRIEABTJE MFhTBRAWES 819

LXXXI 11.-Behaviour OJ Solutioias of Propyl AlcoJzol t o w a d s Semi-permeable Menabmnes.

By ALEXANDER FINDLAY and FREDERICK CHARLES SHORT.

THE problem of the nature of solution is one which has during the past twenty years claimed the attention of many chemists and physicists, and the phenomena of osmosis and the behaviour of semi- permeable membranes have entered largely into its discussion. By reason of the importance of these phenomena and their bearing on tbe fundamental nature of solution, the crucial experiment carried out by S. U. Pickering, and described by him in the following manner, seemed endowed with special interest. Professor Pickering states (Bey., 1891, 24, 3639) : “Eine Lijsung von Alkohol in Wasser (57 p. Ct.) wurde in ein poroses GefGss gebracht und dieses in Wasser gestellt; es zeigte sich, dass das Wasser in die Losung eintrat. Das porose Gefass mit der Lijsung wurde darauf in Alkohol getaucht und ich fand, dass der Alkohol in die Losung eindrang. Das porke Gefass war also sowohl fur Wasser als auch fur Alkohol durchlassig, nicht aber f ur eine Mischung beider Flussigkeiten ; eine solche 3lischung muss daher aus grosseren Rlolekulen bestehen als jede der reinen Flussigkeiten, und diesem Umstande muss die Existenz des osmotischen Druckes zngeschrieben werden, nicht aber der Un- durchliissigkeit des Diaphragmas f iir die geliiste Snbstanz, wiq man bisher angenommen ha;. Ich habe bis jetzt nur einige wenige vorliiufige Versuahe angestellt, sie lassen aber keinen Zmeifel uber die in Kede stehende Thatsache.”

I n the passage just quoted, it is stated that alcohol (by which one would understand ethyl alcohol) was employed ; but Professor Picker- ing has elsewhere mentioned (for example, Nature, 1897, 55, 224) that the alcohol used was propyl alcohol.

Although various allusions to the above experiments are to bo found in the literature, no further details are given as to the way in which the experiments were carried out or as to the height to which the liquid rose in the manometer (if one was used), nor is it stated whether the rise was a temporary or a permanent one. The different references made by Pickering to his experiments, also, vary with regard to whether a simple porous pot or a semi-permeable membrane was employed.” Such being the case, the importance attaching to these results made it desirable to repeat the experiments, as far as possible, especially with the view of discovering whether similar

++ Professor Pickering has explained to me in a letter that EL porous pot only was used, the porosity of which was, however, very small.-A. F.

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820 FINDLAY AND SHORT: BEHAVIOUR OF SOLUTIONS OF

results could be obtained with other alcohols or other substances. Although, in absence of all experimental details, we cannot be sure that the conditions under which we worked were the same as those under which Pickering’s experiments were carried out, we can probably assume that our experiments were at least carried out in a similar manner, and might have been expected to yield similar results. It may, however, be a t once stated that, working as we did, we entirely failed to obtain any confirmation of the anomalous behaviour described by Pickering.

For our first experiments, which were repeated at a later date with a different pot, we also made use of a porous pot without semi- permeable membrane. The pots used were ordinary cylindrical, white clay pots, such as are sold for battery purposes, and had a capacity of about 100 C.C. The pots were not specially prepared, except tha t they were soaked in distilled water, and the air removed from the pores by reduction of pressure. After the pot had been roughly dried by means of filter paper, it was filled with a solution of propyl alcohol (Kahlbaum’s ‘‘ commercial ”) containing 57 per cent. of alcohol by volume. It was then furnished with a tightly-fitting india-rubber bung and an open manometer tube having a bore of about 2 mm. When the pot with the solution was placed in pure water or in the commercial propyl alcohol, the liquid in the manometer tube, which stood initially from one to two feet above the top of the pot, fell in each case ; no indication even of a temporary rise was obtained, except, indeed, in one case when air had accidentally got inside the pot.

As the experiments with porous pots alone gave entirely negative results, it was decided to employ a semi-permeable membrane, espe- cially as Pickering has himself stated that a better result might in that case be expected. As semi-permeable membrane we used copper ferrocyanide, which, as Tammann’s experiments would show ( 7Viede- mann’s Annalen, 1888, 34, 299), is semi-permeable to an aqueous solution of propyl alcohol.

A series of experiments was first of all carried out in duplicate, using two pots of about 170 C.C. capacity, furnished with a copper ferrocyanide membrane, prepared some time previously according to the directions given by Pfeffer. The air was removed from the pores, as described above, and the pots then filled with a solution of propyl alcohol, furnished with a manometer tube and placed in a beaker containing distilled water and standing in a thermostat at 25’. Although solutions of propyl alcohol of different concentration were employed, namely, 10, 30, 70, and 90 per cent. of alcohol by volume, the result was the same in each case ; when the semi-permeable mem- brane was surrounded by water, the liquid in the manometer rose, and continued to do so until it reached the top of the tube-a rise of more

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PROPYL ALCOHOL TOWARDS SEMI-~ERMEABLE MEMBRANES. 821

than a metre. The pot was generally allowed to stand in water for at least twenty-four hours, sometimes for several days, during which time the liquid continued to overflow from the open end of the manometer tube. The rise of liquid, therefore, did not seem to be a temporary one. The pot was then removed from the water and placed in propyl alcohol. When this was done the liquid in the manometer began to fall almost immediately, and after several hours fell to the bottom of the manometer, or even retreated within the pot. This fall was in no may due to any leak past the cork or manometer tube, since, when the pot mas replaced in water, the liquid again rose in the manometer ; and the rise and fall could be obtained repeatedly by placing the pot alternately in water and propyl alcohol.

Although exactly the same results ware obtained in the duplicate experiments, a similar series of experiments was also carried out with a third smaller pot which had been furnished with a freshly-prepared copper ferrocyanide membrane. The same results were obtained as in the preceding experiments. Special experiments were also carried out with a 75 per cent, solution of propyl alcohol, but with no different result,

As will be seen, our restilts are in no way in harmony with those obtained by Pickering; nor can we, in consequence of the uncertainty as to the precise conditions under which thoso results were obtained, offer any definite explanation of the discrepancy.

Apart, however, from our failure to reproduce Pickering’s results, it is somewhat difficult t o find a theoretical basis for the possibility of such results. Assuming that the rise of liquid which was obtained by Yickering was only a temporary one, an explanation of the behaviour might be found i n differences in the velocity of diffusion of the water and propyl alcohol. The discrepancy between the results obtained by Pickering and by ourselves might, then, be due to differences in the porosity of the pots. This appears to us to be much more plausible than the explanation given by Pickering, which assumes the forma- tion of a molecule so large as to be incapable of passing through the pores of the pot. The comparison of a semi-permeable membrane with a sieve may be a legitimate one 60 long as the membrane is semi- permeable and its “pores” are of molecular dimensions; but it is difficult to imagine that this can be the case with what one ordinarily terms a porous membrane, the pores of which are not of molecular size.

Another direction in which one might look for an explanation of Pickering’s results is in the phenomena of capillarity. If the walls of the porous pot are regarded as consisting of a large number of very minute capillary tubes, a rise of liquid in the pot might be due to surface tension effects. If the surface tension of the liquid inside the pot is greater than that outside, pasjage of liquid from the outside to

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822 CHIKASHIGB : OXYMERCURIC PEkCHLORATSS AND THE

the inside will take place, and a rise will therefore be obtained in the manometer tube, This might account for the rise in the case where the pot was surrounded with propyl alcohol, the surface tension of which is less than that of the solution, but it would not account for the rise in the case when water was outside.

We are forced, therefore, to conclude that the anomalous behaviour of solutions of pi-opyl alcohol in water described by Pickeriug, unless confirmed by other experiments, is to be accounted for most probably (if the possibility of experimental error is excluded) by differences in the velocity of diffusion. If this be so, the rise of liquid in the pot could only have been temporary, and the experiment would hence lose all its significance fo r the problem of solution (compare Proc., 1905, 21, 170).

CHEMICAL DEPARLMIENT, UNIVERSITY OF BIRMINGHAM.

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