Osmotic pressure / by Alexander Findlay.

  • Alexander Findlay
Date:
1913
Catalogue details

Licence: In copyright

Credit: Osmotic pressure / by Alexander Findlay. Source: Wellcome Collection.

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    atmospheres), it must be remembered that the property of semi- permeability is a specific and not an universal one. Early Quantitative Measurements of Osmotic Pressure. Although the discovery of semi-permeable precipitation mem- branes by M. Traube made it possible, in principle, to determine quantitatively the value of the osmotic pressure of a solution, a practical difficulty had still to be overcome. The membranes obtained by Traube were much too fragile to withstand the pressures produced with any but the most dilute solutions, and were, indeed, employed by Traube merely for comparative measurements. To overcome this practical difficulty, W. Pfeffer18 introduced the device of forming the membrane in the wall of porous earthenware pots, whereby measurements of osmotic pressure amounting to several atmospheres were rendered practicable. The apparatus used by Pfeffer was of a simple character. It consisted of a small cylindrical pot of porous ware in the walls of which a precipitate of copper ferrocyanide was allowed to form by diffusion of solutions of copper sulphate and potassium ferro- cyanide from opposite sides of the cell walls. By means of a suit- able collar this pot was connected with a closed manometer ; and a tube was also provided to allow of the cell being filled with the solution, the osmotic pressure of which was to be determined. With this apparatus Pfeffer carried out a considerable number of osmotic pressure determinations, and although a number of pre- cautions which have since been found to be necessary to secure accuracy, were not adopted, these measurements were the first, and for a long time remained the only quantitative determinations of osmotic pressure. As such they played a most important part in the history of solutions. The results of the classical experiments of Pfeffer are familiar to all, and the following values of the osmotic pressure of solu- tions of cane sugar are given merely for convenience of reference. TABLE I.—Osmotic Pressure of Solutions of Cane Sugar at Tempera- tures between i3’2° and l6*I°. Grams of Cane Sugar Pressure in Pressure in ioo c.c. of Water. cm. of Mercury. Concentration. 1 53’8 53'8 2 ioi*6 50*8 4 208*2 52*1 6 307*5 51*3
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