The Properties of Dilute Solutions

Van't Hoff showed that the depression of the freezing point is, like the elevation of the boiling point, proportional to the num ber of dissolved molecules. He obtained for the molecular de pression of the freezing point, i.e., the depression produced by one gram-molecule of a solute in i,000 grams of solvent, an ex pression similar to that giving the molecular elevation, viz., A T=0.002 where Lf is the heat absorbed in the fusion of one gram of solvent and T the absolute temperature of its freez ing point. Table III. gives the molecular depressions of a few typical solvents, as calculated in this way.

The researches of Raoult, Beckmann and others showed that, whereas many substances gave rise to molecular depressions which agreed well with the calculated values, others behaved abnormally. Thus in aqueous solutions, the depressions produced by non electrolytes are in good agreement with the calculated values, but those of strong electrolytes are considerably greater (see Table VI.) In benzene solutions, substances of a similar nature give rise to the normal depressions, but substances of a polar nature, e.g., the alcohols, give smaller values.

Osmotic Pressures.

In 1748 the Abbe Nollet published the results of his experiments on the diffusion of substances through animal membranes. He found that if a vessel, f ull of spirits of wine, is closed with a bladder and immersed in water, the bladder expands and a considerable pressure, which may ultimately burst the bladder, is set up inside. Further experiments of this kind were made from time to time, but it was not until 1867 that Moritz Traube showed that similar effects could be obtained with artificially prepared membranes. W. Pfeffer in 1877 was the first to make measurements of the pressures set up. His membrane consisted of a deposit of copper ferro-cyanide, formed in the walls of a porous clay vessel by the interaction of solutions of copper sulphate and potassium ferro-cyanide. This membrane allows water to pass through it freely, but prevents the passage of dissolved substances such as cane sugar. Pfeffer's apparatus consisted (fig. 1) of the prepared porous pot, completely filled with a sugar solution and attached to a mercury manometer or pressure gauge. When the pot was immersed in water, the latter tended to diffuse through the walls into the sugar solution so as to dilute it, but the sugar was unable to diffuse outwards. The volume of sugar solution thus tended to increase (which forced the mercury up the manometer tube), giving rise to a pressure inside the cell which increased until it was sufficient to prevent any more water diffusing into the solution. The hydrostatic pres sure inside the cell when equilibrium is reached is the osmotic pressure of the solution.

In 1885 van't Hoff pointed out that, according to Pfeffer's measurements, dilute solutions obey the laws of gases. The osmotic pressure produced by a substance in solution is the same as the gas pressure which would be exerted if the same number of molecules were present in the same volume in the gaseous state. On this basis he built up a comprehensive theory of dilute solutions, and, making use of the laws of energy, derived the foregoing relations for the boiling points and freez ing points of solutions. On ac count of this analogy between gases and solutions, the osmotic pressure has frequently been ascribed to the bombardment of the membrane by solute mole cules which are unable to pass through it, and exert on it the same pressure as if they were present in the same space in the gaseous state. Its true origin, however, appears to be the tendency of the solvent to dif fuse into the solution. When the two liquids are separated by a membrane which the solute molecules cannot pass, and are at the same pressure, the solvent will flow through the membrane into the solution. The osmotic pressure is the pressure which must be applied to the solution to stop this flow. That it is equal to the equivalent gas pressure is a consequence of the inter-relations of the properties of solvent and solute which are necessitated by the laws of energy. Exact measurements of the osmotic pressures of solutions have been made by H. N. Morse and J. C. W. Frazer (1902-12) in America and by the earl of Berkeley and E. G. J. Hartley in England (1906-09). These in vestigators found that in dilute solutions the osmotic pressure agrees with the corresponding gas pressure within the limits of experimental error; in more concentrated solutions deviations occur similar to those in gases at high pressures.

A simple means of recognizing solutions having the same osmotic pressure was discovered in 1888 by the botanist, Hugo de Vries. He observed the behaviour of the cells of certain plants, particularly Tradescantia discolora, in aqueous solutions. These cells consist of a woody framework, lined by a membrane con taining a protoplasmic fluid which is permeable by water but not by dissolved substances. When such a cell is placed in a solution having the same osmotic pressure as that of the cell contents, it retains its normal appearance ; if the osmotic pressure of the solution is greater or less the cell membrane expands or shrinks. It is thus possible to identify solutions of different substances which have the same osmotic pressure; that is, which are isotonic. Saline solutions which are isotonic with human blood are em ployed in surgery to replace a serious loss of blood.