PHYSICO-CHEMICAL INTRODUCTION 



21 



up a lessened amount of oxygen compared with water, it is capable 

 of taking up far more, the absorption of oxygen of blood as a 

 whole not being a purely physical, but mainly a chemical, process. 

 Besides salts, acids, bases, and soluble substances like cane-sugar, 

 have a similar effect in lowering the solubility of gases. Recent 

 study has demonstrated the fact that the relative effect on the 

 solubility of gases of different salts is nearly independent of the 

 gas employed. Therefore the diminished solvent power of a salt 

 solution as compared to water is mainly determined, 

 not by the nature of the gas, but by some factor in 

 the relationship of the water to the salt. It has also 

 been suggested that the lowered solvent power is due 

 to hydration of the dissolved salt, and thus some of 

 the water is no longer free to absorb the gas. 



The process of diffusion is not confined to gases. 

 Solutions exhibit the phenomenon of diffusion. If 

 water be carefully added to a strong blue solution of 

 copper sulphate with as little mixing as possible, a 

 process of diffusion begins which does not cease until 

 the salt concentration is the same throughout the 

 liquid, and the whole liquid coloured blue. Naturally 

 the process takes time, the rate of movement being 

 much less than in the case of gaseous diffusion. By 

 placing a graduated tube on top of the vessel the 

 rate of diffusion may be roughly measured (Fig. 13). 

 Just as with a gas, the movement may be regarded 

 as due to the pressure of the dissolved substance ; the 

 molecules are said to be driven from a place of high 

 concentration to one of low concentration under the 

 influence of osmotic pressure. 



To measure such pressure it is necessary to have a 

 membrane which will allow free passage to the solvent, 

 but not to the substance; it must be "semi-perme- 

 able," cs it is termed. Then with pure water on one 

 side of the membrane, and a watery solution of a sub- 

 stance e.g., sugar on the other, since diffusion of 

 the dissolved substance is barred, the system seeks 

 to get into equilibrium, as far as possible, by water 

 passing through the membrane to the solution. Such 

 a membrane may be formed by filling a porous vessel 

 of unglazed porcelain, which has been well soaked 

 in water, with a solution of copper sulphate (2-5 grains per litre), 

 and introducing it into a solution of potassium ferrocyanide (2-1 

 grains per litre). The salts diffuse into the porcelain, and, meeting 

 in the interior, form a filmy deposit of copper ferrocyanide. After 

 standing a considerable time, the pot is taken out, thoroughly 

 washed, and soaked in water. Or a similar membrane may be formed 

 by taking a glass tube about 1 centimetre in diameter, one end of 

 which has been dipped in gelatin to which a little potassium dichro- 



FIG. 13. To 

 DEMONSTRATE 

 DIFFUSION IN 

 LIQUIDS. 



The flask b and 

 the superim- 

 posed gradu- 

 ated tube are 

 filled with 

 water. Some 

 crystals of cop- 

 per sulphate 

 are intro- 

 duced. The 

 diffusion of the 

 blue salt can 

 be seen and the 

 rate measured 

 in the cali- 

 brated tube. 



