252 ILLINOIS STATE ACADEMY OF SCIENCE 



in question, then this simple form of the solubility law 

 does not express the true solubility. 



Hildebrand/ in a series of very able papers, has shown 

 that the degree to which a given binary mixture of non- 

 polar substances departs from the formula for ideal mix- 

 tures is closely related to the magnitude of the differ- 

 ences in internal pressures of the components. In the 

 fourth paper of the series, he has described a method 

 for evaluating solubility data, and has indicated how the 

 solubility of many substances may be approximately cal- 

 culated providing the solubility of the given substance 

 has been determined in solvents having a similar internal 

 pressure to that of the solvent in question. 



In evaluating solubility data Hildebrand plots the 

 common logarithm of the mole fraction of solute against 

 the reciprocal of the absolute temperature of the melting 

 point of the system. The experimental solubility points 

 when plotted in this manner should, if there are no sec- 

 ondary molecular effects, lie on a straight or only slight- 

 ly curved line over fairly wide ranges of temperature. 

 When the solubility curves of a given solute in a variety 

 of solvents are plotted in this way, there is obtained a 

 series of lines, which converge to a point at the melting 

 temperature of the solute where N = 1.0(Log N = 0.0) 



According to the hypothesis put forward by Hilde- 

 brand, the nearer the internal pressures of the liquified 

 solute is to that of the solvent in question, the nearer will 

 the experimental curve approach to the ideal solubility 

 curve calculated from the latent heat of fusion of the 

 solute. Therefore, if two solvents should be found to 

 have exactly the same internal pr.essures, then the mole- 

 cular solubility of each solute should be the same for the 

 two solvents. Hildebrand' has prepared a table of rela- 

 tive internal pressures from which, having a series of 

 solubility curves for each solute, the solubility curve of 

 any such solute may be located approximately for any 

 other solvent, the position of which, in the table of rela- 

 tive internal pressures, is known. 



The obvious disadvantages of this method of calculat- 

 ing solubilities are: first, the internal pressures are 

 known for only a relatively small number of substances. 



