HETEROGENEOUS EQUILIBRIA 267 



employed. On examination of the figure, it will be seen that, up to a 

 concentration of 0.1 N of added salt, the points lie very nearly upon a 

 straight line, and, furthermore, that the solubility values due to the 

 addition of different electrolytes conform very nearly to the same straight 

 line. It cannot be said that Equation 70 actually holds for the mixture; 

 nevertheless, the effect of different electrolytes upon the solubility of 

 thallous chloride is much more uniform in character when treated in this 

 way than when treated according to the isohydric principle. Up to 

 0.1 N concentration of added salt, the solubilities differ only a few per 

 cent from the. linear relation. 



The conclusion to be drawn, however, is not so much that the ion 

 product and the concentration of the un-ionized fraction as calculated 

 according to Equation 52 remain constant for a salt in equilibrium with 

 its solution as that the values obtained for the concentrations of the 

 various molecular species present in the mixture depend upon the law 

 assumed to govern the equilibrium in the mixture. The conclusion 

 reached by many writers, that the concentration of the un-ionized frac- 

 tion decreases greatly with increasing concentration of the added electro- 

 lyte, 29 is a consequence of the assumption of the isohydric principle as 

 a basis for calculating the concentrations of the various molecular species 

 present. As was shown by Bray and Hunt, 30 the specific conductances 

 of mixtures of sodium chloride and hydrochloric acid, calculated on the 

 basis of the isohydric principle, are throughout greater than the measured 

 ones. It follows, therefore, that the concentrations of the ions as calcu- 

 lated according to this assumption are greater than the true ones. Con- 

 sequently, the concentration of the un-ionized fraction, which is obtained 

 by difference, is obviously found too low. It is not probable that the 

 concentration of the un-ionized fraction of an electrolyte in equilibrium 

 with its solutions will be entirely unaffected by the addition of other 

 electrolytes, since, as we have seen in a preceding section, the solubility 

 of non-electrolytes is influenced by the addition of electrolytes. We 

 might expect, however, that the change in the concentration of the un- 

 ionized fraction would not differ greatly from that of non-electrolytes 

 under similar conditions. This conclusion is further borne out by the 

 results of Kendall 31 on the solubility of organic acids in the presence of 

 other acids. 



In the case of salts which are more soluble, the effect of a second 

 electrolyte upon the solubility is, in general, much smaller and, in some 



Noyes, J. Am. Chem. 8oc. 33, 1643 (1911) ; Stieglitz, ibid.. 30, 946 (1908) : Arrhenius. 

 Ztachr. f. phys. Chem. 31, 224 (1899). 



80 Bray and Hunt, J. Am. Chem. Soc. 33, 781 (1911). 

 "Kendall, loc. cit. 



