HETEROGENEOUS EQUILIBRIA 



275 



the effect of the intermediate ion, but, in addition, the concentration of 

 the intermediate ion cannot be determined with any degree of certainty, 

 even in solutions in pure water. Nevertheless, as Harkins has pointed 

 out, the solubility curves may be accounted for in a general way on the 

 assumption that intermediate ions are present in solutions of electrolytes 

 of higher type. 



0,0/Z 



ao o-t o* 0.3 0.4 of 0.6 0.7 o* o>9 



Concentration of added salt in equivalents per liter. 



FIG. 56. Solubility of Lanthanum lodate in Water in the Presence of Other 



Electrolytes. 



It will be sufficient to consider, here, the solubility of a ternary elec- 

 trolyte of the type MX 2 , which ionizes according to the equation: 



As we have already seen in connection with the solubility of binary elec- 

 trolytes in the presence of other electrolytes, the experimental results 

 in the case of fairly dilute solutions are. in reasonably good agreement 

 with the assumption that the concentration of the un-ionized fraction 

 of the salt, as well as the ion product, remains constant on the addition 

 of other electrolytes. If a similar assumption is made in the case of a 

 ternary electrolyte, it leads to the following equations for the solubility 

 of the salt in the presence of an electrolyte with a common univalent ion, 

 a common divalent ion, and without a common ion. 

 With a common univalent ion, 



(71) 



S- 



- 



where K is the ionization constant of the reaction given above. In this 

 equation the solubility appears as an explicit function of the concentra- 



