HETEROGENEOUS EQUILIBRIA 279 



The simple explanation offered above must obviously not be pressed 

 too far, particularly in the more concentrated solutions. On the addi- 

 tion of a salt of the type MY 2 , there is a possibility that complexes of 

 the form MXY may result. In all likelihood, however, at low concen- 

 trations, these are not present to a large extent. 



While solutions of highly soluble salts, as well as solutions of non- 

 electrolytes, exhibit a great variety of properties which bring out clearly 

 the individual characteristics of the various substances involved, in solu- 

 tions of difficultly soluble salts, the solubility curves show remarkable 

 regularities, indicating that the observed behavior of these solutions lies 

 in properties common to electrolytes in general, at these concentrations. 

 The solubility effects are readily explained on the assumption that the 

 concentration of the un-ionized fraction, as well as the ion product, 

 remains substantially constant on the addition of a second electrolyte. 

 The great decrease in the concentration of the un-ionized fraction, which 

 many investigators have assumed to be correct, is doubtful. It appears 

 probable that this result follows from a failure of the applicability of 

 the isohydric principle to mixtures of electrolytes. The solubility in- 

 crease observed in the case of salts of higher type on the addition of 

 salts with a common polyvalent ion makes it appear probable that 

 intermediate ions are present in relatively large amounts in solutions of 

 salts of higher type at higher concentrations. 



Heterogeneous equilibria from a thermodynamic point of view will 

 be discussed in another chapter. 



