Chapter X. 



Heterogeneous Equilibria in Which Electrolytes 

 Are Involved. 



1. The Apparent Molecular Weight of Electrolytes in Aqueous Solu- 

 tion. If an electrolyte is dissolved in a solvent in equilibrium with a 

 second phase, the thermodynamic potential of the solvent is displaced, 

 and a displacement in equilibrium results. On the addition of an elec- 

 trolyte to water, therefore, we should expect a change in the solubility 

 of substances in this solvent; or, in case water itself appears as a second 

 phase, we should expect a displacement in the freezing point, boiling 

 point, etc. 



The earlier experiments on the freezing point of aqueous salt solutions 

 indicated a fairly close agreement between the ionization as determined 

 by conductance measurements and that as determined from freezing point 

 measurements. These data have been examined and collected by Noyes 

 and Falk. 1 In solutions of the binary salts the agreement is, on the 

 whole, fairly close in dilute solutions, although in the more concentrated 

 solutions deviations, which exceed possible experimental errors, make 

 their appearance. In solutions of potassium chloride the two methods 

 yield practically identical results up to concentrations as high as 0.1 

 normal. 



In order to calculate the molecular weight of a substance from the 

 freezing point of its solution, the laws governing the equilibrium in the 

 mixture must be known. Since the general case has been worked out 

 only for dilute solutions, it is obvious that the ionization of electrolytes, 

 and the molecular condition of substances in general, may not be deter- 

 mined from freezing point determinations at higher concentrations. 

 Washburn and Maclnnes 2 showed that, while the freezing point curve 

 for potassium chloride corresponds very nearly with that of a solution 

 of sugar in water up to fairly high concentrations, those for solutions of 

 lithium chloride and caesium nitrate exhibit deviations at fairly high 

 dilutions. The deviations in the case of the last named salts lie in oppo- 

 site directions from the theoretical curve of ideal solutions. They found, 



1 Noyes and Falk, J. Am. C hem. 8oc. S3, 1437 (1911). 

 'Washburn and Maclnnes, J. Am. Chem. Soc. 33, 1686 (1911). 



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