THEORIES RELATING TO ELECTROLYTIC SOLUTIONS 335 



As may be seen by reference to Table CXXX, the activity coefficients 

 of electrolytes of the same type do not differ greatly at low concentra- 

 tions. The activity coefficient increases as the concentration decreases 

 at low concentrations. In solutions of strong acids and bases and of the 

 alkali metal chlorides, the activity coefficients pass through a minimum 

 at high concentrations. This, however, does not appear to be a general 

 property of electrolytes, since silver nitrate does not exhibit a minimum 

 up to concentrations of 5 molal. The higher the type of salt, the more 

 rapidly does the activity coefficient decrease with increasing concentra- 

 tion. While, as a rule, salts of the same type exhibit the same values 

 of the activity coefficients, a number of exceptions occur, such as cadmium 

 chloride, whose activity coefficient at 0.1 M is 0.219, while that of barium 

 chloride at the same concentration is 0.50. Aqueous solutions of electro- 

 lytes are remarkable for the uniformity of the phenomena presented. 

 At low concentrations, many properties of these solutions differ only in- 

 appreciably for different electrolytes of the same type. The same rela- 

 tion is found in the case of the activity coefficients. At higher concen- 

 trations, however, different electrolytes exhibit considerable variations. 



At low concentrations, the values of the reduced activity coefficients 



-fr- approach those of the ionization coefficient y = T-. The significance 

 c s A 



of this result is uncertain, since, even at the lowest concentrations, aqueous 

 solutions of strong electrolytes do not conform to the requirements of 

 the law of mass action. 



A comparison of the activity coefficients of solutions of pure electro- 

 lytes with those of mixtures cannot be effected without some further 

 assumption. A priori, we should not expect the activity coefficient of a 

 given salt in a mixture of electrolytes to correspond closely with that 

 in a solution of the pure substance. From Harned's measurements on 

 the electromotive force of concentration cells with mixe'd electrolytes, 

 Lewis and Randall draw the conclusion that "in any dilute solution of a 

 mixture of strong electrolytes, of the same valence type, the activity 

 coefficient of each electrolyte depends solely upon the total concentra- 

 tion." Where the mixture contains salts of different valence types, they 

 have introduced a new concentration function defined by the equation: 



+... 



2 



where C s is the total molal concentration of an ionic constituent in the 

 solution and \j is the number of charges which it carries. This quantity, 



