734 HARMED ART. M 



and the ionization constant by 



K = ''-^^^ "^'^^^ = y.' ^^^ = 7x^ K., (53) 

 THAc whac w — mn 



where m is the original concentration of the weak acid, and 7^ its 

 activity coefficient in the salt solution. Since we determine 

 /wh, Kc becomes known at various salt concentrations. We 

 have yet to find its value at infinite dilution or when 7^ equals 

 unity. This can be done very simply by the use of a function 

 which gives the variation of 7 with the total ionic concentration, 

 li, in dilute solutions; namely, 



logio 7^^ = - Vm + a/^, (54) 



where a is an empirical constant. If we take the logarithm of 

 equation (53) , we obtain 



logio K = logio Kc + logio 7x^ (55) 



Substituting for logio 7x^ and rearranging terms, we find that 

 logio Kc — \/ n = logio K — an. (56) 



Therefore, if we plot [logic Kc — \/ m]) which has been determined 

 against /j., we obtain a straight line in dilute solutions, and the 

 value of the function on the left is equal to logio K when /x equals 

 zero. By this means we have an independent measure of the 

 dissociation constant, the ionic activity coefficient, and dissocia- 

 tion of a weak acid in a salt solution. The same or very similar 

 methods will also afford very valuable evidence concerning 

 similar properties of weak bases, and ampholytes.* 



These considerations, although very brief, serve to show the 

 extent and power of the method of cell measurements when 

 applied to the study of all kinds of electrolytes. It would be 

 far beyond the scope of this discussion to treat the various 



* A thorough discussion of this subject is to be found in the contribu- 

 tions of: Harned and Robinson, /. Am. Chem. Soc, 50, 3157 (1928); 

 Harned and Owen, ibid., 52, 5079 (1930); 52, 5091 (1930); Owen, ibid., 

 64, 1758 (1932); Harned and Ehlers, ibid., 54, 1350 (1932). 



