LAWS OF ELECTROLYTIC DISSOCIATION 



59 



which tells us that ai is at its maximum when [H+] is equal to the 

 geometric mean of the two dissociation constants, or when 



pH = 



pki + pka 



The relation of this maximal value becomes evident when it is 

 introduced into the equation for ai 



ai max = 



1 + 2 



VE 



TABLE 12 

 Table of dissociation constants of polybasic acids 



VI 



When 2W— is very much smaller than unity, then the maximal 



value of ai becomes practically = 1 and at the same time the maxi- 

 mum of the curve is broad and fiat, while in other cases a more 

 pointed maximum is obtained, and its value is less than 1. 



When k2 is much smaller than ki (fig. 5) the ascending and de- 

 scending portions of ai represent a part of the dissociation curve 

 and of the dissociation-residue curve as well, and the projections 

 of the two 0.5 ordinates on the abscissa give the values of pki and 

 pk2. Both of these points are marked by circles on the curve in 

 figure 5. 



