DISSOCIATION OF ACIDS AND BASES 145 



Consequently 



^ - • ' -^ 



— = a — a = A = p 



kb _ ka\ 

 oh' h) 



a 



B}^ substituting the value of p from (7) p. 142. 



kb • ^^' ka 



kw h' , „, 



^ = \ , ■ . .,. ^^ rrr (10) 



kb • h- / i' \ ka / i' \ 



In the special case, when no true salt-formation occurs, i.e., when 

 either no extraneous ions are present, or when their affinity for the 

 ampholyte ions is infinitely small (when i" = i^ = 0, or ki = ku = co ) 

 expression (10) becomes 



kb ka 



^ oh' h 



The isoelectric point is, therefore, that [H+] at which X = 0. 

 Hence, from the general expression for X as well as for the special 

 expression for Xo we derive the same value for the isoelectric point 

 as we did originally. 



LP. - y^i K 



This means that the isoelectric point is not deflected by true salt-forma- 

 tion, which is a very remarkable conclusion. And since it has just 

 been demonstrated that the value of pmax is deflected, it follows that 

 in the case of true salt-formation the values of pmax and I. P. are not 

 identical. 



42. The extension of the conception of the reduced dissociation 



constant 



The buffer equation in its last interpretation stated that in the 

 presence of an excess of neutral salt ((4), page 129). 



[H-.] = k' f^'"^ ^'^^^ 



[salt of the acid] 



