142 HYDROGEN ION CONCENTRATION 



a' 



"T = a' — the degree of dissociation of the anions 



SI 



~ = ffi — the degree of dissociation of the cation salt 



Sll 



~ = (7ii — the degree of dissociation of the anion salt 



The following equilibrium conditions obtain: 



a'.h' = ka.a (1) 



a*. oh' = kb.a (2) 



a'.i' = ki.s, (3) 



a".i' = kii.Sn (4) 



a + a' + a' + Si + Si = a (5) 



h'.oh' = kw. (6) 



a 



Hence for p = - 

 a 



1 



p = - 



^ h- ( —\ - 



kw \ kj^y h 



i"Q 



(7) 



The p-curve is formally the same as that without salt-formation, 

 except that its parameters have a slightly different meaning. The 

 path of the curve is such as it would be if the two dissociation con- 

 stants of the ampholyte were increased by the presence of the electro- 

 lyte in excess (just as in the case of the p-curve of a simple acid or 

 base) . 



A further analytic study yields for the [H+] (or h) at the maximum 

 value of p 



V 



[H+] at pn,ax = l~-k^ -^ ^i^ (8) 



kb ^ / i' ^ 



+ ^. 



This value should be compared with that for h at pmax given in 

 (1) page 65. And for p^ax itself we find 



1 



Pmax — 



1 + 



v^t-3(-a 



(9) 



