LAWS OF ELECTROLYTIC DISSOCIATION 55 



16. The dissociation of polybasic acids 



A dibasic acid may be defined as a molecule possessing two 

 ionogenic H-atoms. The dissociation of each of the two H-ions 

 must be considered separately. Thus oxalic acid is dibasic, but the 

 tendency to dissociate is different in the two H-ions. The acid 

 dissociates in two stages with each of these stages having a distinct 

 dissociation constant. If the acid radical remaining after the 

 removal of both H-atoms be designated as S and the whole acid 

 molecule as SH2, then the chemical equations representing the two 

 stages are: 



1st stage: SH2 -^ SH" + H+ 

 2nd stage: SH" -^ S" + H+ 



To each of these two reactions the mass law applies in the follow- 

 ing way: 



[SH-] [H^ 

 [SH2] 



[Si [H+] 



= ki 



[SH-] 



= k 



In order to arrive at a clear understanding of dissociation, the 

 conceptions of the degree of dissociation and of dissociation-residue 

 will be discussed also in this connection. Let the concentration of 

 undissociated acid be designated as [A], that of the primary ions 

 as [A~] and of the secondary ions as [A=], then ai the degree of 

 dissociation of the first stage may be defined as 



[A-] 



ai = 



[A] + [A-] + [Ai 

 and ai the degree of dissociation of the second step as: 



"' [A] + [A-] + [Ai 

 and the dissociation-residue, p, as 



A 



p = 



[A] + [Ai + [Ai 



