( 173 ) 



Vjj , the unknown dissociation constant Kn is found by the following 

 calculation: 



If in the solution of the standard acid we call the dissociated 

 part « then 



the concentration of the //-ions, therefore, 



Ch = 



Ka 



— 1 + I / [ — : L 1 



Ka Va ^ 



For the acid with an unknown dissociation constant Kb , we may 



calculate the same from 



as Cji and T^/j are known. It is, however, simpler to make the 

 calculation as follows : 

 From 



Ch 



Kb 



1 — 



K V and ^ = C 



we find : 



C'z= 



K 

 V 



CK. 



As both acids are isohydric in dilutions of, respectively, Va and 

 Vb, Ch will be the same in both, therefore 



from which 



Ki Kb 



-^ — Ch Ka = — ^ 

 Va Vb 



Ka = Kb 



CiiKn 



Va 1 - Ch Vb 

 Vb ' I- Ch Va 



Test ExperimeiNts. 



In order so show the accuracy of the process, I have made three 

 determinations. In the first one I have determined the dissociation 

 constant of benzoic acid, taking the constant of salicylic acid as 

 known. In the second experiment I have determined the dissociation 

 constant of anlhranilic acid, using the figure obtained for benzoic acid. 

 In a third experiment, the dissociation constant of propionic acid has 

 been also determined with the aid of benzoic acid. 



Detenninatioii of the dissociation constant of benzoic acid. 

 Given : 



/v, = 0.00102 vs= 150 



vi — 100 



12* 



