92 HYDROGEN ION CONCENTRATION 



where S~ is the concentration of the acid anions and S is that of the 

 total acid. 

 According to the mass law 



[S-] X [H+] 

 IS] - [S-] 



If in the above expression we substitute for the concentration 



g- 



[S~] its equivalent — , where v is the volume and S~ is the absolute 



molar amount of the anions, and by recalling that in a pure acid 

 solution [S~] = [H+], we obtain 



= k 



S - S- (S - S-) V (1 - a) V 



2. The second method depends upon the determination of the 

 degree of hydrolysis of the sodium salt of the weak base or the 

 chloride of the weak acid studied. The relation of this factor to k 

 follows obviously from the preceding chapter. The degree of 

 hydrolysis may be estimated by two methods: 



a. By determining the [H+] from the catalytic action on the 

 hydrolysis of sucrose, methyl acetate, etc. 



b. By the electrometric determination of the [H+]. This method 

 has been little employed. 



3. The third method depends upon the determination of [H+] 

 in a definite mixture of the acid with its Na-salt, for which purposes 

 various methods may be employed. The theoretical basis of this 

 procedure is given by formula (1) on page 44. 



Each of these methods has its own field of applicability. But 

 there are conditions under which either the experimental data 

 become uncertain, or, where the equations which are but approxuna- 

 tions applying within certain limits are no longer applicable. It is 

 beyond the scope of this book to deal with this question in greater 

 detail. Neither are all the possible methods exhausted with those 

 given above. Thus, for instance, a very good method for an acid 



