LAWS OF ELECTROLYTIC DISSOCIATION 87 



of the solubility is made on the approximate assumption that the 

 salts of the slightly soluble acids or bases are totally dissociated. 

 As soon as the presence of appreciable amounts of undissociated 

 salts is taken into account, then certain corrections must be intro- 

 duced, which will be discussed in a later chapter dealing with strong 

 electrolytes and the ionic activity theory. All of the above deduc- 

 tions apply only to such cases in which the concentration of the 

 undissociated electrolytes in the solution in question remains quite 

 small. 



46 



24. The hydrolysis of salts 



It has been assumed throughout above that all salts are completely 

 electrolytically dissociated. But the salts have also another form 

 of dissociation. Since a salt is produced by the general reaction 



Acid + base ;=i salt + H2O 



we must assume, in accord with the theory of equilibrium, that 

 this reaction also takes place in the opposite direction, i.e., that the 

 salt partially "dissociates hydrolytically" or "hydrolyses" to form 

 acid and base. It is obviously imperative to establish the degree 

 or extent of this hydrolysis. 



First case. The salt of a strong acid and a strong base. If the 

 acid be designated as AH, the base as BOH and the salt as C, then 

 according to our assumption we have present in solution only the 

 ions A~, B+, H+ and 0H~. Since A~ and B+ in a strong acid and 

 base have no affinity for the H+ and 0H~ ions, it follows that they 

 have no effect upon the [H+] and [0H~], that is to say, in the aqueous 

 solution the hydrion concentration is not affected by such a salt. 

 These salts are not at all hydrolyzed, and the solution of a salt of 

 this type is neutral in reaction. These salts are therefore called 

 neutral salts. 



Second case. The salt of a strong acid and a weak base whose 

 dissociation constant is kb. In this case the salt is hydrolyzed to a 

 small extent with the formation of a little free acid and free base. 

 But since the base dissociates less than the acid, the solution has a 



*^ Taken chiefly from N. Bjerrum. Die acidimetrische and allialimetrische 

 Titration. Stuttgart. 1914. 



