DETERMINATION OF HYDROLYTIC DISSOCIATION OP SALT-SOLUTIONS. 251 



volumetrically, and from this the hydrolysis can be easily calculated if 

 the distribution coefficient for the substance in question has been 

 previously determined. 



The values found at different dilutions agreed very closely with those 

 required by Arrhenius' ' dilution formula.' So far the method has not 

 been applied much, but it seems to offer advantages over previous methods 

 in several respects. Particularly for solutions which decompose on stand- 

 ing, it seeuis almost the only available method. It remains to be seen 

 whether this method is capable of the same sensitiveness as that of Shields. 

 If so, it would have the advantage of greater simplicity and rapidity. 



The foregoing are, then, the chief methods which have been used up to 

 the present for the determination of hydrolysis. 



It will be evident from the abovementioned theory of hydrolytic 

 dissociation that the extent of the hydrolysis depends on the strength of 

 the weak acid or base present in the salts. The relation between the 

 strength of the acid or base and the hydrolysis of its salts can be 

 expressed by a simple mathematical formula. 



The dissociation constant is, of course, determined by the elestric 

 conductivity. It is only recently, however, that the electric conductivity 

 of such weak acids has been determined with sufficient accuracy to confirm 

 the validity of this formula This was the work of Walker and Cormack.^ 

 The hydrolysis of the alkali salts calculated from the dissociation constants 

 which they found for phenol and other weak acids agreed very closely 

 with that experimentally found by the saponification method. This forms 

 perhaps the most convincing proof of the soundness of Arrhenius' views 

 as opposed to such theories as the one mentioned earlier, in which the 

 acidity was attributed to the formation of hydrates. 



In this way, therefore, it would be possible to calculate the strengths 

 of acids and bases whose electric conductivity is immeasurably small by 

 determining the hydrolysis of their salts. 



This, of course, rests on the assumption that no intramolecular 

 rearrangement takes place when salts are formed, which is not always the 

 case. In the case of various dye stuffs, for instance, where the salt forma- 

 tion is accompaniea by a change of constitution, we should find that the 

 relation between the strength of the acid and the hydrolysis of its salts 

 did not hold. If the measurements are experimentally possible, such 

 intramolecular rearrangements may be detected in this way. This is a 

 method which has been applied by Hantzsch to prove differences of consti- 

 tution between certain acids and the salts that they form. 



In several cases he found that although the acids were very weak 

 indeed, and should therefore give strongly hydrolysed sodium salts, yet 

 the sodium salts showed only a slight hydrolysis. In the case of 

 dinitroethane, for instance, he found that both the free dinitroethane and its 

 sodium salt reacted neutral, and from this he concluded that the salt forma- 

 tion was accompanied by a change of constitution from CH,.CH(N(^.>).) to 



/NO, /NO., 



CHg . C{ forming the salt CH, . C< 



^NOOH ^NOONa 



Fields of research like this offer inducements for the more accurate 

 determination of hydrolysis on the one hand and of the affinity constants 

 of very weak acids on the other. 



> J.C.S., 11, 5 (1900). 



