DETERMINATION OF HYDKOLYTIC DISSOCIATION OV SALT-SOLUTIONS. 243 



about a separation of the components. Thus, the hydrocyanic acid may 

 bo partially removed from a solution of sodium cyanide by a current of 

 pure air, the phenol may be partially extracted from a solution of sodium 

 phenolate by ether, and so on. 



Quantitative Jlethods. — When we attack the problem of ascertaining 

 quantitatively to what extent this hydrolytic dissociation of salts occurs, 

 it is at once evident that the hydrolysis cannot be determined by any 

 direct measurement of the free acid or alkali in the system. If we attempt 

 to titrate the solution of a salt like potassium cyanide, the equilibrium 

 is at once disturbed, and as we neutralise the free potassium hydrate in 

 the system by the addition of acid, more potassium hydrate is supplied 

 from the potassium cyanide to take its place. As we have seen, the 

 neutral point is in many cases only reached when enough acid has been 

 added to completely split up the salt. We must therefore resort to some 

 indirect means of estimating the free acid or alkali in the system without 

 disturbing the equilibrium. 



We will pass over such methods as the determination of the heat of 

 neutralisation, as these have led to very incorrect ideas as to the extent of 

 the hydrolysis. For instance, determinations of the heat of neutralisa- 

 tion of hydrocyanic acid led to the belief that a solution of sodium 

 cyanide was split up to the extent of 80 per cent, into free hydrocyanic 

 acid and sodium hydrate, whereas in reality its hydrolysis only amounts 

 to about 1 per cent, in i\f normal solution. 



In fact, the hydrolysis proves in most cases to be much smaller than 

 was formerly imagined. Even salts like sodium phenolate, which react 

 strongly alkaline, are only hydrolysed to the extent of 2 or 3 per cent, in 

 about ,'iy normal solution. 



The quantitative methods which have hitherto been used are mostly 

 based on the measurement of the velocity of reactions, brought about by 

 the free alkali or acid in the solution. Of these reactions the chief have 

 been the saponification of esters and the inversion of cane sugar. 



S'aponijication of Esters. — If we take an ester such as ethyl acetate 

 and dissolve it in pure water, it will remain for \ve(?ks practically 

 unaffected. If, however, we add acid or alkali, saponification sets in, and 

 proceeds with a velocity depending on the amount of acid or alkali added. 

 The A-elocity can be measured by means of titrations. 



If we treat the ester with a hydrolysed salt, saponification will like- 

 wise take place by virtue of the free acid or alkali which the solution 

 contains. We must distinguish between the case in which the saponification 

 is brought about by free acid and that in which it is brought about by 

 alkali. The action of acids in saponifying esters is purely catalytic ; the 

 amount of acid remains unchanged throughout the reaction ; this is, there- 

 fore, the simplest case, and we will consider it first. 



For the measurement of the velocity, known quantities of ester and 

 acid are brought together in aqueous solution and kept at constant 

 temperature. At measured intervals of time a part of the solution is 

 removed by means of a pipette and quickly titrated. This tells us how 

 much of the ester has been converted to acetic acid and alcohol in a given 

 time. From the results of these titrations the whole course of the i-eaction 

 can be followed. 



By the law of mass action, the velocity of the reaction at any moment 

 is proportional to the product of the concentrations of the reacting sub- 

 stanc'es (ilie ester and acid). Tha velocity diminishes, therefore, as the 



