244 REPORT— 1901. 



ester is used up. If Cj and Cg be the two concentrations, and t be the 

 time, 



dC 

 Velocity = — , =KCiC2, where K is a constant. 



If we always take the same amount of ester, the velocity of the 

 reaction is proportional to the amount of acid added. The general 

 method is therefore to determine by a preliminary experiment the velocity 

 (.'i saponification brought about by a known amount of pure acid, and 

 afterwards to determine its velocity as brought about by the acid in the 

 hydrolysed salt. If we have found the velocity of saponification brought 

 about by a known quantity of acid, then we can conversely calculate 

 from the velocity of saponification which the hydrolysed salt brings 

 about, how much free acid it contains, that is, the extent of its hydrolysis, 

 remembering always that the velocity of the reaction is proportional to 

 the amount of free acid present. 



It should be mentioned that this proportionality does not hold quite 

 strictly in the catalysis of esters by means of acids. There are deviations 

 from it which are not fully understood. It differs in strong and weak 

 solutions of acids, apart from the difference which one would expect from 

 incomplete ionisation. The presence of neutral salts also has a consider- 

 able influence on the velocity. Consequently the results obtained by this 

 method are not to be taken as very accurate. 



Since the velocity varies throughout the whole course of the reaction, 

 we cannot take a direct measurement of the initial velocity of saponi- 

 fication, as the velocity changes so quickly that no trustworthy results 

 cfiuld be obtained in this way. The calculation is carried out by means of 

 the well known equation 



which holds for monomolecular reactions. 



A. is the initial concentration of the ester, x is the amount saponified 

 in time t, and K is a constant. The titrations taken during the whole 

 course of the reaction are used to determine K. By comparing the 

 constant K obtained for the hydrolysed chloride of a weak base with that 

 obtained for pure hydrochloric acid, the amount of free hydrochloric acid 

 iu the solution of the salt can be easily calculated, and liencetlic degree of 

 hydrolysis. 



The first exiicriments in this direction were carried out by AValker in 

 1889.' He determined the velocities of saponification of methyl acetate by 

 tlu' hydrochlorides of very weak bases, such as tliiazol, and thus deter- 

 mined the degrees of hydrolysis. 



A similar method was worked out for the salts of very weak acids by 

 Sliields, in 189.3.'- He determined th^. hydrolysis of the alkali sails of 

 phenol, carbonic acid, boric acid, &c. Iu this case it is not fref acid that 

 we have to determine, but free alkali, and the matter is complicated by 

 the fact that the free alkali is removed from the system as the reaction 

 proceeds, so that the equilibrium of the hydrolysis, as, for instance, 

 KCN + HOH -t KOH -f- HON, is continually changing. It would lead 

 us too far to go into the details of how this is taken into account. It is 



1 ZeiUclir.Juriihys. Cum., 4, 319 (ISSD). ■ Ibid., 12, 167 (1883). 



