DETERMINATION OF HYDROLYTIC DISSOCIATION OF SALT-SOLUTIONS. 241 



has undergone a decomposition into free potassium hydrate and free 

 hydrocyanic acid 



KCN + HOH=KOH + HCN. 



Similarly we find that other salts, as, for instance, ferric chloride, react 

 acid in aqueous solution. 



Even Rose,' who was probably the first to notice these phenomena, 

 recognised that this was the result of a secondary reaction, which was 

 brought about by the water. An analogy was sought in the decomposition 

 of acid chlorides and the breaking up of organic complexes such as saccha- 

 rose, in which the elements of water are taken up, and for this reason 

 the name ' hydrolysis ' was, rather unfortunately, applied indiscriminately 

 to the two phenomena. 



The nature of the decomposition formed the subject of considerable 

 discussion, but it was not until Arrhenius brought the theory of electro- 

 lytic dissociation to bear on it that a satisfactory explanation was found. 

 As this theory is almost universally accepted at the present time, it is not 

 necessary to make more than a passing reference to a theory which at 

 one time ofiered some opposition to that of Arrhenius. This was an 

 assumption that the salts in question formed hydrates in aqueous solution, 

 and that these hydrates possessed acid or basic properties. 



Thus Werner - attempted to explain the acid reaction of copper 



chloride in aqueous solution on the assumption that it formed a hydrate of 



Cl\ /H O 

 the formula qj y Cu s' jj^q which was acid in character. In this way, of 



course, it would be possible to account for the acid or alkaline reaction of 

 all hydrolysed salts. Potassium cyanide would form a hydrate of a basic 

 nature and so on. 



It is an unsatisfactory feature of this theory that it makes the 

 assumption of innumerable hydrates whose existence in aqueous solution 

 is still to be proved ; but apart from this it is shown that the acid or basic 

 reaction is the result of a dissociation and not of a formation of hydrates 

 by the fact that the acid and basic components can be easily separated. 

 This separation can be sometimes etiected by mere warming, as in the 

 case of iron or aluminium acetate, in many other cases by dialysis. 



In the case of diphenylamine hydrochloride repeated washing suffices 

 to completely remove the hydrochloric acid, and in the case of many 

 organic salts, as, for instance, sodium phenolate, one of the components 

 can be partially removed by extraction with ether. 



In 1890 Arrhenius^ brought forward a simple explanation of the 

 hydrolysis of salts on the basis of the theory of electrolytic dissociation. 

 AH that was necessary in order to bring the phenomenon of hydrolysis 

 into complete harmony with the ionic theory was to consider water as an 

 electrolyte, to suppose that it is to a slight extent dissociated into 

 hydrogen and hydroxyl ions. Later investigations have completely justified 

 this assumption. Compared with the weakest of acids, the ionisation of 

 water is almost infinitesimal, but it has been determined with a consider- 

 able amount of accuracy. "Water consists, then, of a solution of hydrogen 

 aaid hydroxyl ions of such a strength that ten million litres of water 

 contain approximately one gram equivalent of free ions. This means that 

 water can act at the same time as a weak acid and a base. 



1901. 



' JaK^c'ilcr., 1852, 310. - Zeitschr.fUr anorg. Chem.i 9, 408. 



^ ^ ZeiiscJir. fur phy's. Chem., 5, 16 (1S90). 



