DETERMINATION OF HYDROLYTIC DISSOCIATION OF SALT-SOLUTIONS. 249 



excess of acid has deen tried, but, so far, without much success. The 

 method is probably capable of much better development. 



The commonest method for the determination of this value /.t^ is a 

 somewhat indirect one. It is a well-known fact that almost all salts 

 are fairly completely ionised in aqueous solution. Thus the molecular 

 conductivity is not very far removed from its limiting value, even at 

 moderately high concentrations, and hence does not rise very much when 

 we increase the dilution. It has been found empirically that the amount 

 by which the molecular conductivity of binary electrolytes increases 

 between any two given dilutions is nearly constant. The conductivity is 

 generally measured at dilutions ranging from 32 litres to 1024 litres. It 

 has been found that in the case of binary electrolytes which are not 

 hydrolysed the molecular 'conductivity at these two dilutions differs by 

 approximately 10 units at .25°. 



Thus the sodium salts of the fatty acids, being scarcely at all 

 hydrolysed, gi\e ditferences which all approximate to 10 units. The 

 sodium salts of dibasic acids give a difference of about 20 units and so on. 

 In general, the difference. A, is given by 



^^ = ^^'1024— A'32 = 10"l'i2- 



where ?*, and n.2 are the valencies of the two ions. With hydrolysed 

 salts we get a very different state of affairs. Here we find the differences 

 to be abnormally large, for the following reason. At the highest con- 

 centrations the hydrolysis will not come into play very much, and the 

 values found will approximate more or less to the true values. As we 

 increase the dilution, however, the hydrolysis inci-eases more and more, 

 and at the highest dilution a considerable part of the conductivity found 

 will be due to free acid or base, and this will, therefore, as we have seen, 

 be greater than the true conductivity of the salt. Hence the difference A 

 will be greater than 10 units. 



If, therefore, we find that the difference A is abnormally great, the 

 excess may be attributed to hydrolysis, and the extent of the hydrolysis 

 may be calculated by making use of the equation mentioned above : 



M = (l— .T)/Ji-fX/iHCl. 



The method cannot be said to be very satisfactory unless the extent of 

 the hydrolysis is very large. First, the measurement of the electric 

 conductivi';y at a dilution of 1021 litres does not permit of an accuracy 

 of within about 1 per cent. ; and secondly, this value A is by no means so 

 constant even for salts which are not hydrolysed as might be desired. It 

 frequently shows deviations of 2 or 3 units, and so a hydrolysis of even 

 1 per cent, or so might pass unnoticed. We saw that the hydrolysis of 

 sodium acetate could be fairly accui-ately measured by the velocity of 

 saponification of ethyl acetate. In jL normal solution it amounts to 0-008 

 per cent. If we calculate what difference this would make to the conduc- 

 tivity, we find that the abnormality of the A value should be about 0-15 

 unit. It will be at once seen that anything approaching this accuracy is 

 out of the question by the electric method. Indeed, if we compare the 

 values actually found for sodium acetate bv two such eminent 



