DETERMINATION OF HYDROLYTIC DISSOCIATION OF SALT-SOLUTIONS. 247 



IV. — Hydrolysis of inorganic chlorides {inversion method). 



Hydrolysis of Chloride 



in— solution, 

 10 



01 



0-3 



1-8 



61 



2-7 



0-3 



0-1 



10 



3 



The chlorides of the alkali metals and of the alkaline earths, as also 

 those of yttrium, scandium, manganese, cobalt, and erbium, showed no 

 appreciable hydrolysis. 



A method somewhat similar to the inversion method was recently 

 suggested by Wood.^ He allowed diastase to act on starch in presence 

 of a hydrolysed salt. Acids or alkalies retard the action of the diastase, 

 and the retardation was taken as a basis of measurement of the amount 

 of acid or alkali present. The action is very much affected by changes of 

 temperature. So far only rough approximations have been obtained in 

 this way. 



Electric Conductivity, — The electric conductivity has for a long time 

 been looked on as a useful method for the determination of hydrolytic 

 dissociation. Its capabilities in this direction have, in my opinion, been 

 considerably overestimated. The method used for the determination is as 

 follows : — It is well known that almost all salts are fairly completely 

 ionised when dissolved in water at a moderate dilution. Their electric 

 conductivities, which form a measure of their ionisations, do not differ 

 from one another by a great deal in solutions of equivalent concentration. 



The free acids and bases, on the other hand, have all possible conduc- 

 tivities, ranging from almost nothing in the case of the very weak acids 

 and bases to values very much greater than those of the salts in the case 

 of the strong acids. 



If, then, we take the solution of a salt such as aniline hydrochloride, 

 which is considerably split up into free aniline and hydrochloric acid in 

 aqueous solution, the observed conductivity will be partly due to the salt 

 C5H5NH2HCI, and partly to the free HCl which is split off by hydro- 

 lysis. The free aniline which is present in the system will not contribute 

 appreciably towards the conductivity. 



Since the conductivity of hydrochloric acid is very much greater than 

 that of aniline hydrochloride, we shall be able to draw some conclusion 

 from the conductivity as to the amount of free hydrochloric acid which is 

 present in the system. If a/; be the molecular conductivity which aniline 

 hydrochloride would have if it were not hydrolysed, //hci bs that of hydro- 

 chloric acid, and x the fraction of the salt which is hydrolysed, the 

 observed molecular conductivity (M) will be 



M^(l — a;)fti, due to unsplit salt, 

 + '^/'hcd ^^^ to ^^^^ HCl. 



' Amer. Chem. Journ,, 16, 313. 



