in Aqueous Salt-Solutions. 367 



us a measure of the quantity of free alkali which they contain. 

 The essential condition for saponification to take place is the 

 presence of hydrogen or hydroxyl ions. Now Walker's 

 method was based on the presence of hydrogen ions, whilst 

 in the experiments about to be described we are dealing with 

 hydroxyl ions. This places us, as it were, on vantage ground ; 

 for, since hydroxyl ions saponify much more rapidly than 

 hydrogen ions, it is thus possible to work with more dilute 

 solutions where perturbing influences are reduced to a 

 minimum. 



To return once more to the typical example of the salts 

 under investigation, potassium cyanide, let us for a moment 



consider what takes place when we dissolve this body in water. 



+ 

 Besides the undissociated KON we get a great number of K 



and CN ions, the water itself, too, is slightly dissociated into 

 + - + 



H and OH ions. Now the H ions coming into contact with 



the CN ions unite with them to form HCN which is uncharged, 

 being practically undissociated, whilst the hydroxyl ions re- 

 main free and counterbalance the potassium ions. 



The water goes on continually supplying hydrogen and 

 hydroxyl ions, which are disposed of in this way, until equili- 

 brium takes place. If we now make up a small inventory of 

 the principal constituents of the solution we get : — 

 + 



1 and 2, K and CN ions. 



3, KCN undissociated. 



4, HOH undissociated (say). 



5, HCN undissociated (say), and, corresponding to this, 



6 and 7, K and OH ions. 



6 and 7 taken together represent the quantity of free potash 

 present in the solution. The task which now lies before us is 

 comparatively easy, but before proceeding to deduce fbrmulse 

 for the calculations it is as well to point out what will be 

 proved later on, namely, that potassium cyanide itself is not 

 an active agent of saponification. Attention may also be 

 directed to the fact that the dissociation of the hydrogen 

 cyanide is so excessively slight in presence of the salt, that it 

 cannot exercise any appreciable influence on the velocity of 

 saponification : the truth of this will be the more readily ad- 

 mitted if we bear in mind that hydrogen ions are much less 

 active than hydroxyl ions. What has already been said 

 regarding the influence of the salt on the dissociation of the 

 hydrogen cyanide applies equally well to the water. 



When ethyl acetate is saponified by a solution of potassium 



2C2 



