PRESIDENTIAL ADDRESS. 353 



ionised portion of the electrolyte which exhibits abnormal behaviour and not 

 the ions, has been reached on other grounds by Noyes and others, and I hope 

 in what follows to deduce reasons in its support. 



One is apt, because the ions are in general the active constituents of an 

 electrolyte, to lay too much stress on their behaviour in considering the 

 equilibrium in an electrolytic solution. We are justified in attributing the fact 

 that acetic acid is a weak acid, whilst trichloracetic acid is a powerful one, 

 rather to the properties of the un-ionised substances than to the properties of 

 the ions. The divergence of trichloracetic acid from the simple dilution law 

 may similarly be due to an inherent property of the un-ionised acid, a single 

 cause being not improbably at the bottom of both the great tendency to sjalit 

 into ions in water and also the abnormal behaviour towards dilution. 



However that may be, I think the following reasoning goes far to show 

 that the non-ionised portion of the electrolyte is that which is primarily abnormal 

 in its behaviour, the ions acting in every way as normal. The dilution formulae 

 of Ostwald or of van't Hoff is essentially equilibrium formulae. One side of the 

 equilibrium represents the interaction of the ions to form the non-ionised sub- 

 stance, the other side represents the splitting up of the non-ionised substance 

 into ions. In order to fix our ideas, we may consider a salt which obeys the 

 empirical dilution-formula of van't Hoff. If c„ represents the molar concen- 

 tration of the un-ionised portion, and c,- the molar concentration of each ion, 

 then according to van't Hoff's empirical formula, 



-s 



1 = const, 

 c 2 



If the law of mass-action were obeyed we should have, on the other hand, 

 Ostwald's dilution formula, 



C' 2 



— = const. 



According to this last formula, the activity of each substance concerned varies 

 directly as its molar concentration, and a normal result is obtained on dilution. 

 According to van't Hoff's formula as stated above, the activity of none of the 

 substances concerned varies directly as its concentration ; but since the con- 

 stancy of the expression is the only test of its accuracy, there are obviously other 

 methods of stating the relation which will throw the abnormal behaviour either 

 on the ions or on the non-ionised substance. Thus, if we write the equivalent 

 form 



. 1-5 



V 



' = eonst., or -i — = const., 



the unionised substance is here represented as behaving normally, and the ions 

 abnormally ; whilst if we write the formula in the form 



.2 



i const., 



ld3 



the ions are represented as behaving normally, and the non-ionised substance 

 abnormally. Now it is very important that a choice should be made amongst 

 these three expressions, all equivalent amongst themselves so far as the mere con- 

 stancy of the expression is concerned, as tested by measurements of electrolytic 

 conductivity. Looked at from the kinetic point of view we have in the first form, 



dx - hr ■< 



dx j, „ 

 -dt= U "'' 

 both direct and reverse actions abnormal. In the second form, we have 



dt ~ U> 



dx j, 



-dt= kc - 



1911. 



A A 



