﻿332 Prof. J. N. Mukherjee on 



little or no potential difference in contact with pure water, 

 and the considerations developed by Williams are not 

 applicable. 



Case 1. — The surface is inert. 



We shall assume that the atoms in the surface do not 

 exert any chemical affinity on hydrogen and hydroxyl ions 

 as such, or on the dissolved acid (or alkali) with which it may 

 be in contact. The adsorbed water molecules behave as a 

 solid layer, being held by strong chemical forces (Haber, 

 loc. cit. ; Hardy, Proc. Roy. Soc. lxxxiv. B. p. 217 (1911)). 

 It is clear that the surface will be neutral in contact with 

 pure water. The molecules of water in the adsorbed layer 

 are in thermodynamic equilibrium with those in the bulk of 

 the liquid. It is reasonable to imagine that a transfer of an 

 electron is taking place between the hydrogen atom and the 

 hydroxyl group in the water molecules in the surface layer, 

 as it does in the molecules in the liquid. That is, the water 

 molecules are dissociating into ions at a definite rate. Let 

 ki nx" be the number of water molecules (in the adsorbed 

 layer) passing into the ionized phase per unit area per 

 second. For equilibrium, as many hydrogen and hydroxyl 

 ions are uniting to form neutral water molecules. Since the 

 adsorbed water molecules behave as a solid layer, recombina- 

 tions would take place mostly between adjacent hydrogen 

 and hydroxyl ions. The recombination will be extremely 

 rapid. It can be assumed that at any instant the number of 

 hydrogen or hydroxyl ions actually remaining free in the 

 surface will be a negligible fraction of the total number of 

 water molecules. 



The neutralization of the ions being formed in the surface 

 layer can also be brought about by impinging hydrogen or 

 hydroxyl ions present in the liquid. In contact with pure 

 water the probability of such collisions is small, for the 

 concentration of hydrogen and hydroxyl ions is extremely 

 small. Thus neutralization of the ions being formed in the 

 surface layer is possible in two ways : 



(1) H s ° + OH, 1 — ^HOH— the subscript "s" refers 



to ions in the surface layer ; 



(2) (a) H.o + OH/— ->HOH, 



(6) H/>+ OH, 1 — *HOH— the subscript «f» refers 

 to the freely moving ions in the liquid. 

 In contact with pure water, neutralizations according 

 to scheme 2 are small in number. Also 2 (a) and 2 (b) are 

 equally probable. Consequently the numbers of H s ° and 

 OH, 1 remaining in the surface at any instant will be equal, 

 and the surface will be neutral. 



