﻿the Adsorption of Jons. 329 



(r/>. (4)). Besides, one cannot get any idea lis to why the 

 anion is generally more strongly adsorbed at low con- 

 centrations. 



The facts can, however, be explained as follows : — 



The negative charge of surfaces in contact with water is to 

 be sought for in the chemical natures of the anions and the 

 cations. The simpler electrolytes (excluding dyes and 

 complex organic ions) have cations whose chemical behaviour 

 can be referred simply to the tendency of the component 

 atom (e g. of the alkali and alkaline earth metals) to pass 

 into the ionic state. These ions do not form any complex 

 ions. They form only one type of compounds that are stable 

 in aqueous solutions, namely, electrolytes with the atom 

 existing as a positively charged ion through the loss of one 

 or more electrons. On the other hand, the anions in general 

 form types of compounds other than electrolytes, and also 

 form complex ions. It is, therefore, possible to imagine that 

 anions are subject to the chemical affinity of the surface 

 atoms and that the chemical action on the cations is relatively 

 small. Complex cations like those of the basic dyes should, 

 for the same reason, be easily adsorbable. This is a well- 

 known fact. 



If now, the assumption is made that the chemical affinity 

 acting on the anion of the electrolyte added is stronger than 

 the electrostatic attraction of the surface on the cation, the 

 observed variation of the charge with the concentration of the 

 electrolyte is easily accounted for. This case corresponds to 

 a strongly marked maximum of a negative charge at a low 

 concentration of the electrolyte. 



The initial charge of a surface in contact with pure water 

 -can be due either : 



(a) to the strong adsorption of an ion of a minute quantity 



of suitable electrolyte associated with the solid, 

 (7>) or to the adsorption of hydroxy! ions from water. 



On the addition of an electrolyte the density of the electric 

 charge will increase at low concentrations because of the 

 chemical adsorption of the anion. The electrical adsorption 

 of the cation is smaller as the chemical adsorption has been 

 assumed to be stronger. Besides, the electric charge of the 

 surface is also not at its maximum. As the surface becomes 

 more and more covered by the anions the rate of adsorption 

 das/dc — where " da" is the increase in the amount adsorbed 

 per unit surface due to an increase in the concentration 

 ^ dc" — rapidly decreases. Also, the electric charge repels 

 the anions, and those only can strike on it that have sufficient 



