﻿340 Prof. J. N. Mukherjee on 



oppositely charged ion cannot be accounted for. The activity 

 of the cations is generally in the following order : — 



Th > Al > Ba > Sr > Ca > H > Cs > Rb > K > Na > Li. 



The postulates that alkali metal salts become undissociated 

 at low concentration of the cation and that their solubility 

 products are of the above order for a large number of diverse 

 chemical substances, are contrary to experience. Regarded 

 from the chemical point of view the generality of these ob- 

 servations cannot be explained. Besides, the conception of 

 the suppression of the dissociation of a salt cannot explain the 

 reversal of the charge which is met with when the oppositely 

 charged ion is polyvalent. 



The view of electrical adsorption put forth by the writer 

 gives a definite correlated account of these various facts. 



The Role of Electrostatic Forces in the Absorption of Ions. 



(a) Adsorbent in contact with a single electrolyte : — 

 Let us consider an adsorbent, P, in contact with an electro- 

 lyte A B . It is assumed that the substance P only adsorbs 

 the anion B by chemical affinity. For simplicity it is also 

 assumed that " P " is a pure chemical substance of definite 

 composition. The amount of B adsorbed per unit area will 

 depend on the concentration of A. B and on the strength 

 of the chemical affinities acting on B _ . Corresponding to 

 the number of anions adsorbed an equivalent number of 

 cations A remain in the solution. These are held near the 

 surface by electrostatic forces, and form the second mobile 

 sheet of the double layer (cp. the earlier paper referred to). 

 If the concentration of the electrolyte is sufficient, some of 

 them will be fixed on the surface by electrostatic forces. 

 These ions of opposite charge fixed on the surface by electro- 

 static forces will be spoken of as electrically adsorbed in 

 the sequel. The chemical adsorption of an ion thus concen- 

 trates both ions at the surface in equal amounts. That is, 

 the primary adsorption is an equivalent adsorption of both 

 ions. Analytical methods cannot differentiate between the 

 two adsorptions, but electro-osmotic and cataphoretic experi- 

 ments can (cp. (d) above). 



If the adsorption of the anion is due to strong chemical 

 forces, perceptible amounts of the electrolyte A B ~ will be 

 adsorbed at very low concentrations. Even saturation may 

 be reached at low concentrations. In such cases, if the 





