﻿the Adsorption of Ions. 341 



adsorbent with adsorbed electrolyte is suspended in pure 



water the adsorbed electrolyte will not be set free (e). 



Since the primary adsorption of the ions is due to chemical 



affinity, the influences of the nature of the adsorbent and of 



the electrolyte (b) are intelligible. 



(/>) The addition of a second electrolyte : — 



The general case when both electrolytes, A B~ and 



C D , are present in all possible concentrations will be too 



complex. It will be assumed for the sake of simplicity 



that— 



(1) the substance P adsorbs chemically the anion B^ 

 strongly, and that the concentration of the electrolyte A B 

 in the liquid is negligible. We are thus dealing with an 

 adsorbent with an amount of adsorbed electrolyte in contact 

 with a second electrolyte solution ; 



(2) tiie atoms on the surface of the adsorbent P do not 

 exert any chemical affinity on the ions C and D~. 



This particular case corresponds with most actual systems, 

 and the electrolyte A + B~ plays the part of the " Aktiver 

 Electrolyt " of Michaelis. 



Let us now consider the effects of the electrostatic forces 

 on the ions C and D~. A cation C , when it- diffuses into 

 the double layer owing to thermal energy, will be attracted 

 to the surface. Considering the kinetic equilibrium between 

 the ions in the second sheet of the double layer (A and C ) 

 and those in the liquid, it is evident that the relative propor- 

 tion of A and ions in the double layer will depend on 

 (i.) their respective concentrations in the bulk of the liquid, 

 and (ii.) their valency. The same consideration applies to 

 the electrically-adsorbed ions A + or C + . At sufficiently 

 large concentrations the whole of the mobile second layer 

 and electrically-adsorbed ions will be formed by the ions 

 C . There will thus be an exchange of ions, and the amount 

 of exchange will depend on the concentration of the second 

 electrolyte. When the displacement is complete the amounts 

 exchanged will be equivalent to the amonnt of B* ions 

 primarily adsorbed and independent of the nature of the 

 replacing ion C + — a fact often observed (cp. iLinder and 

 Picton, loc. cit. ; Whitney and Ober, loc. cit. etc.). 



The ions will be positively adsorbed. 



The relationship between the charge of the surface and 

 the positive adsorption of the oppositely-charged ion is also 

 obvious. The amount of () + ions absorbed depends on the 



