﻿342 Prof. J. N. Mukherjee on 



amount of the negative ions chemically adsorbed. Anything 

 that increases the total amount of adsorbed negative ions 

 will increase the positive adsorption of C • 



The reverse case, when the positively-charged ions A + are 

 adsorbed chemically instead of the ions B~? and no other 

 ions are chemically acted on by the surface, is obvious. 

 Negative ions will now be positively adsorbed and exchanged. 

 This state of affairs corresponds with the statements made in 

 (a) and (c) above. 



Taking the same case again, we shall consider the effect 

 of the electrostatic forces on the ion D • An ion D~ , diffusing 

 into the double laver, will be driven out of it. So long ;is 

 the potential of the double layer is sufficiently strong, a 

 volume of the liquid equal to "SI," where " S " is the extent 

 of the surface and " I " is the thickness of the double layer — 

 will be free from the ion D . In other words, the concen- 

 tration of D~ increases in the bulk of the liquid and a 

 negative adsorption will take - place. This will increase with 

 the concentration of the electrolyte so long as the potential 

 of the double layer is sufficiently strong. Since with in- 

 crease in concentration the potential fall^, the negative 

 adsorption will reach a maximum. At concentrations when 

 the surface becomes electrically neutral, there should be no 

 negative adsorption due to electric forces. It is difficult to 

 determine negative adsorption at high concentrations as the 

 osmotic pressure opposes it. Also, the variations in concen- 

 tration due to negative adsorption become relatively small. 

 The experimental difficulties lie in the analytical estimation 

 of small amounts. Only ions which can be estimated in 

 extremely small amounts are suitable for experiment. 



Estrup (Koll. Zeitscli. xi. p. 8 (1911)) has actually 

 observed a negative adsorption of the oppositely-charged 

 ion. He estimated the adsorption of the iodate, dichromate, 

 and chromate of ammonium. Michaelis and Lachs (KolL 

 Zeitscli. ix. p. 275 (1911)) did not observe a negative 

 adsorption with potassium chloride. 



Exactly similar observations have recently been made by 

 Bethe (Wiener Mediz. Wochsch. 1916, Nr. 14 ; Koll. Zeitsch. 

 xxi. p. 47 (1917)). He worked with gelatine gel, gelatine 

 sol, and a number of animal cells. The adsorption of a basic 

 dye is greater in weak alkaline solutions than in neutral 

 solutions. The same is the case for an acidic dye in weak 

 acid solutions. In alkaline solutions the adsorption of acid 

 dyes is negative, and the same is the case with basic dyes 

 in acid solutions. Examples of the role of- the electrical 



