﻿338 Prof. J. N. Mukherjee on 



Freundlich and Poser (loc. cit.) found that the nature of the 

 adsorbent plays an important part in determining the ad- 

 sorbability of a dye. 



Both Michaelis and Freundlich agree that at least two 

 types of adsorption of ions can be recognized. 



(c). Exchange or displacement of ions already adsorbed by 

 ions of a second electrolyte (cp. Freundlich, ie Verdrangende 

 Ionenadsorption " and Michaelis, "Austausch-Adsorption "). 

 Michaelis (Z. Electrocliem. xiv. p. 353 (1918)) considers that 

 a substance like mastic, or kaolin (bolus), acts as a " zweier 

 electrode " (a binary electrode). Thus kaolin has a slow- 

 moving anion (silicate ion) anchored on its surface and tends 

 to send hydrogen ions into the solution under a definite 

 electrolytic solution tension. Freundlich points out (and 

 Poser, loc. cit.) that other cations can displace the hydrogen 

 ions and form undissociated complexes (and Elissafoff, Z. 

 Phys. Chem. lxxix. p.. 385 1912). 



(d) An adsorbent which contains some adsorbed electro- 

 lytes need not be necessarily saturated. In this case, besides 

 an exchange of ions, primary adsorption of ions is possible. 

 This also applies to substances which act as binary electrodes 

 in the sense in which the word has been used by Michaelis. 

 He considers that, besides adsorption through exchange of 

 ions, there is only one other type of adsorption, namely, 

 adsorption of both ions in equivalent amounts (" Aquivalent 

 Adsorption"). 



One other fact has been emphasized by these authors. 

 (e) It is the irreversible nature of electro-chemical ad- 

 sorption. The well known instance of the adsorption of 

 hydrogen sulphide by metal sulphides studied by Linder and 

 Picton (T. Ixvii. p. 163 (1895) ; Whitney and Ober, J. Amer. 

 Chem. Soc. xxiii. p. 842 (1901)) can be mentioned. The 

 adsorbed substance does not come out in solution when the 

 adsorbent is brought in contact with pure water. 



(/) Lastly, there is no clearly established instance in 

 which hydrolytic splitting up of neutral salts such as 

 potassium chloride has been observed through adsorption. 



Theories regarding tlie Exchange of Ions. 



The conception of an adsorbent acting as a binary electrode, 

 suggested by Michaelis, is not of much help in explaining the 

 exchange of ions and other peculiarities of the adsorption of 

 electrolytes. The relationship between the adsorption of 

 ions, electro-endosmotic cataphoresis, and precipitation of 

 colloids has been established beyond doubt. The only theory 



