ADSORPTION POTENTIALS AND ELECTROKINETIC PHENOMENA 245 



It would be of great interest to study the adsorption capacity of an 

 indifferent substance, such as cellulose (filter paper, cotton wool), 

 but unfortunately great experimental difficulties arise. It is quite 

 impossible to obtain a cellulose entirely free of ash. Even after the 

 most careful extraction with hydrochloric and hydrofluoric acids 

 it always contains some silica and lime. In studying the adsorption 

 of dyes on cellulose, Michaelis and Rona^- found invariably adsorp- 

 tion by exchange of ions. Thus after the adsorption of methylene 

 blue chloride the cation was found to have been adsorbed while the 

 anion (CI") was found in solution as a neutral salt (CaCl2). Simi- 

 larly, in the adsorption of NII4- eosin, the eosin anion was adsorbed, 

 while at least 70 per cent of the NH4- ion remained in solution as a 

 neutral salt. Since "adsorption by exchange" exercises so great an 

 effect it has been practically impossible to ascertain the extent to 

 which the cellulose itself participates in the adsorption process. 

 It must be, in any case to an extremely minute extent. The abihty 

 of the technical forms of cellulose (cotton, paper etc.) to take dyes 

 must depend largely upon an exchange of ions with its ash con- 

 stituents, and the latter form quite possibly an integral part of the 

 cellulose molecule. The properties of cellulose are very different 

 from those of charcoal, in which, to be sure, there is also no lack of 

 these impurities, but in which the much greater adsorbing capacity 

 of the charcoal itseK renders any effect due to the impurities 

 neghgible. 



It is not the purpose of this book to exhaust the problem of ionic 

 adsorption. The discussion offered above should only serve as an 

 introduction to the subject of adsorption potentials, and it should 

 suffice for this purpose. 



70. The formation of a charge upon phase boundary surfaces due 



to adsorption of ions 



In the Ught of the explanations given in the preceding section 

 we can ascribe to each kind of ion its own specific adsorbabihty. 

 But since the electrostatic forces prevent the adsorption of the two 

 ions of an electrolyte to measurably different extents, what really 

 happens is the estabhshment of an adsorption equilibrium which lies 

 somewhere between the hypothetic equihbria of the positively and 



>2 Michaelis and Rona, Bioch. Zeitschr. 103, 19 (1920). 



