ADSORPTION PHENOMENA AND GIBBS' 

 EQUATION 



By W. C. McC. LEWIS, M.A., D.Sc. 

 Professor of Physical Chemistry, University of Liverpool 



The Nature of Adsorption 



The operation of removing the colouring matter from solu- 

 tions by shaking them with finely ground charcoal or by 

 filtration through sand or other material which exposes a 

 large surface is a familiar one both in the laboratory and on 

 the manufacturing scale. The gradual diminution in effi- 

 ciency, due to clogging of the porous material, arises from the 

 deposition of the extracted matter upon the surface of the 

 charcoal or sand. This removal of certain substances from 

 solution and their condensation upon the surface of the finely 

 divided solid is somewhat loosely termed adsorption. Further 

 examination of the phenomenon has shown, however, that it 

 is complex, involving not only a pure surface-concentration 

 effect, but also a coagulation or flocculation of material at 

 the surface, which may in turn be followed by chemical change 

 in the material, e.g. oxidation. In many cases also we find 

 that the extracted material penetrates into the body of the 

 absorbent substance. To cover both adsorptive and absorp- 

 tive effects the non-committal term " sorption " has been 

 suggested. That charcoal and presumably many other sub- 

 stances exert this twofold effect has been most clearly demon- 

 strated by J. W. McBain (PhU. Mag. [6] 18, 916, 1909) in a 

 paper entitled " The Mechanism of the Adsorption (' sorp- 

 tion ') of Hydrogen by Carbon." McBain shows that the pro- 

 cess takes place in two stages. At first there is an extremely 

 rapid increase of concentration of the hydrogen in a very 

 thin layer at the surface of the charcoal, and later there is a 

 slow diffusion of the gas into the interior of the charcoal. It 

 is only the first of these stages which may properly be called 

 adsorption. Adsorption is therefore one of those terms 



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