﻿the Adsorption of Ions. 325 



or a layer under great internal pressure. The necessity for 

 recognizing the existence of a sort of chemical interaction (as 

 Arrhenius has suggested) becomes evident when one con- 

 siders the specific nature of adsorption processes. This point 

 has heen justly emphasized by Bancroft in recent years. 

 Besides his papers in the ' Journal of Physical Chemistry,' 

 compare 'Applied Colloid Chemistry/ 1921, p. 111). 



The chemical point of view has been put clearly by Lang- 

 muir (J. Amer. Chem. Soc. xxxviii. p. 2221 (1916) ; xxxix. 

 }). 18-18 (1917)). He believes that adsorption is due to the 

 chemical affinities of the surface atoms. Considering the 

 thermodynamic equilibrium between molecules of a gas at 

 the surface and those in the surrounding gas he has deduced 

 the following equations correlating the variation of the ad- 

 sorbed amount with its pressure, 



where " di' is the fraction of the solid surface covered and is 

 a measure of the amount adsorbed, vi is the rate at which 

 the gas would evaporate if unit area of the surface were 

 completely covered, "//," is the number of gas molecules 

 striking unit area of the surface per second and is given by 



//,= 43*75 X 10~ 6 — — ;f- — — , and "p ,} denotes the pressure of 



the gas, "T" its absolute temperature, and " M" its molecular 

 weight. a denotes the fraction of the total number of 

 collisions of the molecules of the gas that leads to a condensa- 

 tion on the surface; it is usually close to unity and evidently 

 can never exceed unity. Some interesting applications of 

 his theory to catalysis of gaseous reactions by solid surfaces 

 are given, This theory explains many phenomena which are 

 otherwise difficult to understand. 



Michaelis and Rona (Bio-Chem. Zeitsch. xcvii. pp. 56, 85 

 (1919)) conclude from the investigations of Michaelis and his 

 co-workers that the assumption of special forces at the sur- 

 face fails to account for the facts and that adsorption is the 

 result of chemical affinity. 



I. The Adsorption of a Constituent Ion by a Precipitate. 



The adsorption of ions is different from the adsorption of 

 neutral molecules or groups in that it introduces a new 

 factor — an electrically charged surface. The variation in 

 the electric charge enables us to follow the net effect of the 

 adsorption of the two ions, as the electric charge depends 



