J. A. Prescott 123 



This equation in various forms has been used for many years and was 

 recognised by Van Bemmelen in 1878 as being applicable to one of his 

 absorption results^ but was not followed up by him. He only insisted 

 that the ratio of the absorption to the equilibrium concentration was 

 never constant. 



It has been found to hold for the absorption of iodine by starch by 



F. W. Kiister^ and for picric acid and silk by J. Walker and 

 J. R. Appleyard^. Freundlich and his co-workers have established 

 the general applicability of this law since 1906*; the law holds in 

 most cases of adsorption so far investigated using as solvents, water, 

 alcohol and other organic solvents and as adsorbents, charcoal, silica 

 and alumina, and various textile fibres. 



A typical example of an adsorption curve is given in fig. 1 from 

 experimental data obtained by the author. This shows the adsorption 

 of oxalic acid by a soil, in the presence of N/20 HNO3 to prevent 

 the chemical precipitation of insoluble oxalates. 25 gms. of a deep 

 Rothamsted subsoil were shaken with 1000 c.c. of varying concentrations 

 of oxalic acid in N/20 nitric acid in a thermostat at 23° C. for one hour. 

 The oxalic acid was determined in the extracts by titration against 

 potassium permanganate solution ; the results obtained are given in 

 Table I. 



The experimental points are seen to lie fairly well on a curve 

 expressed by the formula 



Y -^ 



Y ^ 



which is of the usual adsorption type, ^ = KC^\ The values for P 



usually lie between 2 and 10: there are some exceptions but in the 

 majority of adsorptions so far investigated these are the limits. The 

 value of K varies of course with the units employed to express the 

 results and is proportional to the active surface of the adsorbent. 

 Other formulae have been proposed, notably by S. Arrhenius and by 



G. C. Schmidt, but at the present time the one given above is most 

 generally used and it gives fair agreement so long as the concentrations 

 of the solutions are not too high. 



The rapidity with which the equilibrium is reached is very high ; 

 under favourable conditions the process may be said to be instantaneous. 



^ Sec W. Ostwakl in editorial to Die Absorption. 



- Liebig Annalen, 1894, 283, 360. 



3 Trans. Chem. Sac. 1896, 69, 1334. * See Kapilhrchemie, 1906. 



