390 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Peculiarities of Adsorption. 



(1) Amount of Adsorption. 



The amount adsorbed from a solution does not increase in direct 

 proportion to the increase in concentration. It thus differs from 

 chemical reactions, e.g. 10 times the concentration produces only 4 

 times the adsorption. Relatively more substance is adsorbed from, 

 dilute solution than from a concentrated one. 



(2) Different Adsorption from Different Solvents. 



Usually the adsorption of substances from water is greater than 

 from organic solvents. This peculiarity may be of practical use. Dyes 

 can be removed from aqueous solution completely by charcoal, the 

 particles being concentrated on the surface. On putting the charcoal 

 into alcohol, the dye passes into the alcohol. This is due to the surface 

 concentration of the dye on the charcoal being in excess of that neces- 

 sary to produce equilibrium between the phases. 



(3) Selective Adsorption. 



Substances are not adsorbed to the same extent ; benzoic acid or 

 salicylic acid are more adsorbed than acetic acid by charcoal. This 

 selective adsorption has been put to practical use in capillary analysis, 

 e.g. : 



Strips of filter paper are partly suspended in different solutions ; the 

 liquid rises into the paper ; above a certain height there is only water ; 

 the height to which the dissolved substance rises is different; the 

 more adsorbed substance does not rise so high as the less adsorbed. 



Lead salts on filtering through paper are retained by the surface of 

 the paper and account for loss in the concentration of the solutions. 



(4) Adsorption by Different Adsorbents. 



Though the adsorbents may differ in active surface they adsorb the 

 same relative amount of substance : thus if A adsorbs more X than 

 Y, B also adsorbs more X than Y. 



(5) Reactions accompanying Adsorption. 



Chemical reactions may occur at the same time as adsorption, e.g. : 



Alumina adsorbs the acid of congo red at the ordinary temperature 



without chemical reaction as seen by the colour, which is blue ; on 



warming, chemical reaction takes place ; the alumina becomes red in 



colour, the colour of the salts of congo red. 



(6) Effect of Adsorption on Extraction by Solvents. 



If there is adsorption of one substance by another, repeated extrac- 

 tions must be made to separate them. 



(7) Filtration of Particles through Sand, etc. 



The sand particles having a negative charge will retain a definite 

 quantity of positively charged colloids, such as colloidal ferric hydroxide 

 and some dye-stuffs. This is apparently due only to the discharge of 

 the electric charges on the particles. 



