Adsorption of weak and non-electrolytes by activated carbon

Four activated carbons, one of them commercially available Darco active carbon and the remaining three from a series of coconut charcoals steam activated to varying degrees, are used to study the adsorption of weak and non-electrolytes. Introduction of —CI into the CH₃COOH molecule increases adsorption onto activated carbon while —OH and —NH₂ have the opposite effect. Substitution in the benzene ring shows that adsorption from aqueous solutions is in the order —NH₂, >—OH, >CO₂, >(—OH + —COOH). Effect of polarity of solvent on adsorption capacities is studied in H₂O, CHCl₃ and C₆H₆. Substitution of —Cl into the CH₃COOH molecule invariably increases the adsorption irrespective of the polarity of the solvent. On introducing a specific group in the benzene ring the adsorption is in the order: aniline ? phenol > benzoic acid > salicylic acid in H₂O medium but in CHCl₃ and C₆H₆ media aniline ? phenol > salicylic acid > benzoic acid. However, on comparing the results of individual adsorbates in three media, generally the magnitude of adsorption is H₂O > C₆H₆ ? CHCl₃. Adsorption of lower aliphatic acids (formic to caproic) from aqueous solutions increases regularly as one ascends the homologous series—a behaviour known as Traube's rule; however, of all the surface area available for N₂ adsorption, only a fraction of it is available for adsorption of the aliphatic acids.