The influence of hydrogen-ion concentration upon the adsorption of weak electrolytes by pure charcoal.—Part II

Abstract

In a recent communication results were described which indicate that propionic, hexoic, and succinic acids are adsorbed by purified charcoal only as unionised molecules. There was no evidence that anions of these acids were adsorbed to any measurable extent. Thus the amount of these acids adsorbed from mixed solutions of any one acid and its sodium salt is propor­tional to the amount of unionised acid present as calculated from the known ionisation constants and the hydrogen-ion concentration of the solutions. Further work has led to the conclusion that the presence of an unionised carboxyl group is essential for adsorption to take place. On the other hand, the adsorption curve for the bases n -propylamine and n -butylamine at different hydrogen-ion concentrations did not follow the ionisation curves very closely.Strong preferential adsorption of the unionised molecule was observed but there was quite considerable adsorption from solutions of such acidity that no unionised amine could exist in them. The adsorption of both bases was found to fall off gradually and continuously with increasing acidity from p H 11 to P H 3. In view of the fact that the charcoal used in this work was Norit charcoal purified by treatment with strong halogen acids, it was thought possible that very small traces of these acids remained after the washing to which the char­coal was subjected, and that these traces of acid caused the adsorption of basic ions by direct chemical combination.