Kinetics of acid uptake by weak-base anion exchangers

Abstract

The sorption of acids by unifunctional weak-base anion exchangers of conventional gel type from solutions below about 0·1N in acid follows neither classical ion-exchange kinetics nor those appropriate to ion-exchange accompanied by reaction. In many such resins, the particle diameters of which ranged from 400 to 900 microns, the sorption of a number of acids has been found to occur by a chemically controlled process in which the rate of attainment of equilibrium is independent of particle size, but depends upon, amongst other things, the basic strength of the active group.

This behaviour contrasts with that found by earlier workers for multifunctional resins, and with that found in the present work for unifunctional macroporous resins, both of which show a conventional dependence of rate on particle size corresponding to particle-diffusion controlled kinetics at least down to about 0·001N.

Similar particle-diffusion controlled kinetics, characterised by apparent activation energies of ca. 3 kcal./mole, are shown by the unifunctional gel resins at higher acid concentrations at and above 0·1N. The apparent activation energies for the chemically controlled processes are 2 ½ to 3 times higher.

Change in the valence of the counter-ion affects rates of uptake on conventional gel resins markedly, while on macroporous resins the change is small. The importance of electrolyte invasion as a controlling factor in such processes is emphasised, and an approximate analysis of both particle-diffusion and chemically controlled kinetics has been attempted taking electrolyte invasion parameters into account.