Electrokinetic behaviour and interaction with oxalic acid of different hydrous chromium(III) oxides

Three samples of hydrous chromium(III) oxide have been prepared by differen t procedures, and their electrokinetic mobilities have been measured. In mi neral media, isoelectric points were found to be 8.45 +/- 0.15. This value is shifted from the solution isoelectric point by -0.35 +/- 0.15, due to th e contribution of solvation energy to the energy of ionic adsorption. On th e basis of a simple model, it is suggested that the two successive surface acidity constants of hydrous chromium(III) oxide are pK(a1)(S) = 7.17 and p K(a2)(S) = 9.72. The influence of oxalic acidity on mobility may be described in terms of tw o successive adsorption equilibria, the first one conducive to the neutrali zation of positive protonic charge, and the second one originating a charge reversal. It is shown that the apparent affinity for this latter mode is l ower in one of the samples, suggesting that surface complexation constants may in fact be sensitive to the history of hydrous chromium(III) oxide part icles. The three samples show similar dissolution behaviour but the specific rates differ, in one case by two orders of magnitude. This difference is explain ed in terms of the influence of crosslinking on the reactivity.