Dissolution kinetics of MgO in aqueous, acidic media

Abstract

The method of the rotating disc was used to study the kinetics of MgO dissolution. Single crystals of MgO with the orientation {100}, {101}, and {111} were dissolved in N2-saturated HClO4-NaClO4 solutions of constant ionic strength (I=1.0 mol kg−1). Chemical, mixed and diffusional control of the reaction leading to dissolution has been found between 25 to 90°C and 0.5<pH<3. The dissolution at 90°C andpH⩾3 occurred diffusionally controlled with respect to H+ ions. Independent measurements of the limiting current densities for the cathodic reduction of H+ ions were carried out in solutions of the same composition and confirmed this kind of control. In the chemical control region the dissolution rates were proportional to the Mg2+ ion densities of the single crystal surfaces investigated. The dissolution rates relative to {111}, at 40°C andpH=1, were found to be: 1.0±0.1 (1.0) {111}; 1.3±0.2 (1.2) {101} and 1.7±0.2 (1.7) {100}. Figures in parentheses refer to the relative Mg2+ ion densities of the surface. Reproducible results of high quality can only be obtained when “fresh” dislocations are removed by chemical polishing.