Colloid-chemical properties of manganese dioxide

Abstract

A synthetic, poorly ordered manganese dioxide (Mn_(01.90)-MnO_(1.95)) has been studied with respect to colloidal stability and the ability to sorb or exchange Mn⁺², with concomitant release of H⁺ ions. MnO₂ suspensions of concentrations 10⁻⁵ to 10⁻³M were observed to exist as colloidal dispersions at neutral and alkaline pH values for periods of several months or more. Deductions based upon acid-base titrations of MnO₂ suspensions indicate that the zero point of charge with respect to net OH⁻ and H⁺ ions bound is somewhat below pH 3 and that H⁺ and OH⁻ function as potential-determining ions. Colloidal MnO₂ is destabilized by relatively small concentrations of Mn⁺² ions; H⁺ ions are released in the process. At pH 7.5 the capacity of the oxide for Mn⁺² sorption is 0.5 mole of Mn⁺² per mole of MnO₂; capacities of ca. 2 moles per mole are realized at pH values near 9. The uptake of Mn⁺² and release of H⁺ occurs rather rapidly, times of the order of 5 minutes or less being required for 50 % exchange. The observed nonstoichiometric products of oxidation of dilute Mn⁺² would seem to be interpreted satisfactorily as resulting from the uptake of Mn⁺² ions in varying proportions by hydrous manganese dioxide. The characteristics of colloidal MnO₂ appear to provide an instructive model for interpreting the behavior of a number of other colloidal hydrous metal oxides.