Kinetics of reductive dissolution of colloidal manganese dioxide

The temporal characteristics of electron transfer to and proton uptake by colloidal manganese oxides in acidic aqueous solutions have been examined in the time regime of milliseconds to tens of seconds. Time-resolved conductometric measurements coupled with electron-transfer-rate results indicate that electron deposition into the colloidal oxides and protonation occurs simultaneously; i.e., there is no polarization of the colloid during the reduction process. Reduction of the manganese oxides leads to their dissolution with quantitative amounts of Mn(II) produced in the bulk aqueous phase. A detailed examination of both thermal- and radiation-induced dissolution of the metal oxides indicates that Mn(III) centers present in the colloid are continually depleted to produce Mn(II). The driving force for this reaction is the chemical potential gradient between the colloid and the aqueous solution due to the homogeneous nature of mixed Mn(IV)/Mn(III) oxides. Equilibrium between the solution and the colloid is established in both acidic and alkaline solution by either conproportionation or disproportionation of Mn(III) depending only on pH or pMn.