The surface chemistry of hydrous manganese dioxide

An experimental investigation has shown that H+ and OH− are potential determining ions for the δMnO2 surface. The pH (ZPC) was determined using electrophoretic mobilities and Na+ and K+ adsorption and found to be 2.25. Alkalimetric titration curves failed to provide a direct determination of the pH (ZPC), however, when combined with the Na+ adsorption data they provide a means for estimating the surface charge. Surface charge values of approximately 100 μ C/cm2 were found at pH 8, considerably higher than the charge on SiO2 which has a similar pH (ZPC). Below pH 3.5 manganese was released to solution and the experimental data suggest that this is due to the reduction of MnO2. δMnO2 is thermodynamically unstable with respect to H2O below that pH.