The membrane potentials of γ-MnO2 and Mn3O4, which were electrochemically prepared on Pt mesh, were measured in neutral aqueous solutions of KCl, KNO3, NaCl and Na2SO4 at 25°C to examine the transport numbers of the electrolyte ions in the Mn oxides. In these solutions, the cation transport numbers, t+, in γ-MnO2 were 0.62∼0.73, which were larger than those (0.38∼0.51) in the bulk solutions. On the other hand, the anion transport numbers, t–, in Mn3O4 were 0.62∼0.79, which were larger than those (0.49∼0.62) in the bulk solutions. The above results indicated that γ-MnO2 was cation-selective, whereas Mn3O4 was anion-selective. The difference in selective ion permeability between γ-MnO2 and Mn3O4 was explained in terms of the difference in sign of surface electric charge, depending on zero point of charge (zpc) of Mn oxides and pH of the used solutions.
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