Acid‐Base Dissociation of Surface Hydroxyl Groups on Manganese Dioxide in Aqueous Solutions

The surface charge on in solutions was measured as a function of pH and ionic strength by titration. With increasing pH, the surface charge changed from positive to negative through zero at a certain pH (PZC). The absolute value of the charge increased with increasing ionic strength. The surface charge was interpreted in terms of the dissociation of acid and base surface hydroxyl groups on , that is an ion exchange: ,. The charged sites adsorb counterions electrostatically. The equilibrium conditions of these reactions were derived by considering a suppression of dissociation by the formed charged sites:, , where and are acid‐base dissociation constants and and are constants expressing the suppression of dissociation by the charged sites. The relationship between the surface charge, pH, and ionic strength was analyzed by a nonlinear least squares method with equations for the equilibrium and concentration conditions, and values for the constants were established. The suspension pH and point of zero charge of were derived as a function of these constants, and they were consistent with the measured values.