Coagulation of colloidal manganese dioxide by divalent cations

The coagulation of colloidal MnO2 by several divalent cations, both in the absence and presence of a complexing ligand (l-histidine), has been studied. The efficiency as coagulating agents increases in the order Mg2+<Ni2+<Zn2+<Cu2+<Co2+<Mn2+. For a given cation, the coagulating efficiency increases with increasing pH and with increasing temperature, and decreases with increasing concentration of l-histidine. Apparent values for the equilibrium constants of complexation of divalent cations by l-histidine have been obtained. The apparent equilibrium constant of formation of the complex increases in the order Mg2+<Mn2+<Zn2+<Cu2+<Ni2+. The value corresponding to Co2+ could not be determined because l-histidine acted as a catalyst for the reduction of colloidal MnO2 by that cation.