A radiochemical study of the kinetics of exchange between manganese oxides and some cations in solution

The kinetics of the exchange between⁵⁶Mn-labelled manganese dioxide and cations in aqueous solution was studied by measuring the β⁻ activity acquired by the solution. The results of the exchange between a chemical γ MnO₂ and a divalent M²⁺ ion (M=Mn, Co, Cu or Zn) or a trivalent M³⁺ ion (M=Ga, Fe, In, Rh or Al) indicate a fast initial process followed by a diffusion—controlled exchange. It is assumed that M²⁺ ions exchange with Mn²⁺ ions and M³⁺ ions exchange with Mn³⁺ ions in MnO₂. The process depends on the radii of the host and substituent ions and on consideration of crystal field stabilisation energies. It seems that the γ MnO₂ studied contains more Mn³⁺ than Mn²⁺ ions. The possibility of the exchange between Mn ions and cations of a different charge cannot be ruled out. The exchange between Co²⁺ ions and MnO₂ was enhanced in presence of pyrophosphate, which stabilises Mn(III) as a complex. The fraction of Mn in different samples of MnO₂ exchanged with a given cation depends on the type and not on the surface area of the sample.