Kinetics of the exchange between MnO or Mn3O4 and some divalent cations in aqueous solution

The kinetics of the exchange between MnO or Mn₃O₄ and Co²⁺, Cu²⁺, Ni²⁺, or Zn²⁺ ions in solution, was determined by measuring the γ-activity of⁵⁶Mn acquired by the solution after shaking with the neutron irradiated solid. The results indicated a fast exchange followed by a slower apparently diffusion-controlled exchange. The exchange capacity increased in the series: Co²⁺<Ni²⁺<Cu²⁺=Zn²⁺ for MnO and Ni²⁺<Co²⁺<Zn²⁺<Cu²⁺ for Mn₃O₄. The trends could not be satisfactorily explained by the ionic radii or crystal field stabilisation-energies. In the case of MnO, the results were discussed in terms of the estimated standard enthalpy change of the exchange reaction.