MANGANESE OXIDES AS A REDOX BUFFER OF NATURAL-WATERS

Potentials measured with platinum electrodes in solutions equilibrated with manganese oxides MnO1.33 - MnO1.99 at 20 degrees C depend on PH as Eh = 0.926 - 0.05787pH. This equation corresponds to the reaction M nO2 + H+ + e = = MnOOH occurring in the solid phase. The Eh-pH correla tion is independent of the ionic composition of the medium, the presen ce (or absence) of oxygen in water, and the concentration of humic aci ds (in the range 0.5-4 mg/l). The correlation is valid for natural iro n-manganese concretions and MnO2 : FeOOH (1 : 1 to 1 : 5) mechanical m ixtures, when the phase ratio solid : liquid remains less than S : L = 1 : 1000. A value of 129.7 +/- +/- 0.5 kcal/mol is recommended for th e Gibbs free energy of formation of beta-MnOOH. Of inorganic redox sea water systems, MnO2/MnOOH is the only couple whose Eh values agree wit h the platinum-electrode potentials measured in oxidized pelagic red c lays.