THE CHEMOSTAT - A NOVEL-APPROACH TO THE SYNTHESIS OF MANGANESE-DIOXIDE

The development and use of the chemostat, an apparatus which controls pH, Eh and temperature during redox reactions, is described. Its appli cation is illustrated for the synthesis of the birnessite variety of m anganese dioxide, via 2(x-1)MnO4- + (7-2xMn2+ + (8-3x)H-2 <-> 5MnO(x) + 2(8-3xH+ where x is the oxygen index of the manganese dioxide. The c hemostat controls the pH and Eh of the precipitation reaction by the j udicious addition of KOH and KMnO4 solutions, respectively, as well as maintaining a constant temperature in the precipitation cell. Using t he control conditions outlined herein, the average errors associated w ith the pH, Eh and temperature were +/- 0.2 pH units, +/- 0.025 V and +/- 0.5 degrees C, respectively. The birnessite samples that were prod uced were characterized in terms of X-ray diffraction, x in MnOx and o pen circuit voltage measurements. These properties were found to have a strong dependence on the synthetic conditions.