Potassium hypomanganate
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Through Manganese, Redox and Potassium permanganate
Properties
Potassium hypomanganate is oxidized in water to potassium manganate:
- 2 K3MnO4 + H2O + 0.5 O2 → 2 KOH + 2 K2MnO4
However, it undergoes disproportionation in acidic solutions producing manganese dioxide and potassium permanganate.
In the absence of moisture, it is stable up to 900 °C. Above that temperature, it decomposes to potassium oxide, manganese(II,III) oxide, and oxygen.
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Preparative routes
The solid salt can be produced by the reaction of potassium carbonate and manganese carbonate in the presence of oxygen at 800 °C. However, in the industrial process of producing potassium permanganate, it is produced by fusing manganese dioxide and potassium hydroxide. The resulting hypomanganate further reacts with water to produce manganate.
A solution of potassium hypomanganate is produced:
- by two-electron reduction of potassium permanganate with excess potassium sulfite;
- MnO−4 + SO2−3 + H2O → MnO3−4 + SO2−4 + 2 H+
- by the single-electron reduction of potassium manganate with hydrogen peroxide in 10 M potassium hydroxide solution;
- 2 MnO2−4 + H2O2 + 2 OH− → 2 MnO3−4 + O2 + 2 H2O
- by the single-electron reduction of potassium manganate with mandelate in 3–10 M potassium hydroxide solution;
- 2 MnO2−4 + C8H7O−3 + 2 OH− → 2 MnO3−4 + C8H5O−3 + 2 H2O
- by disproportionation when manganese dioxide is dissolved in a concentrated solution of potassium hydroxide;
- 2 MnO2 + 3 OH− → MnO3−4 + MnOOH + H2O
The compound is unstable due to the tendency of the hypomanganate anion to disproportionate in all but the most alkaline solutions.