Potassium hypomanganate

Potassium hypomanganate is the inorganic compound with the formula K3MnO4. Also known as potassium manganate(V), this bright blue solid is a rare example of a salt with the hypomanganate or manganate(V) anion, where the manganese atom is in the +5 oxidation state. It is an intermediate in the production of potassium permanganate and the industrially most important Mn(V) compound.

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.

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;
 * MnO4− + SO3(2−) + H2O → MnO4(3−) + SO4(2−) + 2 H+


 * by the single-electron reduction of potassium manganate with hydrogen peroxide in 10 M potassium hydroxide solution;
 * 2 MnO4(2−) + H2O2 + 2 OH− → 2 MnO4(3−) + O2 + 2 H2O


 * by the single-electron reduction of potassium manganate with mandelate in 3–10 M potassium hydroxide solution;
 * 2 MnO4(2−) + C8H7O3− + 2 OH− → 2 MnO4(3−) + C8H5O3− + 2 H2O


 * by disproportionation when manganese dioxide is dissolved in a concentrated solution of potassium hydroxide;
 * 2 MnO2 + 3 OH− → MnO4(3−) + MnOOH + H2O

The compound is unstable due to the tendency of the hypomanganate anion to disproportionate in all but the most alkaline solutions.