Potassium hypomanganate
# Potassium hypomanganate

Potassium hypomanganate is the inorganic compound with the formula $K_3MnO_4.$ 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 $K_3MnO_4$ + $H_2O$ + 0.5 $O_2$ → 2 KOH + 2 $K_2MnO_4$

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;

    MnO−4 + SO2−3 + $H_2O$ → 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 + $H_2O_2$ + 2 $OH^−$ → 2 MnO3−4 + $O_2$ + 2 $H_2O$
* by the single-electron reduction of potassium manganate with mandelate in 3–10 M potassium hydroxide solution;

    2 MnO2−4 + $C_8H_7O−3$ + 2 $OH^−$ → 2 MnO3−4 + $C_8H_5O−3$ + 2 $H_2O$
* by disproportionation when manganese dioxide is dissolved in a concentrated solution of potassium hydroxide;

    2 $MnO_2$ + 3 $OH^−$ → MnO3−4 + MnOOH + $H_2O$

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

