Potassium hypomanganate explained

Potassium hypomanganate is the inorganic compound with the formula . 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.^[1]

Properties

Potassium hypomanganate is oxidized in water to potassium manganate:^[2]

2 K₃MnO₄ + H₂O + 0.5 O₂ → 2 KOH + 2 K₂MnO₄However, 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.^[3]

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;^{[4] [5]}

• by the single-electron reduction of potassium manganate with hydrogen peroxide in 10 M potassium hydroxide solution;^[6]

• by the single-electron reduction of potassium manganate with mandelate in 3–10 M potassium hydroxide solution;

• by disproportionation when manganese dioxide is dissolved in a concentrated solution of potassium hydroxide;^[4]

The compound is unstable due to the tendency of the hypomanganate anion to disproportionate in all but the most alkaline solutions.^{[4] [5]}

Notes and References

1. Kenneth Pisarczyk . Manganese Compounds . Kirk-Othmer Encyclopedia of Chemical Technology . 2005 . 10.1002/0471238961.1301140716091901.a01.pub2 . en.
2. Diploma. Tupec. Josef. 2001. Interoperational analytical control during the production of potassium permanganate.
3. H. Peters . K.-H. Radeke . L. Till . Über das thermische Verhalten von Kaliummanganat(V), -(VI) und -(VII) . Zeitschrift für anorganische und allgemeine Chemie . 1966 . 346 . 1-2 . 1-11 . 10.1002/zaac.19663460102 . About the thermal behavior of potassium manganate(V), -(VI) and -(VII) . de.
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