Potassium manganate

Structure

K₂MnO₄ is a salt, consisting of K⁺ cations and MnO₄²⁻ anions. X-ray crystallography shows that the anion is tetrahedral, with Mn-O distances of 1.66 Å, ca. 0.03 Å longer than the Mn-O distances in KMnO₄. It is isostructural with potassium sulfate.

Synthesis

The industrial route entails treatment of MnO₂ with air:

2 MnO₂ + 4 KOH + O₂ → 2 K₂MnO₄ + 2 H₂O

The transformation gives a green-colored melt. One can test an unknown substance for the presence of manganese by heating the sample in strong KOH in air. The production of a green coloration indicates the presence of Mn. This green color results from an intense absorption at 610 nm.

In the laboratory, K₂MnO₄ can be synthesized by heating a solution of KMnO₄ in concentrated KOH solution followed by cooling to give green crystals:

4 KMnO₄ + 4 KOH → 4 K₂MnO₄ + O₂ + 2 H₂O

This reaction illustrates the relatively rare role of hydroxide as a reducing agent. Solutions of K₂MnO₄ are generated by allowing a solution of KMnO₄ in 5–10 M KOH to stir for a day at room temperature followed by removal of MnO₂, which is insoluble. The concentration of K₂MnO₄ in such solutions can be checked by measuring their absorbance at 610 nm.

The one-electron reduction of permanganate to manganate can also be effected using iodide as the reducing agent:

2 KMnO₄ + 2 KI → 2 K₂MnO₄ + I₂

The conversion is signaled by the color change from purple, characteristic of permanganate, to the green color of manganate. This reaction also shows that manganate(VII) can serve as an electron acceptor in addition to its usual role as an oxygen-transfer reagent. Barium manganate, BaMnO₄, is generated by the reduction of KMnO₄ with iodide in the presence of barium chloride. Just like BaSO₄, BaMnO₄ exhibits low solubility in virtually all solvents.

An easy method for preparing potassium manganate in the laboratory involves heating crystals or powder of pure potassium permanganate. Potassium permanganate will decompose into potassium manganate, manganese dioxide and oxygen gas:

2KMnO₄ → K₂MnO₄ + MnO₂ + O₂

This reaction is a laboratory method to prepare oxygen, but produces samples of potassium manganate contaminated with MnO₂.

Reactions

Manganate salts readily disproportionate to permanganate ion and manganese dioxide:

3 K₂MnO₄ + 2 H₂O → 2 KMnO₄ + MnO₂ + 4 KOH

The colorful nature of the disproportionation has led the manganate/manganate(VII) pair to be referred to as a chemical chameleon. This disproportionation reaction, which becomes rapid when [OH−
] < 1M, follows bimolecular kinetics.^[1]