Potassium hypomanganate

Potassium hypomanganate

Names
IUPAC names potassium manganate(V) potassium tetraoxidomanganate(3−)
Properties
Chemical formula	K3MnO4
Molar mass	236.23 g mol−1
Appearance	bright blue solid
UV-vis (λmax)	670 nm (ε = 900 dm3 mol−1 cm−1)
Related compounds
Other anions	Potassium manganate Potassium permanganate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Potassium hypomanganate, K₃MnO₄, also known as potassium manganate(V), is a bright blue salt and a rare example of a manganese(V) compound. It is formed:

• by the reduction of potassium permanganate with excess potassium sulfite;

MnO⁻
₄ + SO²⁻
₃ + H₂O → MnO³⁻
₄ + SO²⁻
₄ + 2 H⁺

• by the 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 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 H₂O

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

2 MnO₂ + 3 OH⁻ → MnO³⁻
₄ + MnO(OH) + H₂O

The hypomanganate anion is unstable with respect to disproportionation in all but the most alkaline of solutions: estimated electrode potentials at pH 14 are

MnO²⁻
₄ + e⁻ ⇌ MnO³⁻
₄   E = +0.27 V

MnO³⁻
₄ + e⁻ + 2 H₂O ⇌ MnO₂ + 4 OH⁻   E = +0.96 V

The disproportionation is believed to pass through a protonated intermediate, with the acid dissociation constant for the reaction HMnO²⁻
₄ ⇌ MnO³⁻
₄ + H⁺ being estimated as pK_a = 13.7 ± 0.2. However, K₃MnO₄ has been cocrystallized with Ca₂Cl(PO₄), allowing the study of the UV–visible spectrum of the hypomanganate ion.