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.[2]

Potassium hypomanganate
Names
IUPAC names
potassium manganate(V)
potassium tetraoxidomanganate(3−)
Identifiers
3D model (JSmol)
ChemSpider
  • InChI=1S/3K.Mn.4O/q3*+1;-3;;;;
    Key: BABZALCEXXHDEO-UHFFFAOYSA-N
  • O=[Mn-3](=O)(=O)=O.[K+].[K+].[K+]
Properties[1]
K3MnO4
Molar mass 236.229 g·mol−1
Appearance bright blue solid
Density 2.78 g/cm3
Melting point 900 °C (1,650 °F; 1,170 K) (decomposes)
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

Properties

Potassium hypomanganate is oxidized in water to potassium manganate:[3]

2 K3MnO4 + H2O + 0.5 O2 → 2 KOH + 2 K2MnO4

However, it undergoes disproportionation in acidic solutions producing manganese dioxide and potassium permanganate.[3]

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.[4]

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.[3] 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.[2]

A solution of potassium hypomanganate is produced:

MnO−4 + SO2−3 + H2O → MnO3−4 + SO2−4 + 2 H+
2 MnO2−4 + H2O2 + 2 OH− → 2 MnO3−4 + O2 + 2 H2O
  • by the single-electron reduction of potassium manganate with mandelate in 3–10 M potassium hydroxide solution;[1]
2 MnO2−4 + C8H7O−3 + 2 OH− → 2 MnO3−4 + C8H5O−3 + 2 H2O
  • by disproportionation when manganese dioxide is dissolved in a concentrated solution of potassium hydroxide;[5]
2 MnO2 + 3 OH− → MnO3−4 + MnOOH + H2O

The compound is unstable due to the tendency of the hypomanganate anion to disproportionate in all but the most alkaline solutions.[5][6]

References
  1. Lee, Donald G.; Chen, Tao (1993), "Reduction of manganate(VI) by mandelic acid and its significance for development of a general mechanism of oxidation of organic compounds by high-valent transition metal oxides", J. Am. Chem. Soc., 115 (24): 11231–36, doi:10.1021/ja00077a023.
  2. Kenneth Pisarczyk (2005). "Manganese Compounds". Kirk-Othmer Encyclopedia of Chemical Technology. doi:10.1002/0471238961.1301140716091901.a01.pub2.
  3. Tupec, Josef (2001). Interoperational analytical control during the production of potassium permanganate (PDF) (Diploma thesis).
  4. H. Peters; K.-H. Radeke; L. Till (1966). "Über das thermische Verhalten von Kaliummanganat(V), -(VI) und -(VII)" [About the thermal behavior of potassium manganate(V), -(VI) and -(VII)]. Zeitschrift für anorganische und allgemeine Chemie (in German). 346 (1–2): 1–11. doi:10.1002/zaac.19663460102.
  5. Cotton, F. Albert; Wilkinson, Geoffrey (1980), Advanced Inorganic Chemistry (4th ed.), New York: Wiley, p. 746, ISBN 0-471-02775-8.
  6. Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 1221–22. ISBN 978-0-08-022057-4..
  7. Lee, Donald G.; Chen, Tao (1989), "Oxidation of hydrocarbons. 18. Mechanism of the reaction between permanganate and carbon-carbon double bonds", J. Am. Chem. Soc., 111 (19): 7534–38, doi:10.1021/ja00201a039.
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