Potassium manganate

Potassium manganate is the inorganic compound with the formula K2MnO4. This green-colored salt is an intermediate in the industrial synthesis of potassium permanganate (KMnO4), a common chemical.[1] Occasionally, potassium manganate and potassium permanganate are confused, but they are different compounds with distinctly different properties.

Potassium manganate
Names
IUPAC name
Potassium manganate(VI)
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.030.592
EC Number
  • 233-665-2
UNII
Properties
K2MnO4
Molar mass 197.132 g/mol
Appearance green crystals (darkening with time)[1]
Density 2.78 g/cm3, solid
Melting point 190 °C (374 °F; 463 K) (decomposition)
decomposes
Acidity (pKa) 7.1
Structure
isomorphous with K2SO4
tetrahedral anion
Hazards
Main hazards oxidizer
R-phrases (outdated) R8 R36/37/38
S-phrases (outdated) S17 S26 S36/37/39
NFPA 704 (fire diamond)
Related compounds
Related compounds
 KMnO4
MnO2
K2CrO4
K2FeO4
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

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.[2] It is isostructural with potassium sulfate.

Synthesis

The industrial route entails treatment of MnO2 with air:[1]

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:[3]

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

This reaction illustrates the relatively rare role of hydroxide as a reducing agent. 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
See also: Manganate

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.

Literature cited
  1. Reidies, Arno H. (2002). "Manganese Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a16_123.
  2. Palenik, G. J. (1967). "Crystal Structure of Potassium Manganate". Inorg. Chem. 6: 507–511. doi:10.1021/ic50049a015.
  3. Nyholm, R. S.; Woolliams, P. R. (1968). "Manganates(VI)". Inorg. Synth. Inorganic Syntheses. 11: 56–61. doi:10.1002/9780470132425.ch11. ISBN 978-0-470-13242-5.

Other references
  • Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.

See category for a list.

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