Potassium manganate
Names
IUPAC names
Potassium manganate
Potassium manganate(VI)
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.030.592 Edit this at Wikidata
EC Number
  • 233-665-2
UNII
  • Key: XKYFHUXZPRFUTH-UHFFFAOYSA-N
  • InChI=1S/2K.Mn.4O/q2*+1;;;;2*-1
  • [O-][Mn](=O)(=O)[O-].[K+].[K+]
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
Occupational safety and health (OHS/OSH):
Main hazards
oxidizer
GHS labelling:
GHS03: OxidizingGHS07: Exclamation mark
Warning
H272, H315, H319, H335
P210, P220, P221, P261, P264, P271, P280, P302+P352, P304+P340, P305+P351+P338, P312, P321, P332+P313, P337+P313, P362, P370+P378, P403+P233, P405, P501
NFPA 704 (fire diamond)
Related compounds
Related compounds
Potassium permanganate
Manganese dioxide
Potassium chromate
Potassium ferrate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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 each compound's properties are distinct.

Structure and bonding

K2MnO4 is a salt, consisting of K+ cations and MnO2−4 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. The compound is paramagnetic, owing to the presence of one unpaired electron on the Mn(VI) center.

Synthesis

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

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

The transformation gives a green-colored melt. Alternatively, instead of using air, potassium nitrate can be used as the oxidizer:

2 KOH + KNO3 + MnO2 → K2MnO4 + H2O + KNO2

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:

2 KMnO4 → K2MnO4 + MnO2 + O2

This reaction is a laboratory method to prepare oxygen, but produces samples of potassium manganate contaminated with MnO2. The former is soluble and the latter is not.

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.[1]

See also

Sources

  1. ^ a b c Reidies, Arno H. (2002). "Manganese Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a16_123. ISBN 978-3527306732.
  2. ^ Palenik, G. J. (1967). "Crystal Structure of Potassium Manganate". Inorg. Chem. 6 (3): 507–511. doi:10.1021/ic50049a015.
  3. ^ Nyholm, R. S.; Woolliams, P. R. (1968). "Manganates(VI)". Inorganic Syntheses. Vol. 11. pp. 56–61. doi:10.1002/9780470132425.ch11. ISBN 978-0-470-13242-5. ((cite book)): |journal= ignored (help)