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 each compound's properties are distinct.

Potassium manganate

Names
IUPAC names Potassium manganate Potassium manganate(VI)
Identifiers
CAS Number	10294-64-1 Y
3D model (JSmol)	Interactive image
ChemSpider	141385 Y
ECHA InfoCard	100.030.592
EC Number	233-665-2
PubChem CID	160931
UNII	5PI213D3US Y
CompTox Dashboard (EPA)	DTXSID10893107
InChI Key: XKYFHUXZPRFUTH-UHFFFAOYSA-N InChI=1S/2K.Mn.4O/q2*+1;;;;2*-1
SMILES [O-][Mn](=O)(=O)[O-].[K+].[K+]
Properties
Chemical formula	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)
Solubility in water	decomposes
Acidity (pKa)	7.1
Structure
Crystal structure	isomorphous with K2SO4
Coordination geometry	tetrahedral anion
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	oxidizer
GHS labelling:
Pictograms
Signal word	Warning
Hazard statements	H272, H315, H319, H335
Precautionary statements	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)	1 0 1 OX
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). N verify (what is YN ?) Infobox references

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.

See also

• Category:Manganates for a list.

Sources

1. 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)

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

External links

• National Pollutant Inventory - Manganese and compounds Fact Sheet