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Potassium manganate



Potassium manganate
General
Systematic name Potassium manganate(VI)
Molecular formula K2MnO4
Molar mass 197.13 g/mol
Appearance dark green crystals
CAS number [10294-64-1]
Properties
Density and phase 2.78 g/cm3, solid
Solubility in water highly soluble
Melting point 190 °C
Acidity (pKa) 7.1
Structure
Coordination
geometry
tetrahedral anion
Crystal structure isomorphous with K2SO4
Hazards
MSDS External MSDS
Main hazards oxidizer
NFPA 704
R/S statement R: 8-36/37/38
S: 17-26-36/37/39
Supplementary data page
Structure and
properties
n, εr, etc.
Thermodynamic
data
Phase behaviour
Solid
Spectral data UV λmax (ε) = 610 nm
(1500 M-1cm-1)
IR νMn-O = 830 cm1-
Related compounds
Related compounds KMnO4
MnO2
K2CrO4
K2FeO4
Except where noted otherwise, data are given for
materials in their standard state (at 25 °C, 100 kPa)
Infobox disclaimer and references

Potassium manganate is the chemical compound with the formula K2MnO4. This green salt is an intermediate in the industrial synthesis of potassium permanganate, a common chemical. Occasionally, potassium manganate and potassium permanganate are confused, but they are different compounds with distinctly different properties.

Additional recommended knowledge

Contents

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.[1]

Synthesis

The industrial route entails treatment of MnO2 with air:

MnO2 + 2 KOH + ½ O2 → K2MnO4 + H2O

The transformation gives a green-colored melt. In fact, 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 laboratory, K2MnO4 can be synthesized by heating a solution of KMnO4 in concentrated KOH solution followed by cooling to give green crystals:[2]

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

This reaction illustrates the relatively rare role of hydroxide as a reducing agent. Solutions of K2MnO4 are generated by allowing a solution of KMnO4 in 5-10 M KOH to stir for a day at room temperature followed by removal of MnO2, which is insoluble. 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:

KMnO4 + KI → K2MnO4 + ½ I2

The conversion is signaled by the color change from purple, characteristic of permanganate, to the green color of manganate. This reaction also illustates the fact 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.

Reactions

At lower pH's, the manganate ion will disproportionate to permanganate ion and manganese dioxide:

3 K2MnO4 + 2 H2O → 2 KMnO4 + MnO2 + 4 KOH

The colorful nature of this reaction 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 Manganate

Literature cited

  1. ^ Palenik, G. J. “Crystal Structure of Potassium Manganate” Inorganic Chemistry 1967, Volume 6, pp 507 - 511. DOI: 10.1021/ic50049a01
  2. ^ Nyholm, R. S.; Woolliams, P. R. "Manganates" Inorganic Syntheses, 1986 volume XI, pages = 56–61

Other references

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

See category for a list.

 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Potassium_manganate". A list of authors is available in Wikipedia.
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