Potassium manganate

Chembox new
Name = Potassium manganate
IUPACName = Potassium manganate(VI)
Section1 = Chembox Identifiers
CASNo = 10294-64-1
Section2 = Chembox Properties
Formula = K₂MnO₄
MolarMass = 197.13 g/mol
Appearance = dark green crystals
Density = 2.78 g/cm³, solid
Solubility = highly soluble
MeltingPt = 190 °C
pKa = 7.1
Section3 = Chembox Structure
Coordination = tetrahedral anion
CrystalStruct = isomorphous with K₂SO₄
Section7 = Chembox Hazards
ExternalMSDS =
MainHazards = oxidizer
RPhrases = 8-36/37/38
SPhrases = 17-26-36/37/39
Section8 = Chembox Related
OtherCpds = KMnO₄
MnO₂
K₂CrO₄
K₂FeO₄

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 "per"manganate are confused, but they are different compounds with distinctly different properties.

tructure

K₂MnO₄ is a salt, consisting of K⁺ cations and MnO₄^2- 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. [cite journal | author = Palenik, G. J. | title = Crystal Structure of Potassium Manganate | journal = Inorg. Chem. | year = 1967 | volume = 6 | pages = 507–511 | doi = 10.1021/ic50049a01]

ynthesis

The industrial route entails treatment of MnO₂ with air::MnO₂ + 2 KOH + ½ O₂ → K₂MnO₄ + H₂OThe 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: [cite journal | doi = 10.1002/9780470132425.ch11 | author = Nyholm, R. S.; Woolliams, P. R. | title = Manganates(VI) | journal = Inorg. Synth. | year = 1968 | volume = 11 | pages = 56–61] :4 KMnO₄ + 4 KOH → 4 K₂MnO₄ + O₂ + 2 H₂OThis 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 I₂The 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 KOHThe 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.ref|leesee also Manganate

Literature cited

Other references

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

See for a list.

External links

* [http://www.npi.gov.au/database/substance-info/profiles/52.html National Pollutant Inventory - Manganese and compounds Fact Sheet]