Potassium manganate is the
inorganic compound with the
formula . This green-colored salt is an intermediate in the industrial synthesis of
potassium permanganate
Potassium permanganate is an inorganic compound with the chemical formula KMnO4. It is a purplish-black crystalline salt, that dissolves in water as K+ and , an intensely pink to purple solution.
Potassium permanganate is widely used in the c ...
(), a common chemical.
Occasionally, potassium manganate and potassium ''per''manganate are confused, but these compounds's properties are distinct.
Structure and bonding
is a salt, consisting of
cations and
anion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
s.
X-ray crystallography
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
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 . 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 with air:
:2 MnO
2 + 4 KOH + O
2 → 2 K
2MnO
4 + 2 H
2O
The transformation gives a green-colored melt. Alternatively, instead of using air,
potassium nitrate
Potassium nitrate is a chemical compound with the chemical formula . This alkali metal nitrate salt is also known as Indian saltpetre (large deposits of which were historically mined in India). It is an ionic salt of potassium ions K+ and nitr ...
can be used as the oxidizer:
:
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, can be synthesized by heating a solution of
in concentrated KOH solution followed by cooling to give green crystals:
:
This reaction illustrates the relatively rare role of hydroxide as a reducing agent. The concentration of 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
An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. In everyday life, iodide is most commonly encountered as a component of iodized salt, which many governments mandate. Worldwide, iodine de ...
as the reducing agent:
:
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
Barium manganate is an inorganic compound with the formula BaMnO4. It is used as an oxidant in organic chemistry. It belongs to a class of compounds known as manganates in which the manganese resides in a +6 oxidation state. Manganate should not ...
, , is generated by the reduction of with iodide in the presence of
barium chloride
Barium chloride is an inorganic compound with the formula Ba Cl2. It is one of the most common water-soluble salts of barium. Like most other water-soluble barium salts, it is white, highly toxic, and imparts a yellow-green coloration to a flame. ...
. Just like
, 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:
:
This reaction is a laboratory method to prepare oxygen, but produces samples of potassium manganate contaminated with .
Reactions
Manganate salts readily
disproportionate to
permanganate
A permanganate () is a chemical compound containing the manganate(VII) ion, , the conjugate base of permanganic acid. Because the manganese atom is in the +7 oxidation state, the permanganate(VII) ion is a strong oxidizing agent. The ion is a tr ...
ion and
manganese dioxide:
:
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 [] < 1M, follows bimolecular kinetics.
See also
* :Manganates, Manganates for a list.
Sources
*Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. .
External links
National Pollutant Inventory - Manganese and compounds Fact Sheet
{{manganese compounds
Manganates
Potassium compounds