Potassium manganate
Encyclopedia
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. Occasionally, potassium manganate and potassium permanganate are confused, but they are different compounds with distinctly different properties.
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. It is isostructural with potassium sulfate.
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:
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:
The conversion is signaled by the color change from purple, characteristic of permanganate, to the green color of manganate. This reaction also illustrates 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.
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.
, the manganate ion will disproportionate to permanganate
ion and manganese dioxide:
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 [] < 1M, follows bimolecular kinetics.
See category for a list.
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
with the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
K2MnO4. This green-colored salt is an intermediate in the industrial synthesis of potassium permanganate
Potassium permanganate
Potassium permanganate is an inorganic chemical compound with the formula KMnO4. It is a salt consisting of K+ and MnO4− ions. Formerly known as permanganate of potash or Condy's crystals, it is a strong oxidizing agent. It dissolves in water to give intensely purple solutions, the...
(KMnO4), a common chemical. Occasionally, potassium manganate and potassium permanganate are confused, but they are different compounds with distinctly different properties.
Structure
K2MnO4 is a salt, consisting of K+ cations and MnO42− anions. X-ray crystallographyX-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
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. It is isostructural with potassium sulfate.
Synthesis
The industrial route entails treatment of MnO2 with air:- 2 MnO2 + 4 KOH + O2 → 2 K2MnO4 + 2 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
Potassium permanganate
Potassium permanganate is an inorganic chemical compound with the formula KMnO4. It is a salt consisting of K+ and MnO4− ions. Formerly known as permanganate of potash or Condy's crystals, it is a strong oxidizing agent. It dissolves in water to give intensely purple solutions, the...
in concentrated KOH solution followed by cooling to give green crystals:
- 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
Iodide
An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...
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 illustrates 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
Barium chloride
Barium chloride is the inorganic compound with the formula BaCl2. It is one of the most common water-soluble salts of barium. Like other barium salts, it is toxic and imparts a yellow-green coloration to a flame. It is also hygroscopic....
. Just like BaSO4
Barium sulfate
Barium sulfate is the inorganic compound with the chemical formula BaSO4. It is a white crystalline solid that is odorless and insoluble in water. It occurs as the mineral barite, which is the main commercial source of barium and materials prepared from it...
, 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:
- 2KMnO4 → K2MnO4 + MnO2 + O2
This reaction is a laboratory method to prepare oxygen.
Reactions
At lower pHPH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
, the manganate ion will disproportionate to permanganate
Permanganate
A permanganate is the general name for a chemical compound containing the manganate ion, . Because manganese is in the +7 oxidation state, the permanganate ion is a strong oxidizing agent. The ion has tetrahedral geometry...
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 [] < 1M, follows bimolecular kinetics.
Other references
- Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
See category for a list.