Potassium manganate
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| Potassium manganate | |
|---|---|
| Image:Potassium manganate.jpg | |
| General | |
| Systematic name | Potassium manganate |
| 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 |
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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.
Contents |
[edit] 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]
[edit] 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 Mn by heating the sample in strong KOH in air. The production of a green coloration indicates the presence of Mn. This green color is intense, with an extinction coefficient of 1500 M-1cm-1 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:
- 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 permanganate 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.
[edit] 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/permanganate pair to be referred to as a "chemical chameleon." This disproportionation reaction, which becomes rapid when [OH-] < 1M, follows bimolecular kinetics. see also Manganate
[edit] Literature cited
- ^ Nyholm R.S. Woolliams P.R. (1986). ""Manganates(VI)"". Inorganic Syntheses XI: 56–61.
[edit] Other references
- Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
See category for a list.
