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Potassium ferrate

Names
IUPAC name Potassium ferrate(VI)
Other names Potassium ferrate Dipotassium ferrate
Properties
Chemical formula	K2FeO4
Molar mass	198.0392 g/mol
Appearance	Dark purple solid
Density	2.829 g/cm, solid
Melting point	>198 °C (decomposition temp)
Solubility in water	soluble in 1M KOH
Solubility in other solvents	reacts with most solvents
Structure
Crystal structure	K2SO4 motif
Coordination geometry	Tetrahedral
Dipole moment	0 D
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	oxidizer
Flash point	non-combustible
Related compounds
Other anions	K2MnO4 K2CrO4 K2RuO4
Other cations	BaFeO4 Na2FeO4
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). Infobox references

Potassium ferrate is the chemical compound with the formula K₂FeO₄. This purple salt is paramagnetic, and is a rare example of an iron(VI) compound. In most of its compounds, iron has the oxidation state +2 or +3 (Fe or Fe). Reflecting its high oxidation state, FeO₄ is a powerful oxidizing agent.

K₂FeO₄ has attracted interest for applications in "green chemistry" because the by-products of its use, iron oxides, are environmentally innocuous. In contrast, some related oxidants such as chromate are considered environmentally hazardous. However, the main difficulty with the use of K₂FeO₄ is that it is often too reactive, as indicated by the fact that it decomposes in contact with water.

4 K₂FeO₄ + 4 H₂O → 3 O₂ + 2 Fe₂O₃ + 8 KOH

Synthesis and structure

Georg Ernst Stahl (1660 – 1734) first discovered that the residue formed by igniting a mixture of potassium nitrate (saltpetre) and iron powder dissolved in water to give a purple solution. Edmond Frémy (1814 – 1894) later discovered that fusion of potassium hydroxide and iron(III) oxide in air produced a compound that was soluble in water. The composition corresponded to that of potassium manganate. In the laboratory, K₂FeO₄ is prepared by oxidizing an alkaline solution of an iron(III) salt with concentrated chlorine bleach.

The salt is isostructural with K₂MnO₄, K₂SO₄, and K₂CrO₄. The solid consists of K and the tetrahedral FeO₄ anion, with Fe-O distances of 1.66 Å. The poorly soluble barium salt, BaFeO₄, is also known.

Properties and applications

As a dry solid, K₂FeO₄ is stable. It decomposes with evolution of O₂ in neutral water, and especially rapidly in acidic water. At high pH, aqueous solutions are stable. The deep purple solutions are similar in appearance to potassium permanganate (KMnO
₄). It is stronger oxidizing agent than the latter.

Because the side products of its redox reactions are rust-like iron oxides, K₂FeO₄ has been described as a "green oxidant." It has been employed in waste-water treatment as an oxidant for organic contaminants and as a biocide. Conveniently, the resulting reaction product is iron(III) oxyhydroxide, an excellent flocculant.

In organic synthesis, K₂FeO₄ oxidizes primary alcohols.

K₂FeO₄ has also attracted attention as a potential cathode material in a "super iron battery."

References

1. Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
2. Schreyer, J. M.; Thompson, G. W.; Ockerman, L. T. "Potassium Ferrate(VI)" Inorganic Syntheses, 1953 volume IV, pages 164-168.
3. Hoppe, M. L.; Schlemper, E. O.; Murmann, R. K. "Structure of Dipotassium Ferrate(VI)" Acta Crystallographica 1982, volume B38, pp. 2237-2239. doi:10.1107/S0567740882008395.
4. Green, J. R. “Potassium Ferrate” Encyclopedia of Reagents for Organic Synthesis 2001, John Wiley. doi:10.1002/047084289X.rp212.

• Ferrates
• Potassium compounds