Potassium ferrate

Potassium ferrate is the chemical compound with the formula K₂FeO₄. This purple, paramagnetic salt is a rare example of an Fe(VI) compound. In most of its compounds, iron has the oxidation state II (i.e., Fe²⁺) or III (Fe³⁺). Reflecting its high oxidation state, FeO₄^2- is a powerful oxidant. 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. The main difficulty with the use of K₂FeO₄ is that it is often too reactive, as indicated by the fact that it decomposes in water.^[1]

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Redox properties and applications

As a dry solid, K₂FeO₄ is stable. It decomposes with evolution of O₂ in neutral and especially rapidly in acidic water. At high pH, aqueous solutions are stable. The deep purple solutions are similar in appearance to potassium permanganate. K₂FeO₄ is a stronger oxidizing agent than KMnO₄.

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

In organic synthesis, K₂FeO₄ oxidizes primary alcohols.^[2]

It has also attracted attention as a potential cathode material in a "Super iron battery."

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 the compound produced was soluble in water corresponding to the composition of potassium manganate. In the laboratory, K₂FeO₄ is prepared by oxidizing an alkaline solution of an iron(III) salt with concentrated chlorine bleach.^[3]

The salt is isostructural with K₂MnO₄, K₂SO₄, and K₂CrO₄. The solid consists of K⁺ and the tetrahedral FeO₄^2- anion, with Fe-O distances of 1.66 Å.^[4] The poorly soluble barium salt, BaFeO₄, is also known.

References

1. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
2. ^ Green, J. R. “Potassium Ferrate” Encyclopedia of Reagents for Organic Synthesis 2001, John Wiley. doi:10.1002/047084289X.rp212.
3. ^ Schreyer, J. M.; Thompson, G. W.; Ockerman, L. T. "Potassium Ferrate(VI)" Inorganic Syntheses, 1953 volume IV, pages 164-168.
4. ^ 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.