Potassium ferrate

Potassium ferrate

Names
IUPAC name Potassium ferrate(VI)
Other names Potassium ferrate Dipotassium ferrate
Identifiers
Properties
Chemical formula	K2FeO4
Molar mass	198.0392 g/mol
Appearance	Dark purple solid
Density	2.829 g/cm3, 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
Main hazards	oxidizer
R-phrases	8
S-phrases	17-36
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 [77 °F], 100 kPa).
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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 (Fe²⁺ or Fe³⁺). Reflecting its high oxidation state, FeO₄²⁻ is a powerful oxidizing agent.

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 FeO₄²⁻ anion, with Fe-O distances of 1.66 Å. The poorly soluble barium salt, BaFeO₄, is also known.

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, especially in acidic water: