Potassium perferrate, K₂FeO₄

Potassium perferrate, K₂FeO₄, may be prepared in a variety of ways. It results when chlorine acts on ferric hydroxide suspended in potassium hydroxide solution: -

2Fe(OH)₃ + 10KOH + 3Cl₂ = 2K₂FeO₄ + 6KCl + 8H₂O.

The chlorine in the above reaction may be replaced by ozone or sodium hypochlorite.

On adding a piece of potassium hydroxide to ferric chloride solution in a test-tube, together with a few drops of bromine, a brown mass is obtained which dissolves in water, yielding the intense red colour characteristic of potassium perferrate.

Potassium perferrate crystallises from the warm solution in microscopic black prisms or sharp pyramids, and is isomorphous with potassium sulphate, selenate, and chromate. The solution is stable, and may be kept for days without decomposition, particularly if a small quantity of a mineral salt is present. Acids, ammoniacal salts, and reducing agents readily effect its decomposition.