Potassium ferrioxalate

Potassium ferrioxalate, is a chemical compound with the formula K₃[Fe(C₂O₄)₃], where iron is in the +3 oxidation state. It is an octahedral transition metal complex in which three bidentate oxalate ions are bound to an iron centre. Potassium acts as a counterion, balancing the 3- charge of the complex. Crystals of the trihydrated form of the complex, K₃[Fe(C₂O₄)₃].3H₂O, are emerald green in colour. In solution the complex dissociates to give the ferrioxalate anion, [Fe(C₂O₄)₃]^3-, which appears fluorescent green in colour. Potassium ferrioxalate is often used in chemical actinometry.

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Preparation

The complex can be synthesised from the reaction between iron(III) sulphate, barium oxalate and potassium oxalate:

Fe₂(SO₄)₃ + 3 Ba(C₂O₄) + 3 K₂(C₂O₄) → 2 K₃[Fe(C₂O₄)₃] + 3 BaSO₄

The reactants are dissolved in water and heated for around 1.5 hours. BaSO₄ precipitates out leaving behind the newly formed complex in solution. The complex can then be obtained by filtering off the BaSO₄ and cooling the solution so that it crystallises out.

Isomerism

The ferrioxalate complex demonstrates optical activity since there are two non-superimposable stereoisomers of the complex. In accordance with the IUPAC convention, the isomer with the left handed screw axis is assigned the Greek symbol Λ (lambda). Its mirror image with the right handed screw axis is given the Greek symbol Δ (delta)^[1].

Photoreduction

In solution the ferrioxalate complex is decomposed by light. The complex reacts with a photon of light to form Fe(C₂O₄)₂ and CO₂. Iron is reduced (gains an electron) from the +3 oxidation state to +2. This process is called photoreduction:

3 K₃[Fe(C₂O₄)₃] + hv → 2 Fe(C₂O₄)₂ + 4 K₂(C₂O₄) + 2 CO₂ + K[Fe(C₂O₄)₂]

If a solution of green K₃[Fe(C₂O₄)₃] is left in sunlight for a few hours it turns orange due to the formation of Fe(C₂O₄)₂.

References

1. ^ Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.