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.
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.
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).
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.