Potassium ferrioxalate

Potassium ferrioxalate, also known as potassium trisoxalatoferrate(III), 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 center. 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 color. In solution, the salt ionizes to give the ferrioxalate anion, [Fe(C2O4)3]3-, which appears fluorescent green in color. Potassium ferrioxalate is often used in chemical actinometry, i.e. the measure of light flux.
Preparation
The complex can be synthesized by the reaction between iron(III) sulfate, barium oxalate and potassium oxalate:

Fe2(SO
...