Potassium ferri oxalate , 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.
Monoclinic green crystals. Loses 3H
2
O
at 100C, decomposes at 230C. Soluble in water and
acetic acid; incompatible with alkali and ammonia,
since these react to precipitate ferric hydroxide.
The ferrioxalate complex exhibits 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).
Photography and blueprinting.
The complex can be synthesized from the reaction between iron (III) sulfate, barium oxalate and potassium oxalate :
Fe2(SO4)3 + 3 BaC2O4 + 3 K2C2O4 →
2K3[Fe(C2O4)3] + 3BaSO4
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 crystallizes out.
In solution, the ferrioxalate complex undergoes photoreduction. In this process, the complex absorbs a photon of light and subsequently decomposes to form Fe(C2O4)22- and CO2. The iron centre is reduced (gains an electron) from the +3 to the +2 oxidation state while an oxalate ion is oxidised to carbon dioxide :
2 [Fe(C2O4)3]3- + hν → 2 [Fe(C2O4)2]2- + C2O4 2- + 2CO2.
