Description
Potassium ferri oxalate , is a chemical compound with the formula K
3[Fe(C
2O
4)
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, K
3[Fe(C
2O
4)
3]·3H
2O, are emerald green in color. In solution, the salt ionizes to give the ferrioxalate anion, [Fe(C
2O
4)
3]
3- , which appears fluorescent green in color. Potassium ferrioxalate is often used in chemical actinometry.
Chemical Properties
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.
Characteristics
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).
Uses
Photography and blueprinting.
Preparation
The complex can be synthesized from the reaction between iron (III) sulfate, barium oxalate and potassium oxalate :
Fe
2(SO
4)
3 + 3 BaC
2O
4 + 3 K
2C
2O
4 →
2K
3[Fe(C
2O
4)
3] + 3BaSO
4 The reactants are dissolved in water and heated for around 1.5 hours. BaSO
4 precipitates out leaving behind the newly formed complex in solution. The complex can then be obtained by filtering off the BaSO
4 and cooling the solution so that it crystallizes out.
Reactions
In solution, the ferrioxalate complex undergoes photoreduction. In this process, the complex absorbs a photon of light and subsequently decomposes to form Fe(C
2O
4)
22- and CO
2. 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(C
2O
4)
3]
3- + hν → 2 [Fe(C
2O
4)
2]
2- + C
2O
4 2- + 2CO
2.
