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Potassium ferrioxalate
Potassium ferrioxalate
IUPAC name
Other names Potassium ferrioxalate

Potassium trioxalatoferrate (III)

CAS number 5936-11-8
Molecular formula K3[Fe(C2O4)3]


Molar mass 437.20 g/mol - anhydrous potassium trioxalatoferrate (III)

491.25 g/mol - hydrated potassium trioxalatoferrate (III)

Appearance emerald green hydrated crystals
Density 2.13 g/cm3, solid
Melting point

230 °C, 503 K, 446 °F

Solubility in water  ?M (20 °C)
Dipole moment 0 D
R-phrases R20 R21 R22 R34 R36 R37 R38
Related compounds
Related compounds Fe(C2O4)2
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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) sulfate, 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 crystallizes 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)[1].



In solution the ferrioxalate complex is decomposed by light. The complex reacts with a photon of light to form Fe(C2O4)22- and CO2. Iron is reduced (gains an electron) from the +3 oxidation state to +2. This process is called photoreduction:

2 [Fe(C2O4)3]3- + → 2 [Fe(C2O4)2]2- + C2O42- + 2 CO2

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)22- ions.


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


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