Potassium ferrioxalate

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Potassium ferrioxalate
Lime green crystals of potassium ferrioxalate trihydrate
Potassium ferrioxalate
CAS number 5936-11-8 YesY, 5936-11-8 (trihydrate)
Molecular formula K3[Fe(C2O4)3]


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

491.25 g/mol - potassium trioxalatoferrate (III) trihydrate

Appearance emerald green hydrated crystals
Density 2.13 g/cm3
Melting point 230 °C (446 °F; 503 K)
Dipole moment 0 D
R-phrases R20 R21 R22 R34 R36 R37 R38
Main hazards Corrosive. Eye, respiratory and skin irritant.
Related compounds
Related compounds Fe(C2O4)2
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

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.


Crystals of potassium ferrioxalate trihydrate.

The complex can be synthesized from the reaction between iron(III) sulfate, barium oxalate and potassium oxalate:[1]

Fe2(SO4)3 + 3 BaC2O4 + 3 K2C2O4 → 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 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).[2]



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:

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

See also[edit]


  1. ^ John C. Bailar, Eldon M. Jones "Trioxalato Salts (Trioxalatoaluminiate, -ferriate, -chromiate, and -cobaltiate)" Inorganic Syntheses, 1939, vol. 1, Pages: 35–38. doi:10.1002/9780470132326.ch13
  2. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419.