Potassium ferrioxalate

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Potassium ferrioxalate
Lime green crystals of potassium ferrioxalate trihydrate
Potassium ferrioxalate
IUPAC name
Potassium iron(III) oxalate
Other names
potassium ferrioxalate
potassium trisoxalatoferrate(III)
5936-11-8 YesY
5936-11-8 (trihydrate)
K3[Fe(C2O4)3] (anhydrous)
K3[Fe(C2O4)3]·3H2O (trihydrate)
Molar mass 437.20 g/mol (anhydrous)
491.25 g/mol (trihydrate)
Appearance emerald green hydrated crystals
Density 2.13 g/cm3
Melting point 230 °C (446 °F; 503 K)
Dipole moment 0 D
Main hazards Corrosive. Eye, respiratory and skin irritant.
R-phrases R20, R21, R22, R34, R36/37/38
Related compounds
Other anions
Sodium ferrioxalate
Related compounds
Iron(II) oxalate
Iron(III) oxalate
Except where noted otherwise, data is 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, i.e. the measure of light flux.


Crystals of potassium ferrioxalate trihydrate.

The complex can be synthesized by 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 combined in water, the solid precipitate of BaSO4 is removed, and the green trihydrate crystallizes from the cooled solution.

Another common synthesis is reacting aqueous iron(III) chloride hexahydrate and potassium oxalate monohydrate. Filter the solid using filter paper and a Büchner funnel and recrystallize the product to remove impurities.[2]

FeCl3•6H2O + 3 K2C2O4•H2O → K3[Fe(C2O4)3]•3H2O + 3 KCl + 6 H2O


The ferrioxalate complex has D3 molecular symmetry, within which the iron center is octahedral. The six Fe-O bond distances all close to 2.0 Å[3] which indicates that the Fe(III) is high spin; as the high spin complex whould display Jahn–Teller distortions. The ammonium and mixed sodium-potassium salts are isomorphous, as are related complexes with Al3+, Cr3+, and V3+. The ferrioxalate complex displays helical chirality as it can form two non-superimposable geometries. 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).[4]



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− + hv → 2 [Fe(C2O4)2]2− + 2 CO2 + C2O42−[5]

See also[edit]

A number of other iron oxalates are known


  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. ^ Prefomed by Benjamin J. Abram, OSU lab manual, Dr. Richard Nafshun
  3. ^ "Supramolecular interactions in the X-ray crystal structure of potassium tris(oxalato)ferrate(III) trihydrate" Junk, Peter C. Journal of Coordination Chemistry (2005), 58(4), 355-361. doi:10.1080/00958970512331334250
  4. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419. 
  5. ^ New Insight into Photochemistry of Ferrioxalate Ivan P. Pozdnyakov, Oksana V. Kel, Victor F. Plyusnin, Vjacheslav P. Grivin, and Nikolai M. Bazhin The Journal of Physical Chemistry A 2008 112 (36), 8316-8322