Potassium iron(III) oxalate
3D model (JSmol)
|Molar mass||437.20 g/mol (anhydrous)|
491.25 g/mol (trihydrate)
|Appearance||emerald green hydrated crystals|
|Melting point||230 °C (446 °F; 503 K) the trihydrate loses 3H2O at 113 °C|
|Main hazards||Corrosive. Eye, respiratory and skin irritant.|
|R-phrases (outdated)||R20, R21, R22, R34, R36/37/38|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Potassium ferrioxalate is a chemical compound with the formula K
3]. It often occurs as the trihydrate K
3] · 3H2O. Both are crystalline compounds, lime green in colour. , also known as potassium trisoxalatoferrate or potassium tris(oxalato)ferrate(III).
The compound is a salt consisting of ferrioxalate anions, [Fe(C
3]3−, and potassium cations K+. The anion is a transition metal complex consisting of an iron atom in the +3 oxidation state and three bidentate oxalate ions C
4 anions acting as ligands. Potassium acts as a counterion, balancing the −3 charge of the complex. In solution, the salt dissociates to give the ferrioxalate anion, [Fe(C
3]3−, which appears fluorescent green in color.
The ferrioxalate anion is quite stable in the dark, but is decomposed by light and higher-energy electromagnetic radiation. This feature is exploited for chemical actinometry, the measure of light flux, and for the preparation of blueprints.
3 + 3 BaC
4 + 3 K
4 → 2 K
3] + 3 BaSO
The reactants are combined in water, the solid precipitate of BaSO
4 is removed, and the green trihydrate crystallizes from the cooled solution.
3 + 3 K
4 → K
3] + 3 KCl
The structures of the trihydrate and of the anhydrous salt have been extensively studied. which indicates that the Fe(III) is high spin; as the low spin complex would 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).
The ferrioxalate anion is sensitive to light and to higher-energy electromagnetic radiation, including X-rays and gamma rays. Absorption of a photon causes the decomposition of one oxalate to carbon dioxide CO
2 and reduction of the iron(III) atom to iron(II).
The trihydrate loses the three water molecules at the same time when heated at 113 °C.
- 2 K
3] → 2 K
2] + K
4 + 2 CO
This light catalyzed redox reaction once formed the basis of some photographic processes, however due to their insensitivity and the ready availability of digital photography these processes have become obsolete and all but forgotten.
Photometry and actinometry
The discovery of the efficient photolysis of the ferrioxalate anion was a landmark for chemical photochemistry and actinometry. The potassium salt was found to be over 1000 times more sensitive than uranyl oxalate, the compound previously used for these purposes.
The synthesis and thermal decomposition of potassium ferrioxalate is a popular exercise for high school, college or undergraduate university students, since it involves the chemistry of transition metal complexes, visually observable photochemistry, and thermogravimetry.
That was a simple contact-based photographic process that produced a "negative" white-on-blue copy of the original drawing -- a blueprint. The process based on the photolysis of an iron(III) complex that turned it into an insoluble iron(II) version in areas of the paper that were exposed to light.
A number of other iron oxalates are known
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- Performed by Benjamin J. Abram, OSU lab manual, Dr. Richard Nafshun
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- Pablo Alejandro Fiorito and André Sarto Polo (2015): "A New Approach toward Cyanotype Photography Using Tris-(oxalato)ferrate(III): An Integrated Experiment". Journal of Chemical Education, volume 92, issue 10, pages 1721–1724. doi:10.1021/ed500809n
- Mike Ware (2014): Cyanomicon - History, Science and Art of Cyanotype: photographic printing in Prussian blue. Online document at www.academia.edu, published by www.mikeware.co.uk, accessed on 2019-03-29.