Potassium ferrioxalate

Potassium ferrioxalate is a chemical compound with the formula K
3[Fe(C
2O
4)
3]. It often occurs as the trihydrate K
3[Fe(C
2O
4)
3]·3H2O. Both are crystalline compounds, lime green in colour.,[2] also known as potassium trisoxalatoferrate or potassium tris(oxalato)ferrate(III).[2]

Potassium ferrioxalate
Names
IUPAC name
Potassium iron(III) oxalate
Other names
potassium ferrioxalate
potassium trisoxalatoferrate(III)
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.035.398
EC Number
  • 238-954-7
UNII
Properties
K
3[Fe(C
2O
4)
3] (anhydrous)
K
3[Fe( C
2O
4)3]·3H
2O (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) the trihydrate loses 3H2O at 113 °C[1]
Structure
octahedral
0 D
Hazards
Main hazards Corrosive. Eye, respiratory and skin irritant.
R-phrases (outdated) R20, R21, R22, R34, R36/37/38
Related compounds
Other anions
 Sodium ferrioxalate
Related compounds
 Iron(II) oxalate
Iron(III) oxalate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

The compound is a salt consisting of ferrioxalate anions, [Fe(C
2O
4)
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
2O2−
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
2O
4)
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.

Preparation

The complex can be synthesized by the reaction between iron(III) sulfate, barium oxalate and potassium oxalate:[3]

Fe
2(SO
4)
3 + 3 BaC
2O
4 + 3 K
2C
2O
4 → 2 K
3[Fe(C
2O
4)
3] + 3 BaSO
4

The reactants are combined in water, the solid precipitate of BaSO
4 is removed, and the green trihydrate crystallizes from the cooled solution.

Structure

The structures of the trihydrate and of the anhydrous salt have been extensively studied.[4] 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).[5]

Reactions

Photoreduction

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).[6]

Thermal decomposition

The trihydrate loses the three water molecules at the same time when heated at 113 °C.[1]

At 296 °C, the anhydrous salt decomposes into the iron(II) complex potassium ferrooxalate, potassium oxalate, and carbon dioxide:[1]

2 K
3[Fe(C
2O
4)
3] → 2 K
2[Fe(C
2O
4)
2] + K
2C
2O
4 + 2 CO
2

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.

Uses

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.[6][7]

Chemistry education

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.[8]

Blueprints

Before the ready availability of wide ink-jet and laser printers, large-size engineering drawings were commonly reproduced by the cyanotype method.

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.

The complex used in cyanotype is mainly ammonium iron(III) citrate, but potassium ferrioxalate is used too.[9][10]

See also

A number of other iron oxalates are known

References
  1. J. Ladriere (1992): "Mössbauer study on the thermal decomposition of potassium tris (oxalato) ferrate(III) trihydrate and bis (oxalato) ferrate(II) dihydrate". Hyperfine Interactions, volume 70, issue 1, pages 1095–1098. doi:10.1007/BF02397520
  2. A. Saritha, B. Raju, M. Ramachary, P. Raghavaiah, and K. A. Hussain (2012) "Synthesis, crystal structure and characterization of chiral, three-dimensional anhydrous potassium tris(oxalato)ferrate(III)", Physica B: Condensed Matter, volume 407, issue 21, pages 4208-4213. doi:10.1016/j.physb.2012.07.005
  3. Bailar, Jr., John C.; Jones, Eldon M. (1939). Trioxalato Salts (Trioxalatoaluminiate, -ferriate, -chromiate, and -cobaltiate). Inorg. Synth. Inorganic Syntheses. 1. pp. 35–38. doi:10.1002/9780470132326.ch13. ISBN 9780470132326.
  4. Junk, Peter C. (2005). "Supramolecular interactions in the X-ray crystal structure of potassium tris(oxalato)ferrate(III) trihydrate". J. Coord. Chem. 58 (4): 355–361. doi:10.1080/00958970512331334250.
  5. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  6. Hatchard, C. G.; Parker, C. A. (1956). "A new sensitive chemical actinometer. II. Potassium ferrioxalate as a standard chemical actinometer". Proceedings of the Royal Society of London. 235 (1203): 518–36. Bibcode:1956RSPSA.235..518H. doi:10.1098/rspa.1956.0102. S2CID 98652159.CS1 maint: uses authors parameter (link)
  7. Pozdnyakov, Ivan P.; Kel, Oksana V.; Plyusnin, Victor F.; Grivin, Vyacheslav P.; Bazhin, Nikolai M. (2008). "New Insight into Photochemistry of Ferrioxalate". J. Phys. Chem. A. 112 (36): 8316–8322. Bibcode:2008JPCA..112.8316P. doi:10.1021/jp8040583. PMID 18707071.
  8. John Olmsted (1984): "Preparation and analysis of potassium tris(oxalato)ferrate(III)trihydrate: A general chemistry experiment". Journal of Chemical Education, volume 61, issue 12, page 1098. doi:10.1021/ed061p1098
  9. 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
  10. 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.
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