Potassium ferrioxalate

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Potassium ferrioxalate
Lime green crystals of potassium ferrioxalate trihydrate
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 Edit this at Wikidata
EC Number
  • 238-954-7
RTECS number
  • SZ3500000
UNII
UN number 3077
  • InChI=1S/3C2H2O4.Fe.K/c3*3-1(4)2(5)6;;/h3*(H,3,4)(H,5,6);;/q;;;+3;+1/p-2
    Key: VSRUWRBJHJVUDC-UHFFFAOYSA-L
  • O=C(C(=O)[O-]1)[O-][Fe+3]123([O-]C(C(=O)[O-]2)=O)[O-]C(C(=O)[O-]3)=O.[K+].[K+].[K+]
Properties
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 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[2]
Occupational safety and health (OHS/OSH):
Main hazards
 Corrosive. Eye, respiratory and skin irritant.
GHS labelling:
GHS07: Exclamation mark
Warning
H302, H312
P280, P301+P330+P331, P302+P353, P312, P330, P363, P403, P501
Related compounds
Other anions
 Sodium ferrioxalate
Related compounds
Iron(III) oxalate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Potassium ferrioxalate, also called potassium trisoxalatoferrate or potassium tris(oxalato)ferrate(III)

trihydrate K3[Fe(C2O4)3]·3H2O. Both are crystalline compounds, lime green in colour.[4]

The compound is a

ligands. Potassium is a counterion, balancing the −3 charge of the complex. In solution, the salt dissociates to give the ferrioxalate anion, [Fe(C2O4)3]3−, which appears fluorescent green in color. The salt is available in anhydrous form[3] as well as a trihydrate.[5]

The ferrioxalate anion is quite stable in the dark, but it is decomposed by light and high-energy electromagnetic radiation.

Preparation

The complex can be synthesized by the reaction between

potassium oxalate:[4]

Fe2(SO4)3 + 3 BaC2O4 + 3 K2C2O4 → 2 K3[Fe(C2O4)3] + 3 BaSO4

As can be read in the reference above, iron(III) sulfate, barium oxalate and potassium oxalate are combined in water and digested for several hours on a steam bath. Oxalate ions from barium oxalate will then replace the sulfate ions in solution, removing them as BaSO4 which can then be filtered and the pure material can be crystallized.

Structure

The structures of the trihydrate and of the anhydrous salt have been extensively studied.

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).[6]

Reactions

Photoreduction

The ferrioxalate anion is sensitive to light and to high-energy electromagnetic radiation, including

actinometry, the measure of luminous flux, and for preparation of blueprints
. This light-catalyzed redox reaction once formed the basis of some photographic processes. However due to their insensitivity and ready availability of advanced digital photography, these processes are obsolete.

Thermal decomposition

The trihydrate loses the three water molecules at 113 °C.[1]

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

2 K3[Fe(C2O4)3] → 2 K2[Fe(C2O4)2] + K2C2O4 + 2 CO2

Uses

Photometry and actinometry

The discovery of the efficient

actinometry. The potassium salt was found to be over 1000 times more sensitive than uranyl oxalate, the compound previously used for these purposes.[7][8]

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.[9]

Blueprints

Before the ready availability of wide

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 is based on the photolysis of an iron(III) complex which gets converted 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 also used.[10][11]

See also

A number of other iron oxalates are known

References

  1. ^
    doi:10.1007/BF02397520
  2. ^ "5936-11-8 - Potassium trioxalatoferrate(III) trihydrate - Potassium iron(III) oxalate - 31124 - Alfa Aesar". www.alfa.com.
  3. ^
    doi:10.1016/j.physb.2012.07.005
  4. ^
    ISBN 9780470132326
    .
  5. ^
    S2CID 216142329
    .
  6. ISBN 978-0-08-037941-8
    .
  7. ^
    S2CID 98652159
    .
  8. PMID 18707071
    .
  9. doi:10.1021/ed061p1098
  10. doi:10.1021/ed500809n
  11. ^ 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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