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Concept
Potassium ferrioxalate
Summary
Potassium ferrioxalate, also called potassium trisoxalatoferrate or potassium tris(oxalato)ferrate(III) is a chemical compound with the formula . It often occurs as the trihydrate . Both are crystalline compounds, lime green in colour.
The compound is a salt consisting of ferrioxalate anions, , and potassium cations . The anion is a transition metal complex consisting of an iron atom in the +3 oxidation state and three bidentate oxalate ions 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, , which appears fluorescent green in color.
The ferrioxalate anion is quite stable in the dark, but is decomposed by light and high-energy electromagnetic radiation. This photo-sensitive property is used for chemical actinometry, the measure of luminous flux, and for preparation of blueprints.
The complex can be synthesized by the reaction between iron(III) sulfate, barium oxalate and potassium oxalate:
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 which can then be filtered and the pure material can be crystallized.
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 high-energy electromagnetic radiation, including X-rays and gamma rays.
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