Potassium ferrioxalate is a chemical compound with the formula . It often occurs as the trihydrate. Both are crystalline compounds, lime green in colour., also known as potassium trisoxalatoferrate or potassium trisferrate. The compound is a saltconsisting 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 bidentateoxalate 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, 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 sulfate, barium oxalate and potassium oxalate: The reactants are combined in water, the solid precipitate of is removed, and the green trihydrate crystallizes from the cooled solution. Another common synthesis is reacting aqueous iron chloride hexahydrate and potassium oxalate monohydrate.
Structure
The structures of the trihydrate and of the anhydrous salt have been extensively studied. which indicates that the Fe 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. Its mirror image with the right-handed screw axis is given the Greek symbol Δ.
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 and reduction of the iron atom to iron.
Thermal decomposition
The trihydrate loses the three water molecules at the same time when heated at 113 °C. At 296 °C, the anhydrous salt decomposes into the iron complex potassium ferrooxalate, potassium oxalate, and carbon dioxide: 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.
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 complex that turned it into an insoluble iron version in areas of the paper that were exposed to light. The complex used in cyanotype is mainly ammonium iron citrate, but potassium ferrioxalate is used too.
