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 In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...

consisting of ferrioxalate

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

s, , and

potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

cations . The anion is a transition metal oxalate complex consisting of an

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

atom in the +3 oxidation state and three bidentate

oxalate Oxalate (systematic IUPAC name: ethanedioate) is an anion with the chemical formula . This dianion is colorless. It occurs naturally, including in some foods. It forms a variety of salts, for example sodium oxalate (), and several esters such as ...

ligands In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ...

. Potassium is 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 salt is available in anhydrous formA. 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. as well as a trihydrate. The ferrioxalate anion is quite stable in the dark, but it is decomposed by light and high-energy

electromagnetic radiation In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength ...

Preparation

The complex can be synthesized by the reaction between

iron(III) sulfate Iron(III) sulfate or ferric sulfate (British English: sulphate instead of sulfate) is a family of inorganic compounds with the formula Fe2(SO4)3(H2O)n. A variety of hydrates are known, including the most commonly encountered form of "ferric sulf ...

, 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.

Structure

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 . The ferrioxalate complex displays

helical chirality In chemistry, axial chirality is a special case of chirality in which a molecule contains two pairs of chemical groups in a non-planar arrangement about an axis of chirality so that the molecule is not superposable on its mirror image. The axis of ...

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). 2-isomers-of-ferrioxalate

Reactions

Photoreduction

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

X-rays An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

and

gamma rays A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...

. Absorption of a photon causes the decomposition of one oxalate ion to

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

and reduction of the iron(III) atom to iron(II). This photo-sensitive property is used for chemical actinometry, the measure of luminous flux, and for preparation of

blueprint A blueprint is a reproduction of a technical drawing or engineering drawing using a contact print process on light-sensitive sheets introduced by Sir John Herschel in 1842. The process allowed rapid and accurate production of an unlimited number ...

s. 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.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. At 296 °C, the anhydrous salt decomposes into the iron(II) complex potassium ferrioxalate, potassium oxalate, and

: :

Uses

Photometry and actinometry

The discovery of the efficient

photolysis Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by absorption of light or photons. It is defined as the interaction of one or more photons wi ...

of the ferrioxalate anion was a landmark for chemical

photochemistry Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet (wavelength from 100 to 400 Nanometre, nm), visible ligh ...

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

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.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.

Blueprints

Before the ready availability of wide ink-jet and

laser printer Laser printing is an electrostatic digital printing process. It produces high-quality text and graphics (and moderate-quality photographs) by repeatedly passing a laser beam back and forth over a Electric charge, negatively charged cylinder call ...

s, large-size engineering drawings were commonly reproduced by the

cyanotype The cyanotype (from , and , ) is a slow-reacting, photographic printing formulation sensitive to a limited near-ultraviolet and blue light spectrum, the range 300 nm to 400 nm known as UVA radiation. It produces a monochrome, blu ...

method. That was a simple contact-based photographic process that produced a "negative" white-on-blue copy of the original drawing—a

. 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 ferric citrate, but potassium ferrioxalate is also used.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. 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.

References

{{Potassium compounds Iron complexes Iron(III) compounds Potassium compounds Oxalato complexes Ferrates