Sodium ferrioxalate is a chemical compound with the formula . It is also called sodium oxalatoferrate or sodium trisoxalatoferrate. The compound is a

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...

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 conve ...

s, , and

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

cations . The anion is a

transition metal complex A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...

consisting of an

iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...

atom in the +3 oxidation state and three bidentate

oxalate Oxalate (IUPAC: ethanedioate) is an anion with the formula C2O42−. This dianion is colorless. It occurs naturally, including in some foods. It forms a variety of salts, for example sodium oxalate (Na2C2O4), and several esters such as dimethyl ...

ions anions acting as ligands. The ferrioxalate anion is sensitive to light and higher-energy

electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) li ...

, which causes the decomposition of one oxalate to

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

and reduction of the iron(III) atom to iron(II).

Properties

Solubility

This compound is very soluble in hot water, (182 parts per 100 parts solvent by mass), but a lot less soluble in cold water, (32 parts per 100 parts solvent), about the solubility of sodium chloride. It is not appreciably soluble in ethanol or ethanol water mixtures which are more than 50% ethanol by mass. It is somewhat more soluble in water than the corresponding potassium salt.

Preparation

Sodium ferrioxalate can be obtained by mixing solutions of

sodium oxalate Sodium oxalate, or disodium oxalate, is the sodium salt of oxalic acid with the formula Na2C2O4. It is a white, crystalline, odorless solid, that decomposes above 290 °C. Disodium oxalate can act as a reducing agent, and it may be used as a pr ...

and

ferric oxalate Ferric oxalate, also known as iron(III) oxalate, is a chemical compound composed of ferric ions and oxalate ligands; it may also be regarded as the ferric salt of oxalic acid. The anhydrous material is pale yellow; however, it may be hydrated to f ...

, and waiting a few hours for the brown colour of the ferric oxalate to be replaced with the green colour of the complex anion. : 3 + → 2 The equilibrium is attained only slowly at room temperature. The product can then be crystallized by evaporating the solution at just below boiling until small crystals appear, then allowing it to cool. The product may also be precipitated by adding methanol or

ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...

to the solution. Some decomposition of the ferric oxalate may occur during the process, resulting in the canary-yellow insoluble

iron(II) oxalate Ferrous oxalate, or iron(II) oxalate, is an inorganic compound with the formula FeC2O4 where is typically 2. These are orange compounds, poorly soluble in water. Structure The dihydrate FeC2O4 is a coordination polymer, consisting of chains of ...

. Small amounts of

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3%� ...

may be added to keep the iron in the 3+ oxidation state.

Isomerism

Theoretically the two stereoisomers could be separated by crystallization of a diastereomeric salt of the optically inactive racemic mixture of ferrioxalate ions with an optically active cation, such as Methyl-ethyl-propyl-ammonium ion which is one pure enantiomer. Thus Methyl-ethyl-propyl-ammonium ferrioxalate should crystallize out to produce crystals which are non superimposable mirror images. These would be Λ-Methyl-ethyl-propyl-ammonium Λ-ferrioxalate and Λ-Methyl-ethyl-propyl-ammonium Δ-ferrioxalate.

Photoreduction

Some samples of the crystals were exposed to direct sunlight for a few hours, the larger crystals did not appear to be affected, however solutions and small crystals so exposed did change colour to a different shade of green.

References