Ferrocyanide is the

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 ...

CN)₆">cyanide.html" ;"title="e(cyanide">CN)₆sup>4−. Salts of this coordination complex give yellow solutions. It is usually available as the salt potassium ferrocyanide, which has the formula K₄Fe(CN)₆. e(CN)₆sup>4− is a diamagnetic species, featuring low-spin iron(II) center in an octahedral ligand environment. Although many salts of

cyanide In chemistry, cyanide () is an inorganic chemical compound that contains a functional group. This group, known as the cyano group, consists of a carbon atom triple-bonded to a nitrogen atom. Ionic cyanides contain the cyanide anion . This a ...

are highly toxic, ferro- and ferricyanides are less toxic because they tend not to release free cyanide. It is of commercial interest as a precursor to the pigment Prussian blue and, as its potassium salt, an anticaking agent.

Reactions

Treatment of ferrocyanide with ferric-containing salts gives the intensely coloured pigment Prussian blue (sometimes called

ferric In chemistry, iron(III) or ''ferric'' refers to the chemical element, element iron in its +3 oxidation number, oxidation state. ''Ferric chloride'' is an alternative name for iron(III) chloride (). The adjective ''ferrous'' is used instead for i ...

ferrocyanide and

ferrous In chemistry, iron(II) refers to the chemical element, element iron in its +2 oxidation number, oxidation state. The adjective ''ferrous'' or the prefix ''ferro-'' is often used to specify such compounds, as in ''ferrous chloride'' for iron(II ...

ferricyanide). Ferrocyanide reversibly oxidized by one electron, giving ferricyanide: : e(CN)₆sup>4− ⇌ e(CN)₆sup>3− + e⁻ This conversion can be followed spectroscopically at 420 nm, since ferrocyanide has negligible absorption at this wavelength while ferricyanide has an extinction coefficient of 1040 M⁻¹ cm⁻¹.

Applications

The dominant use of ferrocyanides is as precursors to the Prussian blue pigments. Sodium ferrocyanide is a common anti-caking agent. Specialized applications involves their use as precipitating agents for production of citric acid and wine.

Research

Ferrocyanide and its oxidized product ferricyanide cannot freely pass through the

plasma membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

. For this reason ferrocyanide has been used as a probe of extracellular electron acceptor in the study of redox reactions in cells. Ferricyanide is consumed in the process, thus any increase in ferrocyanide can be attributed to secretions of reductants or transplasma membrane electron transport activity. Nickel ferrocyanide (Ni₂Fe(CN)₆) is also used as catalyst in

electro-oxidation Electro-oxidation (EO or EOx), also known as anodic oxidation or electrochemical oxidation (EC), is a technique used for wastewater treatment, mainly for industrial effluents, and is a type of advanced oxidation process (AOP). The most general layo ...

( anodic oxidation) of

urea Urea, also called carbamide (because it is a diamide of carbonic acid), is an organic compound with chemical formula . This amide has two Amine, amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest am ...

. Aspirational applications range from

hydrogen production Hydrogen gas is produced by several industrial methods. Nearly all of the world's current supply of hydrogen is created from fossil fuels. Article in press. Most hydrogen is ''gray hydrogen'' made through steam methane reforming. In this process, ...

for cleaner energy with lower CO₂ emission to

wastewater treatment Wastewater treatment is a process which removes and eliminates contaminants from wastewater. It thus converts it into an effluent that can be returned to the water cycle. Once back in the water cycle, the effluent creates an acceptable impact on ...

. Ferrocyanide is also studied as an electrolyte in flow batteries.Developing Organic Flow Batteries for Energy Storage (arpa-e.gov) http://arpa-e.energy.gov/sites/default/files/documents/files/HarvardFlowBattery_OPEN2012_ExternalProjectImpactSheet_FINAL.pdf

Nomenclature

According to the recommendations of

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

, ferrocyanide should be called "hexacyanidoferrate(II)". Cyanides as a chemical class were named because they were discovered in ferrocyanide. Ferrocyanide in turn was named in Latin to mean "blue substance with iron." The dye Prussian blue had been first made in the early 18th century. The word "cyanide" used in the name is from κύανος ''kyanos'', Greek for "(dark) blue."

Gallery

File:Pigment Berliner Blau.JPG, Fe4 e(CN)6 File:Ag4Fe(CN)6.jpg, Ag4Fe(CN)6 File:Nickel(II) ferrocyanide.jpg, Ni4 e(CN)6 File:Potassium hexacyanidoferrate(II).jpg, K4 e(CN)6

References