Ferrocyanide is the name of 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 convent ...

CN)₆.html" ;"title="cyanide.html" ;"title="e(cyanide">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 Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms. In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of a ...

are highly toxic, ferro- and

ferricyanide Ferricyanide is the anion e(CN)6sup>3−. It is also called hexacyanoferrate(III) and in rare, but systematic nomenclature, hexacyanidoferrate(III). The most common salt of this anion is potassium ferricyanide, a red crystalline material that ...

s are less toxic because they tend not to release free cyanide. It is of commercial interest as a precursor to the pigment

Prussian blue Prussian blue (also known as Berlin blue, Brandenburg blue or, in painting, Parisian or Paris blue) is a dark blue pigment produced by oxidation of ferrous ferrocyanide salts. It has the chemical formula Fe CN)">Cyanide.html" ;"title="e(Cyani ...

and, as its potassium salt, an

anticaking agent An anticaking agent is an additive placed in powdered or granulated materials, such as table salt or confectioneries, to prevent the formation of lumps (caking) and for easing packaging, transport, flowability, and consumption. Caking mechanisms ...

Reactions

Treatment of ferrocyanide with ferric-containing salts gives the intensely coloured pigment

(sometimes called

ferric In chemistry, iron(III) refers to the element iron in its +3 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe3+. The adjective ferric or the prefix ferri- is often used to sp ...

ferrocyanide and

ferrous In chemistry, the adjective Ferrous indicates a compound that contains iron(II), meaning iron in its +2 oxidation state, possibly as the divalent cation Fe2+. It is opposed to "ferric" or iron(III), meaning iron in its +3 oxidation state, such a ...

ferricyanide). Ferrocyanide reversibly oxidized by one electron, giving

: : 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

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 (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

. For this reason ferrocyanide has been used as a probe of extracellular electron acceptor in the study of

redox reaction Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

s in

cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...

. 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 Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recy ...

in electro-oxidation ( anodic oxidation) of

urea Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. Urea serves an important ...

. Aspirational applications range from

hydrogen production Hydrogen production is the family of industrial methods for generating hydrogen gas. As of 2020, the majority of hydrogen (∼95%) is produced from fossil fuels by steam reforming of natural gas and other light hydrocarbons, partial oxidation of h ...

for cleaner energy with lower CO₂ emission to

wastewater treatment Wastewater treatment is a process used to remove contaminants from wastewater and convert it into an effluent that can be returned to the water cycle. Once returned to the water cycle, the effluent creates an acceptable impact on the environm ...

. 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

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 ferrocyanide.jpg, K4 e(CN)6

References