Ferrocyanide
Anion in which a Fe2+ ion is complexed by 6 CN– ions
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Through Cyanide, Coordination complex and Diamagnetism
Reactions
Treatment of ferrocyanide with ferric-containing salts gives the intensely coloured pigment Prussian blue (sometimes called ferric ferrocyanide and ferrous ferricyanide).
Ferrocyanide can be reversibly oxidized by one electron, giving ferricyanide:
- [Fe(CN)6]4− ⇌ [Fe(CN)6]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.
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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. 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 (anodic oxidation) of urea. Aspirational applications range from hydrogen production for cleaner energy with lower CO2 emission to wastewater treatment.
Ferrocyanide is also studied as an electrolyte in flow batteries.
Nomenclature
According to the recommendations of IUPAC, 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."