ferrocyanides

Ferrocyanides are a group of coordination compounds containing the hexacyanoferrate(II) anion, [Fe(CN)6]4-. The iron atom is in the +2 oxidation state and is surrounded by six cyanide ligands. The most common ferrocyanide is potassium ferrocyanide, K4[Fe(CN)6], also known as yellow prussiate of potash. It is a yellow crystalline solid that is soluble in water. Other alkali metal and alkaline earth metal ferrocyanides are also known, as are ammonium ferrocyanide. Ferrocyanides are generally considered to be non-toxic because the cyanide ligands are tightly bound to the iron atom and are not readily released as free cyanide ions.

Historically, ferrocyanides were produced from heating animal matter rich in nitrogen, such as blood, hair, or hooves, with potassium carbonate. This process yielded potassium ferrocyanide. They are used in various industrial applications. For instance, potassium ferrocyanide is used as an anti-caking agent in table salt and road salt, where it prevents the salt crystals from clumping together. It also finds use in the production of pigments, such as Prussian blue, which is an iron(III) hexacyanoferrate(II) complex. Additionally, ferrocyanides are employed in metallurgy and in the case-hardening of steel. The ferrocyanide ion can be oxidized to the ferricyanide ion, [Fe(CN)6]3-, which is the basis for some photographic processes and redox indicators.