ferrocyanide

Ferrocyanide refers to the coordination complex hexacyanoferrate(II), [Fe(CN)6]4−. The iron center is in the +2 oxidation state and is octahedrally coordinated by six cyanide ligands. The complex is highly stable and water-soluble. Ferrocyanide salts include potassium ferrocyanide K4[Fe(CN)6], sodium ferrocyanide Na4[Fe(CN)6], and ammonium ferrocyanide (NH4)4[Fe(CN)6]. The term contrasts with ferricyanide, [Fe(CN)6]3−, in which iron is in the +3 state.

Ferrocyanide can be prepared by reduction of ferricyanide or by metathesis reactions involving iron and cyanide under appropriate conditions. It is a fundamental member of the Prussian blue family of coordination compounds; ferric ferrocyanide forms the deep-blue pigment known as Prussian blue.

Applications of ferrocyanide salts are diverse. In electrochemistry, the ferrocyanide/ferricyanide redox couple is a classic, well-behaved, water-soluble system used in analytical chemistry and as a reference electrode in buffered solutions. Ferrocyanide salts are also used as anticaking agents in table salt and in some processed foods in certain regulatory regimes, with potassium ferrocyanide commonly cited as the yellow prussiate of potash.

Safety and handling: the cyanide ligands are tightly bound within the complex, and ferrocyanide salts typically exhibit low acute toxicity. However, under strong acidification or heating, cyanide can be released, so standard chemical safety precautions apply when handling or heating these materials.

See also: ferricyanide; Prussian blue.