Ferricyanide

Ferricyanide refers to the ferricyanide anion, [Fe(CN)6]3-, a coordination complex in which iron is in the +3 oxidation state and is octahedrally coordinated by six cyanide ligands. The complex is typically paired with alkali metals or other cations to form salts such as potassium ferricyanide (K3[Fe(CN)6]) and sodium ferricyanide (Na3[Fe(CN)6]). In solution the complex is stable and is used widely in chemistry as a colorant and oxidizing agent.

Chemically, ferricyanide is a mild oxidant that accepts electrons to form ferrocyanide, [Fe(CN)6]4-. The ferricyanide/ferrocyanide redox couple has a standard potential near +0.36 V versus the standard hydrogen electrode, making it a standard reference mediator in electrochemical and biochemical experiments. The complex is relatively inert under neutral to basic conditions, though it can release cyanide under strongly acidic or harsh conditions.

Preparation typically involves oxidation of ferrocyanide salts, such as K4[Fe(CN)6], with suitable oxidants (for example, chlorine, hypochlorite, or permanganate). In practice, ferricyanide is valued for its reversibility as a redox mediator and for use in analytical methods, including redox titrations and enzyme assays where it serves as an external electron acceptor.

Applications extend across electrochemistry, analytical chemistry, and biochemistry. The ferricyanide/ferrocyanide couple is employed as a convenient, well-behaved redox system and as an oxidant in various assays. In inorganic chemistry, ferricyanide participates in reactions such as the formation of Prussian blue upon reduction and complexation with iron species.

Safety considerations include recognizing that, although ferricyanide salts have relatively low toxicity, the cyanide ligands warrant caution under acidic or strongly caustic conditions. Handle with standard laboratory safety practices and store in a dry, cool place.