Prussian blue

Article By:

Miller, Joel S. Department of Chemistry, University of Utah, Salt Lake City, Utah.

Last reviewed:January 2020

DOI:https://doi.org/10.1036/1097-8542.801240

The common name for a material prepared by either the addition of trivalent iron (Fe^III) to ferrocyanide [Fe(CN)₆]⁴⁻ or Fe^II to ferricyanide [Fe(CN)₆]³⁻ with the composition of Fe^III₄[Fe^II(CN)₆]₃ · xH₂O, where x ∼ 15. Prussian blue, also known as Turnbull's blue or ferric hexacyanoferrate, was discovered accidentally 300 years ago by a colonist named Diesbach of Berlin, and its structure has been determined by several diffraction methods. Prussian blue has a structure with each Fe^II bound to six C-bounded cyanides (CN⁻), while the nitrogens (N) are bonded to Fe^III; thus, it has a cubic structure with Fe^IICNFe^III linkages along each of the edges of the cube (see illustration). Owing to the presence of more Fe^III than Fe^II, some Fe^II sites are vacant and filled with water. The presence of iron in two different oxidation or valence states in Prussian blue makes it a mixed-valence compound. This gives Prussian blue its intense blue color and its application as a pigment. The reduction of Prussian blue results in the formation of Everitt's salt, K₂Fe^II[Fe^II(CN₆)].

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