Prussian Blue: Artists' Pigment and Chemists' Sponge

The accidental discovery over 300 years ago of the artists' pigment Prussian blue, iron(III) hexacyanoferrate(II), opened up a whole new area of chemistry—that of the cyanide radical. The variable composition of Prussian blue has tantalized chemists, until investigations by X-ray crystallography in the late 20th century determined its structure. The open 'zeolytic' lattice can act as host to small molecules and ions, making the substance useful as an antidote to certain poisons, notably thallium and radiocaesium. The redox chemistry of the 'mixed valence' iron atoms in Prussian blue confers the property of electrochromism on the solid, which can therefore serve in electronic digital displays. Photochemical production of the pigment is the basis for the cyanotype or blueprint reprographic process. Inclusion of redox-active species in the Prussian blue lattice can 'fine-tune' its color, by shifting the electronic charge transfer band in the visible absorption spectrum. Thus, photographic artists unknowingly employ coordination chemistry to tone the hues of their cyanotypes.