Prussian blue

Prussian blue (also known as Berlin blue, Brandenburg blue, Parisian and Paris blue) is a dark blue pigment produced by oxidation of ferrous ferrocyanide salts. It has the chemical formula Fe4[Fe(CN)6]3. It consists of Fe3+ cations, where iron is in the oxidation state of +3, and [Fe(CN)6]4− anions, where iron is in the oxidation state of +2, so, the other name of this salt is iron(III) hexacyanoferrate(II). Turnbull's blue is essentially identical chemically, excepting that it has different impurities and particle sizes—because it is made from different reagents—and thus it has a slightly different color. Prussian blue was created in the early 18th century and is the first modern synthetic pigment. It is prepared as a very fine colloidal dispersion, because the compound is not soluble in water. It contains variable amounts of other ions and its appearance depends sensitively on the size of the colloidal particles. The pigment is used in paints, it became prominent in 19th-century aizuri-e (藍摺り絵) Japanese woodblock prints, and it is the traditional "blue" in technical blueprints. In medicine, orally administered Prussian blue is used as an antidote for certain kinds of heavy metal poisoning, e.g., by thallium(I) and radioactive isotopes of caesium. The therapy exploits Prussian blue's ion-exchange properties and high affinity for certain "soft" metal cations. It is on the World Health Organization's List of Essential Medicines, the most important medications needed in a basic health system. Prussian blue lent its name to prussic acid (hydrogen cyanide) derived from it. In German, hydrogen cyanide is called Blausäure ('blue acid').