Prussian blue

Lore & Background

Prussian blue is a dark blue pigment, also known as Berlin blue, Brandenburg blue, or Parisian blue. It is produced by the oxidation of ferrous ferrocyanide salts and has the chemical formula of an iron(III) hexacyanoferrate(II) salt. In this compound, the cations contain iron in the +3 oxidation state, while the anions contain iron in the +2 oxidation state. The substance is prepared as a very fine colloidal dispersion because it is not soluble in water. Its appearance depends sensitively on the size of these colloidal particles, and it contains variable amounts of other ions. A chemically identical substance, Turnbull's blue, differs only in its impurities and particle sizes due to being made from different reagents, resulting in a slightly different color. Prussian blue was created in the early 18th century and is the first modern synthetic pigment. It is used in paints, became prominent in 19th-century Japanese woodblock prints, and is the traditional "blue" in technical blueprints. In medicine, it is used orally as an antidote for certain heavy metal poisonings, such as thallium(I) and radioactive caesium isotopes, exploiting its ion-exchange properties and high affinity for certain "soft" metal cations. It is on the World Health Organization's List of Essential Medicines. The pigment lent its name to prussic acid (hydrogen cyanide), derived from it. Prussian blue was significant as the first stable and relatively lightfast blue pigment widely used since the loss of knowledge of Egyptian blue, replacing expensive ultramarine from lapis lazuli. It was probably first synthesized by paint maker Johann Jacob Diesbach in Berlin around 1706, believed to have been accidentally created when he used potash tainted with blood to make a red cochineal dye; instead, the blood, potash, and iron sulfate reacted to form iron ferrocyanide, which has a distinct blue hue.

Reader's Guide

Prussian blue is significant as the first stable and relatively lightfast blue pigment widely used since the loss of knowledge regarding the synthesis of Egyptian blue. European painters had previously used indigo dye, smalt, Tyrian purple, and the extremely expensive ultramarine from lapis lazuli. Japanese painters and woodblock print artists likewise lacked a long-lasting blue pigment until they began to import Prussian blue from Europe. In medicine, Prussian blue is used as an antidote for heavy metal poisoning, exploiting its ion-exchange properties and high affinity for certain 'soft' metal cations. It is on the World Health Organization's List of Essential Medicines. The pigment lent its name to prussic acid (hydrogen cyanide), derived from it. From the beginning of the 18th century, Prussian blue was the predominant uniform coat color of the Prussian Army, and continued to be worn by most German soldiers for ceremonial occasions until World War I. Despite being prepared from cyanide salts, Prussian blue is not toxic because the cyanide groups are tightly bound to iron.

Did You Know?

• Prussian blue is used as an antidote for thallium(I) and radioactive caesium poisoning and is on the WHO List of Essential Medicines.
• The pigment lent its name to prussic acid (hydrogen cyanide), called Blausäure ('blue acid') in German.