Prussian blue is a blue pigment used in paints and formerly in blueprints. It has several different chemical names, these being iron (III) ferrocyanide, ferric ferrocyanide, iron (III) hexacyanoferrate (II), and ferric hexacyanoferrate.

The chemical formula is Fe₄[Fe(CN)₆]₃.

Prussian blue can also act as a chelating agent, and is used as a treatment for heavy metal poisoning. In particular, it is used for patients who have ingested radioactive caesium or thallium (also non-radioactive thallium).

As engineer's blue it is mixed with an oily material, and rubbed onto a metallic surface. This is then rubbed with another surface, and the removal of the pigment indicates the position of high-spots. Thus it can be used to indicate the flatness of a surface or the trueness of a bearing assembly.

Joseph Whitworth invented the first practical method of making and polishing accurate flat surfaces in 1830. This used engineer's blue and three trial surfaces. This led to an explosion of development of precision instruments using his flat surfaces as a basis for further construction of precise shapes.

Despite the presence of the cyanide ion, Prussian Blue, like other ferro- and ferri-cyanides, is not especially toxic due to the strong binding between the cyanide and ferrous or ferric (iron) ions. However, treatment with acids can liberate the cyanide in the form of hydrogen cyanide, which is extremely toxic.

Because of the above mentioned there is a controversy among pseudo-scientists trying to disprove the holocaust. It is a common argument that because the levels of cyanide (Prussian blue) are higher on the walls of the delousing chambers than the homicide chambers, the homicide chambers were not used to any extent. Anyone that understands the properties of iron bonded cyanide and cyanide gas can easily refute this.

"Prussian Blue" is also the name of both a white-power band consisting of twin preteen girls, and an English rock band that actually holds the copyright to the name.

See also:

• potassium ferrocyanide
• potassium ferricyanide

External links

• http://www.nlm.nih.gov/medlineplus/druginfo/uspdi/202737.html (note: this is not in the public domain, even though it is on a .gov site)
• The FDA's page on prussian blue: http://www.fda.gov/cder/drug/infopage/prussian_blue/Q&A.htm