Prussian Blue

Prussian blue is a dark blue pigment with the idealized formula Fe₇(CN)₁₈. Another name for the color Prussian blue is Berlin blue or, in painting, Parisian blue. Turnbull's blue is the same substance but is made from different reagents.

Prussian blue was one of the first synthetic pigments. It is employed as a very fine colloidal dispersion, as the compound itself is not soluble in water. It is famously complex, owing to the presence of variable amounts of other ions and the sensitive dependence of its appearance on the size of the colloidal particles formed when it is made. The pigment is used in paints, and it is the traditional "blue" in blueprints.

In medicine, Prussian blue is used as an antidote for certain kinds of heavy metal poisoning (caesium and thallium).

Prussian blue lent its name to prussic acid, which was derived from it, and to ferrocyanide (originally meaning "blue compound of iron", from Latin ferrum and Greek κυανεος). As ferrocyanide is made of iron and CN ligands, reinterpreting the component "-cyanide" in the compound word produced the word "cyanide" for compounds containing the CN radical.