Ferric ferrocyanide (CI 77510)

Ferric ferrocyanide, also known by the names Hexacyanoferric ferric nitrate, Ferric ferric cyanoferric nitrate, Ferric ferrocyanide, CI Pigment Blue 27, Berlin Blue, Hamburg Blue, Prussian Blue (CI 77520) or Iron Blue is a highly stable and insoluble blue dye with a strong hiding effect. Its chemical formula is KFe[Fe(CN)6] and it is a complex salt that is obtained by reacting ferric salts with hydrogen cyanide. It was one of the first synthetic blue dyes to be produced on a large scale, replacing more expensive natural products.

Berlin blue is widely used in many applications. It is most commonly used in the manufacture of paints and inks, as a component of some blue pigments in painting, but is also widely used in the cosmetics and pharmaceutical industries. In cosmetics, it is a component of eye shadows, lipsticks or nail polishes, where it adds a blue or violet colour. In the pharmaceutical industry it is often used to coat tablets. It is also one of the dyes used in the food industry for colouring food, where it is referred to as E131. However, it should be noted that its use in the food industry is controversial and banned in some countries, including the USA.