Prussian blue
In histology, potassium ferricyanide is used to detect ferrous iron in biological tissue. Potassium ferricyanide reacts with ferrous iron in acidic solution to produce the insoluble blue pigment, commonly referred to as Turnbull's blue or Prussian blue. To detect ferric (Fe3+) iron, potassium "ferrocyanide" is used instead in the Perls' Prussian blue staining method. The material formed in the Turnbull's blue reaction and the compound formed in the Prussian blue reaction are the same.