chemistry.... K3Fe(CN)6

potassium ferricyanide K3Fe(CN)6 is used to detect ferrous ion Fe2+
In this reaction, potassium ferricyanide reacts with ferrous ion in acidic solution to produce an insoluble blue pigment, which is the hexacyanoferrate(III) ion reacts with Fe2+ ions to form an intense blue complex ion commonly referred to as Turnbull's blue
Fe2+ + [Fe(CN)6] 3- ---> Fe[Fe(CN)6] -

potassium ferricyanide