Potassium ferricyanide  -  K3Fe(CN)6



Potassium ferricyanide, also called potassium hexacyanoferrate (III) is a common compound. It is used extensively in photography as part of image reducers and in many toners. Although this compound contains cyanide groups, it is only marginally toxic. The cyanide groups are coordinated to the iron very tightly and are released only on strong heating, strong UV-radiation or in fairly concentrated acids.