Ferrous potassium cyanide, commonly known as the Yellow blood salt, is the chemical formula for K4[FE (CN) 6] 3H2O coordination compounds. At room temperature, the lemon-yellow single oblique crystal is decomposed at boiling point. It is insoluble in ethanol, but the solubility in water is as high as 300g/l. Ferrous cyanide aqueous solution reacts with acid to emit extremely poisonous cyanogen hydrogen (HCN) gas, but ferrous cyanide has low toxicity. Ferrous potassium cyanide is heated to decompose to obtain potassium cyanide, which reacts with trivalent iron ions to produce pigment Prussian blue. In mainland China, ferrous cyanide is used as a salt additive, although officials say it is usually harmless, but many consumers are not at ease. It should be noted that ferrous cyanide is a food additive (anti caking agent) approved by the EU, and the maximum allowable dosage is 20mg/kg (with anhydrous ferrous cyanide) in the 95/2/EC directive.