2% of suitable food-grade anticaking, free flowing, or conditioning agents. Included in the 2%, salt may contain up to 13-mg/kg sodium ferrocyanide (YPS, yellow prussiate of soda) or up to 25 mg/kg of green ferric ammonium citrate (seldom used),, either of which prevents caking in table salt .
Sodium ferrocyanide is a chemical additive known as E 535 in the EU. It is added to road and food grade salt as an anticaking agent. When combined with iron, it converts to a deep blue pigment called Prussian blue. In photography, it is used for bleaching, toning, and fixing. It is used as a stabilizer for the coating on welding rods. In the petroleum industry, it is used for removal of mercaptans.