Sym: Orthophosphoric acid Phosphoric acid is a corrosive acid that can form three different classes of salts, namely primary phosphates, dibasic phosphates and tribasic phosphates. Phosphoric acid is soluble in water. It is incompatible with strong caustics and it is corrosive to ferrous metals and alloys. It readily reacts with metals to form flammable hydrogen gas. It is used as an anti-oxidant in food, as a flavour additive for sharp taste in food (jellies, preserves) and soft drinks, as a tang (Food Additive 338) and for the manufacture of yeasts and gelatine.