White or yellow crystalline powder or small crystals, with strong SO2smell, specific gravity is 1.4, soluble in water, aqueous solution is acid, contact with strong acid will emit SO2 and generate the corresponding salts, for a long time in air will oxidization to Na2S2O4. Exceed 1500 will decomposition SO2. Usage: 1. For industry it used as organic synthesis, printing, leather, pharmaceutical..