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Sodium pyrosulfite, also known as sodium metabisulfite, sodium dithiopentanoate, or sodium disulfide, is an inorganic salt compound with the chemical formula Na₂S₂O₅ and a molecular weight of 190.10. It appears as a white or slightly yellow crystalline powder or small crystals with a strong sulfur dioxide odor. Its density is 1.4 g/cm³. It is soluble in water and glycerol, slightly soluble in alcohol, and its solubility in water increases with temperature. Sodium pyrosulfite is a strong reducing agent; it reacts with elemental iodine and methanol to form sulfur dioxide, sodium iodide, and monomethyl sulfate. When dissolved in water, sodium pyrosulfite forms stable sodium bisulfite, and its aqueous solution is acidic. It reacts with sulfuric acid to release sulfur dioxide, and reacts with caustic soda or soda ash to form sodium sulfite. Sodium pyrosulfite is easily oxidized to sodium sulfate when exposed to air and decomposes when heated to 150°C.
It is used in the production of sodium hydrosulfite, sulfadiazine, aminopyrine, caprolactam, etc.; and for the purification of chloroform, sulfone, and benzaldehyde. In the photographic industry, it is used as an ingredient in fixing agents; in the fragrance industry, it is used to produce vanillin; in the brewing industry, it is used as a preservative; in rubber coagulants and as a dechlorinating agent after bleaching cotton fabrics; as an organic intermediate; in printing and dyeing, and leather making; as a reducing agent; in electroplating, oilfield wastewater treatment, and as a mineral processing agent; and in food processing, it is used as a preservative, bleaching agent, and leavening agent.