Sodium sulfite is a colorless monoclinic crystal with good water solubility; its aqueous solution is alkaline. It oxidizes significantly in air, readily efflorescing and eventually transforming into sodium sulfate. At high temperatures, sodium sulfite loses its water of crystallization, and further heating may melt it into a mixture of sodium sulfide and sodium sulfate. Anhydrous sodium sulfite has a density of 2.633, and its oxidation rate is slower compared to its hydrated form; it does not readily change in dry air.
Sodium sulfite is mainly used as a reducing agent, preservative, and developer in the production of sodium thiosulfate. In the dyeing and printing industry, it acts as a deoxidizer and bleaching agent, helping to improve the scouring effect and fiber strength of cotton fabrics. In the photosensitive industry, sodium sulfite is used as a developer to promote image development. In the organic industry, it is a reducing agent in the production of m-phenylenediamine, 2,5-dichloropyrazolone, anthraquinone-1-sulfonic acid, 1-aminoanthraquinone, and sodium aminosalicylate, effectively preventing oxidation of semi-finished products.
Sodium sulfite is used in the paper industry as a lignin remover, in the textile industry as a stabilizer for synthetic fibers, and in the electronics industry for manufacturing photoresistors. It is also used in the water treatment industry for treating electroplating wastewater and drinking water. Its packaging and storage must be airtight to prevent contact with air and deterioration due to moisture. It should not be stored or transported together with oxidants, strong acids, or harmful and toxic substances.
Sodium sulfite is chemically reactive and a strong reducing agent. It reacts with sulfur dioxide to form sodium bisulfite, and reacts with strong acids to form the corresponding salts and release sulfur dioxide. Its quality standards are divided into industrial grade and photographic grade, following national standards GB/T 9005-88 and GB/T 10550-89, respectively. Sodium sulfite has a molecular weight of 126.04 and its structural formula is Na₂SO₃·7H₂O.
The production process of sodium sulfite generally includes sulfur dioxide absorption, neutralization with sodium bisulfite, and the preparation of anhydrous sodium sulfite crystals. Its characteristics include stable operation and high desulfurization efficiency, generally reaching 97%–98%, which does not decrease even with significant changes in exhaust gas load.
When using sodium sulfite, relevant safety regulations should be followed. In case of fire, water and various fire extinguishers can be used to extinguish it. Sodium sulfite is hygroscopic and should not be stored for long periods; during transportation, care should be taken to protect it from rain and direct sunlight.
