What Is Sodium Bisulfite?
Sodium bisulfite is an inorganic compound, commonly found as a mixture of two forms: sodium hydrogen sulfite (NaHSO3) and sodium bisulfite (also known as sodium pyrosulfite, Na2S2O5). It is sometimes referred to as acidic sodium sulfite or anhydrous sodium bisulfite. This compound appears as a white, crystalline powder with a slight sulfur odor. It is soluble in water but almost insoluble in ethanol. Its aqueous solutions are slightly acidic.
Uses of Sodium Bisulfite
Sodium bisulfite serves multiple roles, primarily as an antiseptic and bleaching agent in the food industry. Its applications extend to leather processing, dye and intermediate purification, photography, and as a reducing and bleaching agent. In wastewater treatment, detergents, and perfumes, it plays a crucial role. Additionally, in pharmaceuticals and cosmetics, it functions as an antioxidant. Its use in cosmetics is regulated for safe concentration levels. Furthermore, sodium bisulfite reacts with formaldehyde, a chemical known to cause sensitivity and contribute to sick building syndrome, especially in moist environments.
Properties of Sodium Bisulfite
Sodium bisulfite comprises hydrogen sulfite and bisulfite ions, each forming a salt with sodium. In its solid state, it is unstable and gradually oxidizes in air to sulfate, emitting sulfurous acid gas. Commonly sold as a concentrated aqueous solution (34% or higher), it decomposes upon heating, forming sulfurous acid gas and releasing aldehydes and adducts.
Other Information About Sodium Bisulfite
1. How Sodium Bisulfite Is Produced
Production of sodium bisulfite involves refined sulfur (S) and sodium hydroxide (NaOH). The process starts with roasting sulfur to produce sulfurous acid gas, which then reacts with a sodium hydroxide solution. The resultant clear, pale yellow solution is filtered, leading to the formation of sodium bisulfite. Concentration and crystallization processes further yield sodium hydrogen sulfite crystals, which are dehydrated to form sodium bisulfite powder.
2. Safety of Sodium Bisulfite
Sodium bisulfite, while nonflammable, releases toxic sulfur dioxide gas when thermally decomposed. It reacts with acids, producing sulfurous acid gas, and intense heat when combined with oxidants. Its use as a food additive is generally safe, but high concentrations in industrial applications necessitate careful handling to prevent environmental pollution.