Bisulfite

Bisulfite refers to the bisulfite ion, HSO3−, the conjugate base of sulfurous acid (H2SO3). It is produced when sulfur dioxide dissolves in water and is partially neutralized, giving salts such as sodium bisulfite (NaHSO3) and potassium bisulfite (KHSO3). In aqueous solution, sulfurous acid equilibrates with bisulfite and sulfite depending on pH, with bisulfite predominating at mildly acidic to neutral conditions.

Chemically, bisulfite can act as a reducing agent and a nucleophile. It readily adds to carbonyl compounds such as aldehydes (and some ketones) to form stable bisulfite adducts. This reaction is exploited to protect aldehydes during analysis and to decolorize certain dyes. Bisulfite also scavenges reactive oxygen species, contributing to antioxidant effects in some food systems.

Applications and occurrence are widespread. In the food and beverage industry, bisulfites serve as preservatives and antioxidants, helping to prevent enzymatic browning and spoilage; allowed usage levels vary by jurisdiction. In winemaking, bisulfites help preserve aroma and color and act as antimicrobial agents. In the pulp and paper industry, sulfite pulping uses bisulfite solutions to break down lignin. In molecular biology, bisulfite treatment of DNA is a fundamental method for assessing DNA methylation: unmethylated cytosine converts to uracil, while methylated cytosine remains unchanged, enabling downstream sequencing to map methylation patterns.

Safety considerations include irritation to skin, eyes, and the respiratory tract, and some individuals exhibit sulfite sensitivity, with potential asthmatic or allergic reactions. Proper handling, ventilation, and waste disposal are advised.