Bisulfites

Bisulfites are salts and esters of sulfurous acid, containing the bisulfite anion HSO3−. In aqueous solution, HSO3− is the predominant form at acidic to neutral pH, while at higher pH the sulfite ion SO3^2− becomes more prevalent. Common bisulfite salts include sodium bisulfite (NaHSO3) and potassium bisulfite (KHSO3).

Bisulfites are reducing agents and nucleophiles. They readily add to aldehydes and some ketones to form stable bisulfite adducts, a reaction used to protect carbonyl groups or to facilitate analysis of carbonyl-containing compounds. Upon acidification or heating, bisulfites release sulfur dioxide (SO2), which accounts for their preservative and bleaching roles in various industries.

Applications of bisulfites include use as antioxidants and preservatives to prevent browning and spoilage in foods and drinks, notably in wines, dried fruits, and fruit juices. They are also employed in paper production, textile bleaching, and certain organic syntheses as reducing agents or to decolorize solutions. In many cases they function by releasing SO2 under processing conditions.

Safety and regulation: Some individuals are sensitive to sulfites and may experience asthmatic or allergic-like reactions upon ingestion or inhalation. Many jurisdictions require labeling of foods containing sulfites above specified thresholds. Handling bisulfites as powders or solutions can irritate the eyes, skin, and respiratory tract; appropriate ventilation and protective measures are advised.