Hydrogensulfite

Hydrogensulfite, also known as bisulfite, is a chemical species with the formula HSO₃⁻. It is the conjugate base of sulfurous acid (H₂SO₃) and the salt of sulfurous acid with a cation. Hydrogensulfite ions are typically found in aqueous solutions and are often associated with cations such as sodium (NaHSO₃), potassium (KHSO₃), or ammonium ((NH₄)HSO₃). These salts are commonly encountered as white crystalline solids.

In solution, hydrogensulfite exists in equilibrium with dissolved sulfur dioxide and sulfite ions: SO₂ + H₂O ⇌ H₂SO₃ ⇌ H⁺ + HSO₃⁻ ⇌ 2H⁺ + SO₃²⁻. The position of this equilibrium is dependent on pH. At lower pH values, the concentration of sulfurous acid and dissolved sulfur dioxide increases, while at higher pH values, sulfite ions become more prevalent.

Hydrogensulfite has various industrial and practical applications. It is a common reducing agent and is used in chemical synthesis. It is also employed as a preservative in food and beverages due to its ability to inhibit the growth of microorganisms and prevent oxidation. In water treatment, it can be used to remove residual chlorine. Additionally, hydrogensulfite is utilized in the paper industry as a bleaching agent and in photographic processes. Its properties make it a versatile compound with a range of uses.