Sulfite

Sulfite refers to the sulfite anion, SO3^2−, the conjugate base of sulfurous acid (H2SO3). In aqueous solution it exists in a pH-dependent equilibrium with bisulfite (HSO3−) and with sulfur dioxide (SO2) released under acidic conditions. Sulfite can be oxidized to sulfate (SO4^2−) and, under strong reducing conditions, may be converted to sulfide in some environments.

Sulfites occur naturally in trace amounts in some fruits and wines and are produced industrially for multiple applications. They form salts with alkali and alkaline earth metals, such as sodium sulfite (Na2SO3) and potassium sulfite (K2SO3). Sulfite solutions also arise from dissolving sulfur dioxide in water or by treating sulfates with reducing agents.

The most common uses are as antioxidants and preservatives in foods and beverages, including dried fruits, wines, and some processed foods, where they help prevent browning and microbial growth. They are also used in the paper, textile, and pharmaceutical industries, in bleaching processes, and as reducing agents in chemical synthesis and photography.

Safety and regulation: Some individuals are sensitive to sulfites and can experience asthmatic or allergic-like reactions after exposure to sulfite-containing foods or inhalants. Many jurisdictions require labeling of sulfite-containing products above certain concentrations, and permitted levels are limited in foods and beverages.

In chemistry and environmental contexts, sulfites slowly oxidize in air to sulfates and can be reduced by certain microbes to sulfide. They are distinct from sulfates, which contain SO4^2− and are the more oxidized form of sulfur in most environments.