CaSO3H2O

CaSO3·H2O, commonly written as calcium sulfite monohydrate, is a hydrated salt comprising calcium ions, sulfite groups, and one molecule of water per formula unit. It is the monohydrate form of calcium sulfite (CaSO3) and has a pale, white to off-white appearance as a solid. The compound is of interest mainly as an intermediate in sulfur dioxide capture processes and in related chemical-pulping and water-treatment contexts.

Formation and occurrence

Calcium sulfite hydrates readily in aqueous environments. In industrial settings, such as flue-gas desulfurization and lime or limestone slurry processes, CaSO3 can form and hydrate to CaSO3·H2O. The hydrate can accumulate as a solid in process solids or slurries and may convert to the anhydrous salt CaSO3 or oxidize to calcium sulfate (CaSO4) over time when exposed to air.

Properties

Calcium sulfite monohydrate is a solid with relatively low to moderate solubility in water. It is less soluble than many sodium or potassium sulfites, reflecting its sparing interaction with the solvent. The hydrate is thermodynamically less stable than the anhydrous form in surface conditions and can lose water to yield CaSO3. In air, slow oxidation can convert sulfite to sulfate, producing CaSO4 over time.

Reactions and handling

CaSO3·H2O can release sulfurous gases upon strong heating or decomposition and may contribute to sulfur dioxide under certain conditions. It is typically handled as a chemical intermediate or byproduct; proper protective equipment and containment are advised to limit exposure to dust or mists.

Uses and context

The compound occurs mainly as an intermediate in sulfur dioxide scrubbing processes and in the chemical processing pathways related to sulfite pulps and related industries. It is not widely marketed as a standalone reagent, but understanding its hydrate form helps in managing CaSO3 equilibria in industrial systems.