CALCIUM SULFITE

Description

Calcium sulfite,CaS03·O.5H20, is colorless, hexagonal crystals which are nearly insoluble in water. It is formed when sulfur dioxideis passed through a solution of calcium hydroxide, or when solutions of calcium salts and alkali sulfites are mixed. Used as a disinfectant in the sugar industry and breweries, to preserve fruit juices, and as an antichlorine in textile bleaches.

Chemical Properties

CaSO3.2H2O, white precipitate, pKsp = 7.9, formed by reaction of soluble calcium salt solution and sodium sulfite solution, or by boiling calcium hydrogen sulfite solution; calcium hydrogen sulfite, Ca(HSO3)2, formed in solution by saturating calcium hydroxide or carbonate suspension with sulfurous acid.

Uses

A major use of calcium sulfite in the past has been the use of “lime” (Ca(OH)2) in “flue-gas” desulphurization (removal of SO2 gas) at “fossil-fuel” electrical power generating plants. Since calcium sulfite is relatively insoluble in water, it can be removed from the slurry by filtration and converted to gypsum, CaSO4·2H2O. The other usage of calcium sulfite or bisulfite compounds has been in the “wood-pulping” industry.

Preparation

Calcium sulfite can be prepared by the reaction of sodium sulfite with calcium chloride:
CaCl2＋ Na2SO3→CaSO3＋ 2NaCl
The dihydrate, CaSO3·2H2O is formed. This salt is white powder that is soluble in dilute sulfurous acid to form the bisulfite. It may be dehydrated at 150°C to form the anhydrous salt. It is used in the sulfite process for the manufacture of wood pulp.