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any of several salts of sulfurous acid, H2SO3. Two series of sulfites exist: the normal sulfites, with the general formula M2SO3, and the acid sulfites, or hydrogen sulfites, with the general formula MHSO3, where M is a monovalent metal. Normal sulfites—with the exception of the sulfites of alkali metals and ammonium sulfite—are sparingly soluble in water. They dissolve in the presence of SO2. Among the acid sulfites, only those of the alkali metals have been isolated in the free state.
Sulfites in aqueous solution are characterized by oxidation to sulfates and reduction to the thiosulfates, M2S2O3. Sulfites are obtained by the reaction of SO2 with hydroxides or carbonates of the corresponding metals in an aqueous medium. The hydrogen sulfites are more frequently used, and their applications are in the textile industry for dyeing and printing (KHSO3 and NaHSO3), in the paper industry for the production of wood pulp [Ca(HSO3)2], in photography, and in organic synthesis.