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sulfurous acid[′səl·fə·rəs ′as·əd]
H2 SO3, a weak dibasic acid, corespond-ing to the + 4 oxidation state of sulfur. Sulfurous acid is known only in dilute aqueous solutions. Its dissociation constants are K1 = 1.6 X 10-2 and K2 = 1.0 X 10-7(18°C). It gives two series of salts: normal salts—sulfites—and acidic salts—hydrosulfites. Sulfurous acid is a strong reducing agent, and its solutions, when left exposed to the air, are gradually converted into H2 SO4. In reactions with stronger reducing agents, for example, H2 S, the acid behaves as an oxidizing agent. Sulfurous acid is obtained by dissolving SO2 in water. The following equilibria exist simultaneously in aqueous solutions:
The products obtained by adding sulfurous acid to organic dyestuffs are colorless or only slightly colored. This result explains the use of sulfurous acid in bleaching those materials that cannot withstand the action of such strong oxidizing agents as chlorine and hypochlorites.