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In pharmaceuticals, the salts of sulfurous acid, sulfites, are important. They are contained in medicines as preservatives and antioxidants. Sulfurous acid should not be confused with sulfuric acid.
Structure and properties
The sulfurous acid (H2SO3, Mr = 82.1 g/mol) is formed in the reaction of sulfur dioxide (SO2) with water. However, it is extremely unstable and cannot be detected in water. Formal reaction of sulfur dioxide with water:
- SO2 (sulfur dioxide) + H2O (water) H2SO3 (sulfurous acid)
Formal dissociation of sulfurous acid:
- H2SO3 (sulfurous acid) HSO3– (hydrogen sulfite) + H+ SO32- (sulfite) + H+
or
- H2SO3 (sulfurous acid) SO2 (sulfur dioxide) + H2O (water)
The salts are stable and are called sulfites or hydrogen sulfites (bisulfites).
Areas of application
The sulfites are used as preservatives and antioxidants.
