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Sulfurous acid
From Wikipedia, the free encyclopedia
| Sulfurous acid | |
|---|---|
  sulfurous acid has been detected in the gas phase[1] |
|
| General | |
| Systematic name | Sulfurous acid |
| Molecular formula | H2SO3 (aq) |
| Molar mass | 82.07 g/mol |
| Properties | |
| Acid dissociation constant pKa |
1.81 (18°C) 1 6.91 (18°C) 2 |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references |
|
Sulfurous acid (or sulphurous acid in British spelling) is a name given to aqueous solutions of sulfur dioxide. There is no evidence that the sulfurous acid molecule, H2SO3, exists in these solutions. They cannot be isolated as a pure substance, because boiling the sulfurous acid will drive away sulfur dioxide, leaving water. They react with alkalis to form bisulfite (or hydrogensulfite) and sulfite salts.
Raman spectra of solutions of sulfur dioxide in water show only signals due to the SO2 molecule and the bisulfite ion, HSO3−. The intensities of the signals are consistent with the following equilibrium:
-
- SO2 + H2O ⇌ HSO3− + H+
- Ka = 1.54 × 10−2 dm3 mol−1; pKa = 1.81.
- SO2 + H2O ⇌ HSO3− + H+
The hydrogen atom in the bisulfite ion is bonded to the sulfur atom and not to an oxygen atom as is more usual in the case of oxoanions. This has been shown in the solid state by X-ray crystallography and in aqueous solution by Raman spectroscopy (ν(S–H) = 2500 cm−1). It is, however, acidic according to the following equilibrium:
-
- HSO3− ⇌ SO32− + H+
- Ka = 1.02x10−7 dm3 mol−1; pKa = 6.97.
- HSO3− ⇌ SO32− + H+
Solutions of sulfur dioxide ("sulfurous acid") and of bisulfite and sulfite salts are used as reducing agents and as disinfectants. They are also mild bleaches, and are used for materials which may be damaged by chlorine-containing bleaches.
- ^ D. Sülzle, M. Verhoeven, J. K. Terlouw, H. Schwarz, Angew. Chem. Int. Edn. Engl. 27, 1533-4 (1988).
- Sulfite
- Sulfuric acid
- Bisulfite
- Carbonic acid, another hypothetical acid used to represent a dissolved gas