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 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⁺

K_a = 1.54x10⁻² L/mol; pK_a = 1.81.

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⁺

K_a = 1.02x10⁻⁷ L/mol; pK_a = 6.97.

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.

See also

• Sulfite
• Sulfuric acid
• Bisulfite

References

• Simon, A.; & Kriegsmann, H. (1956). Chem. Ber. 89:2442.
• Weast, R. C. (Ed.). Handbook of Chemistry and Physics (53rd Edn.), Cleveland:Chemical Rubber Co., 1972.

External links

Sulfites | Acids

Kyselina siřičitá | Schweflige Säure | Θειώδες οξύ | Acido solforoso | Zwaveligzuur | 亜硫酸 | Kwas siarkowy(IV) | Svavelsyrlighet | 亚硫酸