Sulfurous acid
IUPAC name	Sulfurous acid
Identifiers
CAS number	7782-99-2 Y
Properties
Molecular formula	H2SO3
Molar mass	82.07 g/mol
Hazards
MSDS	ICSC 0074
EU Index	016-011-00-9
EU classification	Corrosive (C)
R-phrases	R20, R34
S-phrases	(S1/2), S9, S26, S36/37/39, S45
Flash point	Non-flammable
Related compounds
Related compounds	Sulfur dioxide Sulfuric acid
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Sulfurous acid is the chemical compound with the formula H₂SO₃. There is no evidence that sulfurous acid exists in solution, but the molecule has been detected in the gas phase.^[1] The conjugate bases of this elusive acid are, however, common anions, bisulfite (or hydrogensulfite) and sulfite.

Raman spectra of solutions of sulfur dioxide in water show only signals due to the SO₂ molecule and the bisulfite ion, HSO₃⁻.^[2] The intensities of the signals are consistent with the following equilibrium:

SO₂ + H₂O HSO₃⁻ + H⁺

K_a = 1.54 × 10⁻²; pK_a = 1.81.

Aqueous solutions of sulfur dioxide, which sometimes are referred to as sulfurous acid are used as reducing agents and as disinfectants, as are solutions of bisulfite and sulfite salts. They are also mild bleaches, and are used for materials which may be damaged by chlorine-containing bleaches.

[edit] References

1. ^ D. Sülzle, M. Verhoeven, J. K. Terlouw, H. Schwarz (1988). "Generation and Characterization of Sulfurous Acid (H₂SO₃) and of Its Radical Cation as Stable Species in the Gas Phase". Angew. Chem. Int. Ed. Engl. 27: 1533–4. doi:10.1002/anie.198815331. 
2. ^ Jolly, William L. (1991), Modern Inorganic Chemistry (2nd ed.), New York: McGraw-Hill, ISBN 0-07-032768-8 

[edit] See also

• Sulfite
• Sulfuric acid
• Bisulfite
• Carbonic acid, another hypothetical acid used to represent a dissolved gas