Sulfurous acid

Sulfurous acid

Names
IUPAC name Sulfurous acid
Identifiers
CAS Number	7782-99-2 Y
3D model (Jmol)	Interactive image
ChEBI	CHEBI:48854 Y
ChemSpider	1069 Y
ECHA InfoCard	100.029.066
KEGG	C00094 Y
PubChem CID	1100
UNII	J1P7893F4J Y
InChI InChI=1S/H2O3S/c1-4(2)3/h(H2,1,2,3) Y Key: LSNNMFCWUKXFEE-UHFFFAOYSA-N Y InChI=1/H2O3S/c1-4(2)3/h(H2,1,2,3) Key: LSNNMFCWUKXFEE-UHFFFAOYAJ
SMILES O=S(O)O
Properties
Chemical formula	H2SO3
Molar mass	82.07 g/mol
Acidity (pKa)	1.857, 7.172
Hazards
Safety data sheet	ICSC 0074
EU classification (DSD)	C (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
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y  (what is YN ?)
Infobox references

Sulfurous acid (also sulphurous 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. The conjugate bases of this elusive acid are, however, common anions, bisulfite (or hydrogen sulfite) and sulfite. Sulfurous acid is an intermediate species in the formation of acid rain from sulfur dioxide.

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

¹⁷O NMR spectroscopy provided evidence that solutions of sulfurous acid and protonated sulfites contains a mixture of isomers, which is in equilibrium:

When trying to concentrate the solution by evaporation to produce waterless sulfurous acid it will decompose (reversing the forming reaction). In cooling down a clathrate SO₂ · 5.75 H₂O will crystallise which decomposes again at 7 °C. Thus sulfurous acid H₂SO₃ cannot be isolated.

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 oxidised to sulfuric acid or sulfate by accepting another oxygen atom.

They are also mild bleaches, and are used for materials which may be damaged by chlorine-containing bleaches.