Sulfurous Acid

Title: Sulfurous Acid
CAS Registry Number: 7782-99-2
Additional Names: Sulfur dioxide soln
Literature References: A soln of ~6% sulfur dioxide in water.
Properties: Colorless, clear, acid liquid; suffocating odor of sulfur dioxide. d ~1.03. Gradually oxidizes in the air to sulfuric acid. Keep in nearly full, tightly closed containers, in a cool place.
Density: d ~1.03
Use: Dental bleach.
Therap-Cat: Antiseptic.
Sulfuryl Chloride Sulfuryl Fluoride Sulindac Sulisatin Sulisobenzone

Sulfurous acid
Sulfurous acid
Ball-and-stick model of sulfurous acid
CAS number 7782-99-2 YesY
PubChem 1100
ChemSpider 1069 YesY
UNII J1P7893F4J YesY
KEGG C00094 YesY
ChEBI CHEBI:48854 YesY
Jmol-3D images Image 1
Molecular formula H2SO3
Molar mass 82.07 g/mol
Acidity (pKa) 1.857, 7.172
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
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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.[1] The conjugate bases of this elusive acid are, however, common anions, bisulfite (or hydrogensulfite) and sulfite. Sulfurous acid is an intermediate species in the formation of acid rain from sulfur dioxide.[2]

Raman spectra of solutions of sulfur dioxide in water show only signals due to the SO2 molecule and the bisulfite ion, HSO3.[3] The intensities of the signals are consistent with the following equilibrium:

SO2 + H2O is in equilibrium with HSO3 + H+
Ka = 1.54×10−2; pKa = 1.81.

17O NMR spectroscopy provided evidence that solutions of sulfurous acid and protonated sulfites contains a mixture of isomers, which is in equilibrium:[4]

[H-OSO2] [H-SO3]

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.