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Sulfurous acid

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Title: Sulfurous acid  
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Subject: Selenous acid, Sulfuric acid, Tellurous acid, Carbonic acid, Oxoacid
Collection: Hydrogen Compounds, Sulfites, Sulfur Oxoacids
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Sulfurous acid

Sulfurous acid
Sulfurous acid
Ball-and-stick model of sulfurous acid
IUPAC name
Sulfurous acid
ChemSpider  Y
Jmol-3D images Image
Molar mass 82.07 g/mol
Acidity (pKa) 1.857, 7.172
Safety data sheet ICSC 0074
Corrosive 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  (: Y/N?)

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 hydrogen sulfite) 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 HSO3 + H+
Ka = 1.54×10−2; pKa = 1.81.

17O [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.


  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".  
  2. ^ McQuarrie and Rock (1987), General Chemistry, 2nd ed., W.H. Freeman and Company, N.Y., p.243, ISBN 0-7167-1806-5
  3. ^ Jolly, William L. (1991), Modern Inorganic Chemistry (2nd ed.), New York: McGraw-Hill,  
  4. ^ Catherine E. Housecroft; Alan G. Sharpe (2008). "Chapter 16: The group 16 elements". Inorganic Chemistry, 3rd Edition. Pearson. p. 520.  

See also

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