Sulfurous acid

Sulfurous acid (also sulfuric(IV) acid, sulphurous acid (UK), sulphuric(IV) acid (UK)) is the chemical compound with the formula H2SO3. There is no evidence that sulfurous acid exists in solution,[1] but the molecule has been detected in the gas phase.[2] 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.[3]

Sulfurous acid
IUPAC name
Sulfurous acid
Other names
Sulfuric(IV) acid
3D model (JSmol)
ECHA InfoCard 100.029.066
EC Number
  • 231-973-1
  • InChI=1S/H2O3S/c1-4(2)3/h(H2,1,2,3) Y
  • InChI=1/H2O3S/c1-4(2)3/h(H2,1,2,3)
  • O[S](=O)O
  • Tautomer: O[S+](O)[O-]
Molar mass 82.07 g/mol
Acidity (pKa) 1.857, 7.172
Conjugate base Bisulfite
GHS labelling:
H314, H332
P260, P261, P264, P271, P280, P301+P330+P331, P303+P361+P353, P304+P312, P304+P340, P305+P351+P338, P310, P312, P321, P363, P405, P501
Flash point Non-flammable
Safety data sheet (SDS) ICSC 0074
Related compounds
Related compounds
Sulfur dioxide
Sulfuric acid
Selenous acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y verify (what is YN ?)
Infobox references

Raman spectra of solutions of sulfur dioxide in water show only signals due to the SO2 molecule and the bisulfite ion, HSO3.[4] The intensities of the signals are consistent with the following equilibrium:

SO2 + H2O ⇌ HSO3 + H+          Ka = 1.54×10−2; pKa = 1.81.

17O NMR spectroscopy provided evidence that solutions of sulfurous acid and protonated sulfites contain a mixture of isomers, which is in equilibrium:[5]

[H−OSO2] ⇌ [H−SO3]

Attempts to concentrate the solutions of sulfurous acid simply reverses the equilibrium, producing sulfur dioxide and water vapor. A clathrate with the formula 4SO2·23H2O has been crystallised. It decomposes above 7 °C. Anhydrous sulfurous acid H2SO3 has not been 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.[6]

See also


  1. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 719. ISBN 978-0-08-037941-8.
  2. 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 (11): 1533–4. doi:10.1002/anie.198815331.
  3. McQuarrie; Rock (1987). General Chemistry (2nd ed.). New York: W.H. Freeman and Company. p. 243. ISBN 0-7167-1806-5.
  4. Jolly, William L. (1991), Modern Inorganic Chemistry (2nd ed.), New York: McGraw-Hill, ISBN 0-07-032768-8
  5. Catherine E. Housecroft; Alan G. Sharpe (2008). "Chapter 16: The group 16 elements". Inorganic Chemistry, 3rd Edition. Pearson. p. 520. ISBN 978-0-13-175553-6.
  6. L. Kolditz, Anorganische Chemie, VEB Deutscher Verlag der Wissenschaften, Berlin 1983, S. 476.
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