Sulfurous acid

From Wikipedia, the free encyclopedia
(Redirected from Sulphurous acid)
Sulfurous acid
Sulfuric(IV) acid
Ball-and-stick model of sulfurous acid
IUPAC name
Sulfurous acid
Other names
Sulfuric(IV) acid
Thionic acid
Sulfinic acid
3D model (JSmol)
ECHA InfoCard 100.029.066 Edit this at Wikidata
EC Number
  • 231-973-1
  • InChI=1S/H2O3S/c1-4(2)3/h(H2,1,2,3) checkY
  • 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:
GHS05: CorrosiveGHS07: Exclamation mark
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).
checkY verify (what is checkY☒N ?)

Sulfuric(IV) acid (United Kingdom spelling: sulphuric(IV) acid), also known as sulfurous (UK: sulphurous) acid and thionic acid,[citation needed] is the chemical compound with the formula H2SO3.

Raman spectra of solutions of sulfur dioxide in water show only signals due to the SO2 molecule and the bisulfite ion, HSO3.[1] 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:[2]

[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.

History and production[edit]

Sulfurous acid is commonly known to not exist in its free state, and due to this, it is stated in textbooks that it cannot be isolated in the water-free form.[3] However, the molecule has been detected in the gas phase in 1988 by the dissociative ionization of diethyl sulfite.[4] 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.[5]


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[edit]


  1. ^ Jolly, William L. (1991), Modern Inorganic Chemistry (2nd ed.), New York: McGraw-Hill, ISBN 0-07-032768-8
  2. ^ 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.
  3. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 719. ISBN 978-0-08-037941-8.
  4. ^ 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.
  5. ^ McQuarrie; Rock (1987). General Chemistry (2nd ed.). New York: W.H. Freeman and Company. p. 243. ISBN 0-7167-1806-5.
  6. ^ L. Kolditz, Anorganische Chemie, VEB Deutscher Verlag der Wissenschaften, Berlin 1983, S. 476.