Chemistry:Sulfurous acid

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Short description: Chemical compound
Sulfurous acid
Sulfuric(IV) acid
Ball-and-stick model of sulfurous acid
Names
IUPAC name
Sulfurous acid
Other names
Sulfuric(IV) acid
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
EC Number
  • 231-973-1
1458
KEGG
UNII
Properties
H
2
SO
3
Molar mass 82.07 g/mol
Acidity (pKa) 1.857, 7.172
Conjugate base Bisulfite
Hazards
Safety data sheet ICSC 0074
GHS pictograms GHS05: CorrosiveGHS07: Harmful
GHS Signal word Danger
H314, H318, H332
P260, P261, P264, P271, P280, P301+330+331, P303+361+353, P304+312, P304+340, P305+351+338, P310, P312, P321, P363, P405, P501
Flash point Non-flammable
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).
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Sulfurous acid (also Sulfuric(IV) acid, Sulphurous acid (UK), Sulphuric(IV) acid (UK)) is the chemical compound with the formula H
2
SO
3
. 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 SO
2
molecule and the bisulfite ion, HSO
3
.[3] The intensities of the signals are consistent with the following equilibrium:

SO
2
+ H
2
O ⇌ HSO
3
+ 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:[4]

[H–OSO
2
]
⇌ [H–SO
3
]

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 4SO
2
 · 23H2O
will crystallise which decomposes again at 7 °C. Thus sulfurous acid H
2
SO
3
cannot be isolated.

Sulfurous acid can be obtained by dissolving sulfur dioxide in water.

Uses

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.[5]

See also

References

  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". Angew. Chem. Int. Ed. Engl. 27 (11): 1533–4. doi:10.1002/anie.198815331. 
  2. McQuarrie; Rock (1987). General Chemistry (2nd ed.). New York: W.H. Freeman and Company. p. 243. ISBN 0-7167-1806-5. 
  3. Template:Jolly2nd
  4. 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. 
  5. L. Kolditz, Anorganische Chemie, VEB Deutscher Verlag der Wissenschaften, Berlin 1983, S. 476.