Raman spectra of solutions ofsulfur dioxide in water show only signals due to theSO2 molecule and the bisulfite ion,HSO−3.[2] The intensities of the signals are consistent with the followingequilibrium:
SO2 + H2O ⇌ HSO−3 + H+Ka = 1.54×10−2; pKa = 1.81.
17ONMR spectroscopy provided evidence that solutions of sulfurous acid and protonated sulfites contain a mixture of isomers, which is in equilibrium:[3]
[H−OSO2]− ⇌ [H−SO3]−
Attempts to concentrate the solutions of sulfurous acid simply reverse the equilibrium, producing sulfur dioxide and water vapor. Aclathrate with the formula4SO2·23H2O has been crystallised. It decomposes above 7 °C.
Sulfurous acid is commonly known not to exist in its free state, and owing to this, it is stated in textbooks that it cannot be isolated in the water-free form.[4] However, the molecule has been detected in the gas phase in 1988 by the dissociative ionization ofdiethyl sulfite.[5] The conjugate bases of this elusive acid are, however, common anions,bisulfite (or hydrogen sulfite) andsulfite. Sulfurous acid is an intermediate species in the formation ofacid rain from sulfur dioxide.[6]
^Perrin, D. D., ed. (1982) [1969].Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution.IUPAC Chemical Data (2nd ed.). Oxford: Pergamon (published 1984). Entry 217.ISBN0-08-029214-3.LCCN82-16524.
^Jolly, William L. (1991).Modern Inorganic Chemistry (2nd ed.). New York: McGraw-Hill.ISBN0-07-032768-8.
^Catherine E. Housecroft; Alan G. Sharpe (2008). "Chapter 16: The group 16 elements".Inorganic Chemistry, 3rd Edition. Pearson. p. 520.ISBN978-0-13-175553-6.
^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.
^McQuarrie; Rock (1987).General Chemistry (2nd ed.). New York: W.H. Freeman and Company. p. 243.ISBN0-7167-1806-5.
^L. Kolditz,Anorganische Chemie, VEB Deutscher Verlag der Wissenschaften, Berlin 1983, S. 476.
