Sulfurous acid

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

SO₂ + H₂O ⇌ HSO₃⁻ + H⁺K_a = 1.54×10⁻²; pK_a = 1.81.

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

[H−OSO₂]⁻ ⇌ [H−SO₃]⁻

Attempts to concentrate the solutions of sulfurous acid simply reverse 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

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. However, the molecule has been detected in the gas phase in 1988 by the dissociative ionization of diethyl sulfite. 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.