Sulfurous acid

Sulfuric(IV) acid (United Kingdom spelling: sulphuric(IV) acid), also known as sulfurous (UK: sulphurous) acid and thionic acid,Template:Citation needed is the chemical compound with the formula Template:Chem2.

Raman spectra of solutions of sulfur dioxide in water show only signals due to the Template:Chem2 molecule and the bisulfite ion, Template:Chem2.<ref>Template:Jolly2nd</ref> The intensities of the signals are consistent with the following equilibrium:

¹⁷O NMR spectroscopy provided evidence that solutions of sulfurous acid and protonated sulfites contain a mixture of isomers, which is in equilibrium:<ref name="InorgChem">Template:Cite book</ref>

Attempts to concentrate the solutions of sulfurous acid simply reverse the equilibrium, producing sulfur dioxide and water vapor. A clathrate with the formula Template:Chem2 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.<ref>Template:Greenwood&Earnshaw2nd</ref> However, the molecule has been detected in the gas phase in 1988 by the dissociative ionization of diethyl sulfite.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite book</ref>

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.<ref>L. Kolditz, Anorganische Chemie, VEB Deutscher Verlag der Wissenschaften, Berlin 1983, S. 476.</ref>

See also

• Bisulfite
• Carbonic acid
• Pulp (paper)
• Sulfite paper pulp process
• Sulfite
• Sulfuric acid