Bisulfite
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2008-05-06T00:38:32Z
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[[Image:Bisulfite-S-H-tautomer-3D-balls.png|thumb|right|200px|A [[ball-and-stick model]] of the most commonly stated structure for the bisulfite ion in solution and in the solid state.]]
'''Bisulfite''' ion ([[IUPAC]]-recommended nomenclature: '''hydrogen sulfite''') is the ion HSO<sub>3</sub><sup>−</sup>. Salts containing the HSO<sub>3</sub><sup>−</sup> ion are termed bisulfites. For example, [[sodium bisulfite]] is NaHSO<sub>3</sub>.
==Reactions==
Bisulfite salts are typically prepared by treatment of alkaline solutions with excess [[sulfur dioxide]]:
:SO<sub>2</sub> + NaOH → NaHSO<sub>3</sub>
HSO<sub>3</sub><sup>−</sup> is the [[conjugate base]] of [[sulfurous acid]], H<sub>2</sub>SO<sub>3</sub>:
:H<sub>2</sub>SO<sub>3</sub> {{unicode|⇌}} HSO<sub>3</sub><sup>−</sup> + H<sup>+</sup>
Sulfurous acid is not an isolable compound and does not appear to exist in solution either. An equilibrium that is much more consistent with [[spectroscopy|spectroscopic]] evidence is given :
:SO<sub>2</sub> + H<sub>2</sub>O {{unicode|⇌}} HSO<sub>3</sub><sup>−</sup> + H<sup>+</sup>
HSO<sub>3</sub><sup>−</sup> is a weakly acidic species with a [[acid dissociation constant|p''K''<sub>a</sub>]] of 6.97. Its conjugate base is the [[sulfite]] ion, SO<sub>3</sub><sup>2−</sup>:
:HSO<sub>3</sub><sup>−</sup> {{unicode|⇌}} SO<sub>3</sub><sup>2−</sup> + H<sup>+</sup>
==Structure==
[[Image:Bisulfite-equilibrium-2D.png|300px|thumb|right|Bisulfite tautomers in equilbrium]] [[Image:Bisulfite-equilibrium-3D-balls.png|300px|thumb|right|Ball-and-stick models of the proposed bisulfite equilbrium. The tautomer on the left has ''C''<sub>s</sub> symmetry, while that of the tautomer on the right is ''C''<sub>3v</sub>.]]
Most evidence suggests that the proton in bisulfite ion is located on sulfur, giving rise to [[molecular symmetry |''C''<sub>3v</sub> symmetry]]. There is, however, some evidence from [[oxygen-17|<sup>17</sup>O]] [[nuclear magnetic resonance spectroscopy|NMR]] spectroscopy to suggest that two tautomeric forms of HSO<sub>3</sub><sup>−</sup> exist in dynamic equilibrium, one having ''C''<sub>3v</sub> symmetry (protonated at sulfur) and other ''C''<sub>s</sub> symmetry (protonated at oxygen)<ref>D. A. Horner, R. E. Connick, ''Inorg. Chem.'' '''25''', 2414-7 (1986).</ref>. The ''C''<sub>3v</sub> structure is supported by [[X-ray crystallography]] and, in aqueous solution, by [[Raman spectroscopy]] (ν(S–H) = 2500 cm<sup>−1</sup>).
==References==
<references/>
[[Category:Bisulfites|*]]
[[Category:Oxoanions]]
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