Bisulfite

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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 HSO₃⁻. Salts containing the HSO₃⁻ ion are termed bisulfites. For example, sodium bisulfite is NaHSO₃.

[edit] Reactions

Bisulfite salts are typically prepared by treatment of alkaline solutions with excess sulfur dioxide:

SO₂ + NaOH → NaHSO₃

HSO₃⁻ is the conjugate base of sulfurous acid, H₂SO₃:

H₂SO₃ ⇌ HSO₃⁻ + H⁺

Sulfurous acid is not an isolable compound and does not appear to exist in solution either. An equilibrium that is much more consistent with spectroscopic evidence is given :

SO₂ + H₂O ⇌ HSO₃⁻ + H⁺

HSO₃⁻ is a weakly acidic species with a pK_a of 6.97. Its conjugate base is the sulfite ion, SO₃²⁻:

HSO₃⁻ ⇌ SO₃²⁻ + H⁺

[edit] Structure

Bisulfite tautomers in equilbrium

Ball-and-stick models of the proposed bisulfite equilbrium. The tautomer on the left has C_s symmetry, while that of the tautomer on the right is C_3v.

Most evidence suggests that the proton in bisulfite ion is located on sulfur, giving rise to C_3v symmetry. There is, however, some evidence from ¹⁷O NMR spectroscopy to suggest that two tautomeric forms of HSO₃⁻ exist in dynamic equilibrium, one having C_3v symmetry (protonated at sulfur) and other C_s symmetry (protonated at oxygen)^[1]. The C_3v structure is supported by X-ray crystallography and, in aqueous solution, by Raman spectroscopy (ν(S–H) = 2500 cm⁻¹).