Bisulfite

Bisulfite ion (IUPAC-recommended nomenclature: hydrogen sulfite) is the ion HSO₃⁻. Salts containing the HSO₃⁻ ion are termed bisulfites also known as sulfite lyes. For example, sodium bisulfite is NaHSO₃.

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 unicode|⇌ 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 unicode|⇌ HSO₃⁻ + H⁺

HSO₃⁻ is a weakly acidic species with a p"K"_a of 6.97. Its conjugate base is the sulfite ion, SO₃²⁻::HSO₃⁻ unicode|⇌ SO₃²⁻ + H⁺

tructure

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) [D. A. Horner, R. E. Connick, "Inorg. Chem." 25, 2414-7 (1986).] . The "C"_3v structure is supported by X-ray crystallography and, in aqueous solution, by Raman spectroscopy (ν(S–H) = 2500 cm⁻¹).

References