bisulfite

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 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 ⇌ 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⁺

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).

Medicine

Bisulfite salts are common additives to the drug epinephrine in order to prevent its oxidation to adrenochrome and resulting inactivation. Bisulfites can sometimes cause an allergic reaction. This is different from the common sulfa drugs allergy.

References

1. ^ D. A. Horner, R. E. Connick, Inorg. Chem. 25, 2414-7 (1986).

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