Sulfites

Sulfites or sulphites are compounds that contain the sulfite ion (or the sulfate(IV) ion, from its correct systematic name), SO2−
3. The sulfite ion is the conjugate base of bisulfite. Although its acid (sulfurous acid) is elusive, its salts are widely used.

Sulfites are substances that naturally occur in some foods and the human body. They are also used as regulated food additives.

The structure of the sulfite an ion can be described with three equivalent resonance structures. In each resonance structure, the sulfur atom is double-bonded to one oxygen atom with a formal charge of zero (neutral), and sulfur is singly bonded to the other two oxygen atoms, which each carry a formal charge of −1, together accounting for the −2 charge on the anion. There is also a non-bonded lone pair on the sulfur, so the structure predicted by VSEPR theory is trigonal pyramidal, as in ammonia (NH₃). In the hybrid resonance structure, the S-O bonds are equivalently of bond order one and one-third.

Evidence from ¹⁷O NMR spectroscopic data suggests that protonation of the sulfite ion gives a mixture of isomers:

Sulfites are used as a food preservative or enhancer. They may come in various forms, such as:

Sulfites occur naturally in all wines to some extent. Sulfites are commonly introduced to arrest fermentation at a desired time, and may also be added to wine as preservatives to prevent spoilage and oxidation at several stages of the winemaking. Sulfur dioxide (SO₂) protects wine from not only oxidation, but also from bacteria. Without sulfites, grape juice would quickly turn to vinegar.

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