Actually, sulfite is
SO3 not SO2...
Although the majority of your reply is accurate, I felt it necessary to mention this "oversight" to prevent misinforming those who would not know the difference between the two.
In reality, the conversion of a metabisulfite to the ion that provides oxidation protection is somewhat more complex, and involves the conversion of free SO2 [erroneously reffered to as
sulfites] into molecular SO2 [the ion that protects].
Potassium metabisulfite, K2S2O5, is a white crystalline powder with a pungent sulfur odour. The main use for the chemical is as an antioxidant or chemical sterilant. It is a sulfite and is chemically very similar to sodium metabisulfite, with which it is sometimes used interchangeably. Potassium metabisulfite is generally preferred out of the two as it does not contribute sodium to the diet.
Potassium metabisulfite has a monoclinic crystal structure which decomposes at 190°C, yielding potassium oxide and sulfur dioxide:
K2S2O5(s) → K2O(s) + 2SO2(g)
Potassium metabisulfite is a common wine or must additive, where it forms sulfur dioxide gas (SO2). This both prevents most wild microorganisms from growing, and it acts as a potent antioxidant, protecting both the color, and delicate flavors of wine.
Typical dosage is 1/4 tsp potassium metabisulfite per 6 gallon bucket of must (yielding roughly 75 ppm of SO2) prior to fermentation, and 1/2 tsp per 6 gallon bucket (150 ppm of SO2) at bottling.
Winemaking equipment is sanitized by spraying with a 1% SO2 (2 tsp potassium metabisulfite per L) solution.
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This article is about the molecule SO3. For the ion SO32-, see Sulfite.
Sulfur trioxide (also spelled sulphur trioxide) is the chemical compound with the formula SO3. In the gaseous form, this species is a significant pollutant, being the primary agent in acid rain. It is prepared on massive scales as a precursor to sulfuric acid.
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Sulfites (also sulphites) are compounds that contain the sulfite ion SO32− (IUPAC name sulfate(IV) ion).
The structure of the sulfite anion 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.