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( Sulfite)
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. Sulfites occur naturally in all wines to some extent.[1] 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. In the US, wine bottled after mid-1987 must have a label stating that they contain sulfites if they contain more than 10 parts per million.[2]
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