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Redox Reactions of Quercetin and Quercetin-5 -sulfonic Acid < with Fe3+ and Cu2+ Ions and with H2O2 M. Kopacz, S. Kopacz, and E. Skuba Rzeshow Polytechnic University, Rzeshow, Poland Received December 16, 2002 Abstract - Redox reactions of quercetin and quercetin-5`-sulfonic acid with Fe3+ and Cu2+ ions and with H2O2 were studied spectrophotometrically. Oxidation of the flavonoids occurs at the 3-OH and 4-OH groups. The redox reactions are largely influenced by pH. With Fe3+ ions, oxidation occurs in strongly acidic (pH 132), and with Cu2+ ions, in weakly acidic (pH 4 35) solutions. Oxidation of quercetin and quercetin-5`-sul- fonic acid with Fe3+ and Cu2+ ions is accompanied by complexation. Hydrogen peroxide oxidizes the fla- vonoids at pH 133.5, and at pH > 4 oxidation is insignificant.
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