Anti- and prooxidative properties of gallic acid in Fenton-type systems

The anti- and prooxidative properties of gallic acid in Fenton-type systems containing H2O2 and Fe(III) were examined in pH 3-10 reaction media and at reaction temperatures of 20-50 degreesC. Although it is a free radical scavenger, gallic acid may exhibit prooxidative properties, as it promotes the production of hydroxyl radicals due to iron chelation. The overall effect is prooxidative if the ratio of the concentrations of gallic acid and Fe(III) in the reaction medium is smaller than 2. If the ratio is greater than 2, the overall effect of gallic acid presence is antioxidative due to free radical scavenging properties. The dependence of rates and of apparent activation energies of gallic acid consumption on pH in Fenton-type systems was also examined, and it is concluded that the rate-determining steps in acidic and alkaline media are different, the overall rate of gallic acid consumption being lowest at pH 7.