The polarographic (direct current, dc, and differential pulse, DP) and voltammetric oxidation of hydrogen peroxide on mercury electrodes has been examined. The electrooxidation of H 2 O 2 involves hydroperoxide radical and superoxide anion radical able to interact with radical scavengers (antioxidants), namely 3-hydroxycoumarin, carvacrol, vanillin and gallic acid. For such interaction, theoretical equations are derived using the convective diffusion approximation and the steady-state conditions, as well as solving the differential equations of the Nernst diffusion-layer approximation by the method of dimensionless variables. The experimental results agree with the theoretical predictions and a kinetic parameter is proposed to evaluate the scavenging activity of antioxidants.An antioxidant is defined as a substance that may delay or prevent the oxidation of a substrate, at relatively low concentrations. 1 Primary antioxidants (prooxidants) prevent the formation of free radicals, particularly reactive oxygen species (ROS). These primary antioxidants include vitamin E, 2 polyphenols, 3 antioxidant enzymes and membranes. 4,5 Secondary antioxidants (antioxidants with radical scavenging activity) operate either interrupting the propagation of free radicals or inhibiting the generation of ROS, as well as preventing the metabolic activation of carcinogens. 6,7 Tertiary antioxidants repair the damage from free radicals or eliminate damaged molecules. 8,9 Although different assays have been used to evaluate the antioxidant activity of natural products, there is not a universal method to cover this task. Direct methods have been proposed, based on the determination of the concentration of antioxidant compounds, 10,11 and indirect methods, as the measure of glutathione levels 12 and the reduction of Fe(III) or Cu(II) ions. 13,14 In addition, enzymatic methods measure the activity of antioxidant enzymes. 15 A good review detailing these methods can be found in reference. 16 The electrochemical measurements have advantages for the determination of antioxidant activity such as their use as a rapid proof of the antioxidant capacity of many organic molecules 17,18 and the capacity to study the interaction between the antioxidant and ROS, as was observed for dihydropyridines and electrogenerated superoxide radical. [19][20][21] The oxidation potentials measured by cyclic voltammetry have been used to compare the antioxidant strength of compounds as phenolic acids, flavonoids, cinnamic acids etc., 22-24 being the glassy carbon electrode, GCE, the most frequently used electrode. Low oxidation potentials are associated with a greater facility of a given molecule for the electrodonation and, thus, to act as an antioxidant, with good correlation with other techniques such as the scavenging of the DPPH • radical. 25 The results show that the DPPH • assay can assess radical scavenging for substances that are thermodynamically capable of reacting with agents with a redox potential below a given value, related to the reduction potential o...
