The oxidation process occurring on mercury electrodes in the presence of hydrogen peroxide has been analized in basic solutions by polarography and voltammetry in their linear scan and differential pulse modes. The process involves Hg(I) ion -in addition to Hg(II) ion -and hydroperoxide radicals, these last formed at trace levels. To demonstrate this radical generation radical scavengers (antioxidants), characterized with the DPPH • radical scavenging assay, were employed. The interaction of the radicals with the antioxidant originate the decrease in signal, but the antioxidant itself does not react with the hydrogen peroxide in the absence of Hg. In the absence of antioxidant, the process is two-electron, being the rate-determining step the second electron transfer; at high concentrations of antioxidant, the oxidation corresponds to a reversible one-electron transfer followed by a chemical reaction between the hydroperoxide radicals and the antioxidant. A reaction scheme is proposed for intermediate antioxidant concentrations.More than forty years ago, the oxidation and reduction processes originated by the H 2 O 2 on mercury electrodes were investigated in acidic media using polarographic and galvanostatic methods. 1 In the study, it was concluded that the anodic reaction corresponds not the H 2 O 2 oxidation, but to the oxidation of Hg to Hg ++ ions and the subsequent formation of a complex with sulfate ions.Kikuchi and Murayama 2 proposed that the anodic wave observed on mercury electrodes in the presence of H 2 O 2 is due to the oxidation of Hg to Hg ++ followed by the formation of a mixed complex with HO 2 − and OH − ions, as is shown in Scheme 1. This assumption was confirmed by Suznkievic and coworkers by using the titration of H 2 O 2 with HgCl 2 in basic solutions. 3 A mechanism involving ROS (reactive oxygen species) generation has been recently proposed. 4 Reactive oxygen species (ROS) are radicals generated by organic and inorganic peroxides, including those originated by H 2 O 2 or water itself, i.e., HO • , HO • 2 , O • 2 − . . . Free radicals are produced in moderate quantities in the organism, and apart from their role as secondary redox messengers, 5 they participate also in important biological functions as the cellular proliferation stimulation, the physiological regulation, etc. 6 The excessive production of ROS of exogenous or endogenous origin imply the occurrence of oxidative stress. 7 Such species play a significant role in the antioxidant stress and damage to DNA, proteins and lipids 8 and in chronic diseases such as Alzheimer's, Parkinson's, Crohn's, cancer, etc. 9,10 The reactivity of radicals is associated to the chain reactions they initiate. Secondary antioxidants (that is, antioxidants with radical scavenging activity) interrupt the propagation of free radicals and prevent the metabolic activation of carcinogens. 11,12 Spices and condiments have been recognized long time ago for their physiological and medicinal properties. 13,14 The antioxidant activity of these compounds, special...

The antioxidant activity of a set of 17 active principles present in foods is studied by using two sets of methods: i) spectrophotometric, DPPH radical scavenging method and CUPRAC assay; ii) electrochemical, using either a mercury electrode or a glassy carbon electrode covered with poly-neutral red and doped with Pt nanoparticles. The modified glassy carbon electrode assesses the scavenging activity of the antioxidants studied, improving the measurement time and easiness respect to the rest of the methods. Compared to Hg electrode, the measurements can be made in conditions close to the physiological conditions (pH 7 and aqueous medium). Using this sensor, antioxidants without activity in the DPPH radical scavenging assay can be studied. Finally, with respect to CUPRAC, the advantages of the modified electrode are the absence of organic solvent, with the consequent closeness to physiological conditions, and the lower measuring time involved. The sensitivity of this sensor is comparable with those shown by DPPH or CUPRAC.

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