Mechanism of Mercury Electrooxidation in the Presence of Hydrogen Peroxide and Antioxidants

Abstract

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...

Evaluation of synergistic and antagonistic effects between some selected antioxidants by means of an electrochemical technique

“…The structure of Eugenol allows the formation of stable radicals than the rest, because the double bound in the lateral chain and the presence of one phenolic –OH and the methoxy group. Thymol and sesamol have only one phenolic –OH, although the dioxol ring of sesamol can be opened giving relatively stable radicals (Estévez Brito et al ., ). Thus, the synergy could be related to the oxidation mechanisms and the possibility of interaction between the radicals.…”

“…Electrochemical methods were also used, for example by monitoring the polarographic oxidation of H 2 O 2 that gives peroxide and hydroperoxide radicals (Palma et al ., ; Estévez Brito et al ., ), which interact with antioxidants (Gorjanović et al ., ). The use of the differential pulse voltammetry increased the accuracy of the measurements (Palma et al ., ).…”

“…Both the generation step and the detection of the interaction are made in the same voltammetric measurement (Palma et al ., ). H 2 O 2 is not active on carbon electrodes, ROS are generated in the Hg oxidation reaction in the presence of hydrogen peroxide (Estévez Brito et al ., ) and the use of Hg electrodes is necessary. The antioxidant capacities of antioxidants appearing in natural samples and some of their mixtures were obtained by this method (Palma et al ., , ) that has been selected in this work.…”

Reduction of Hg2+by Individual Phenolics and Complex Samples and Its Application in Polarographic Antioxidant Assay

“…Palma et al also observed no direct reaction between AOs and peroxide itself 7 and instead hypothesized the formation of hydroperoxide radical at trace levels and its scavenging by AOs. [6][7][8] As alternative, a reaction between AOs and Hg 2+ could take place, allowing the employment of both DC polarography and DP voltammetry in AO activity measurements. Close agreement between HPMC and MRAP results indicates that the reduction leads to a decrease of Hg 2+ concentration available for…”

“…7 Briton et al 8 also proposed a reaction of AOs with the hydroperoxide radical resulting in Hg 2+ concentration decrease and consequent decrease of the area z E-mail: stasago@yahoo.co.uk under current peak. However, the presence of radicals supposed to be formed in the range of potentials of the oxidation peak on the mercury electrode and claimed to be required to achieve the interaction with AOs as the only possibility in the system studied [6][7][8] was not confirmed. Until now, a reaction of AOs with Hg 2+ as the oxidant species capable to react was not considered, although the reduction of Hg 2+ occurring in the presence of certain phenolics and hydroquinones was reported by McKillop and Young 9 a few decades ago.…”