Aromatic hydroxylation reactions by electrogenerated HO radicals : a kinetic study

The oxidation of benzoic acid (BA) and of 4-hydroxybenzoic acid (4-HBA) by galvanostatic electrolysis with simultaneous oxygen evolution, using BDD or Pt as anode materials is studied. Results concerning the oxidation kinetics as well as the identification and quantification of hydroxylated products are reported. First order kinetics are used to describe the consumption rates of both compounds despite of the anode material and of the applied current density. A simple kinetic model that accounts for the anode surface coverage by HO radicals is proposed. Based on this model it is possible to correlate the apparent rate constant of the organic consumption with kinetic parameter related to the organics reactivity and to the degree of the adsorption of HO radicals to the anode surface.FCT (Fundação para a Ciência e Tecnologia); FEDER (European Fund for Regional Development); COMPETE-QREN-EU; CQ/UM (Centro de Química / Universidade do Minho), POPH Programa Operacional Potencial Humano); FSE (Fundo Social Europeu

Hydroxylation of benzoic acid by electrochemical generated hydroxyl radicals

The generation of hydroxyl radicals is an important process in many applications such as organic synthesis [1], oxidative stress studies [2] and detoxification of effluents [3]. Due to the high instability of these radicals they are usually produced in situ by means of different methods, such as disproportionation of peroxynitrous acid or dissociation of hydrogen peroxide by UV action. The generation of hydroxyl radicals based on metal catalyzed decomposition of hydrogen peroxide, Fenton or Fenton-like reactions, is the most spread method. The electrochemical generation of hydroxyl radical, as an intermediate in the oxygen formation from the oxidation of water is a well-known process. The ability of boron doped diamond electrodes (BDD) to produce hydroxyl radicals is extensively described on the oxidation of organics for wastewater [4]. Although the mineralization of several electroactive species is reported, the role of hydroxyl radicals in the initial phase of these reactions is not well established. In this work we present a study on the performance of Pt and BDD anodes on the hydroxylation of benzoic acid (a non-electroactive species) comparatively to electroactive species, such as quinol and p-hydroxybenzoic acid. The reactions were monitored by fluorescence, cyclic voltammetry and HPLC (UV detection).Fundação para a Ciência e a Tecnologia (FCT) - SFRH/BD/64189/200

Effect of the medium composition on the current of steady state voltammograms of neutral and charged species in dimethylformamide/toluene mixtures

Steady state voltammetric currents of both neutral and charged species obtained at microelectrodes are affected by the medium composition. Experiments carried out in N,N-dimethylformamide (DMF) and toluene/DMF mixtures for the reduction of diciano(fluoren-9-ylidene)methane and the methylviologen cation and for the oxidation of ferrocene, in the presence of different electrolyte concentrations indicate that both viscosity of the solution and mass transport by migration may have a strong effect on the measured limiting currents. These observations are particularly important in electroanalysis since the appropriate choice of the medium may substantially improve both the sensitivity and the detection limits of the electrochemical method.FEDER. PRAXIS/2/2.1/QUI /260/94

Evaluation of radical scavenging activity of antioxidants by means of electrogenerated HO radical

Hydroxyl radical is one of the most reactive radicals. The electrochemical generation of hydroxyl radical by oxidation of water can be achieved using different anode materials. The adsorption of HO radicals at the anode surface has a significant effect on their reactivity. Using materials such as BDD, electrogenerated hydroxyl radicals are weekly adsorbed whereas strongly adsorbed radicals are formed at Pt. Although most of the available studies deal with high oxidation power anodes for the degradation of pollutants, the use of anodes with low oxidation power can have important applications particularly when a certain degree of selectivity is required for oxidation. In this context electrochemically-generated hydroxyl radical was applied for the evaluation of antioxidant scavenging activity. A set of species with antioxidant activity was oxidized by galvanostatic electrolyses using a fairly oxidized Pt anode in conditions of O2 evolution. In such conditions the consumption of species was not limited by mass transport but by the oxidation kinetics of both charge transfer and reaction with HO radicals. Information regarding the rate constant of the reaction of antioxidants with electrogenerated HO radicals was obtained by means of the apparent rate constant variation with electrolysis current density. Scavenger activity of the analysed antioxidants estimated by the kinetic parameter kR,HO / kO2 was ordered as follows: gallic acid, trolox > ascorbic acid > caffeic acid. The method was applied successfully in synthetic solutions and in a green tea based beverage

Electrogenerated HO radical reactions: the role of competing reactions on the degradation kinetics of hydroxy-containing aromatic compounds

The rate of degradation of some aromatic compounds, benzoic acid, 2-hydroxybenzoic acid, 4-hydroxybenzoic acid and 2,3-hydroxybenzoic acid, by electrogenerated HO radicals is investigated. The effect of the addition of secondary species, that can either be a reaction product of the target compound or not is analysed. The presence of the actual reaction products is also considered. The action of these secondary species is integrated in a general model that accounts for the dependency of the anode coverage by HO radicals on the concentration and on the reactivity of HO radical scavengers. From the magnitude of the effect of competing reactions, the reactivity of the hydroxybenzoic acid derivatives was estimated as the product between the stoichiometric coefficients and the rate constants. A possible mechanistic interpretation is provided to explain the unexpected high values of the stoichiometric coefficients estimated that largely exceed the number of radicals required for the complete mineralization of the species. (C) 2014 Elsevier Ltd. All rights reserved.- We would like to acknowledge the contribution of Prof. Paula M.T. Ferreira on the mechanistic interpretation of the HO radical stoichiometric coefficients. Thanks are due to FCT (Fundacao para a Ciencia e Tecnologia) and FEDER (European Fund for Regional Development)-COMPETE-QREN-EU for financial support to the Research Centre, CQ/UM [PEst-C/QUI/UI0686/2011 (FCOMP-01-0124-FEDER-022716)]. Raquel Oliveira thanks to FCT, POPH (Programa Operacional Potencial Humano) and FSE (Fundo Social Europeu) for the PhD Grant (SFRH/BD/64189/2009).info:eu-repo/semantics/publishedVersio

Evaluation of the lactic acid consumption in yeast cultures by voltammetric means

The voltammetric study of the lactic acid reduction was performed in media suitable for yeast growth, using platinum microelectrodes. The decrease of the voltammetric peak current from square wave voltammetry, variation of Ip, in cultures of the yeasts Candida utilis and Saccharomyces cerevisiae was related to both the growth time and cellular biomass. Either the imposed variations of medium composition or those resulting from cellular growth did not significantly affect the variation of Ip values. For both yeast species, similar specific growth rates were estimated from the variation of Ip and absorbance at 640 nm.POCTI/QUI/39525/2001

Assessement of Candida utilis growth by voltammetric reduction of acids using microelectrodes

A voltammetric method for the evaluation of the microbial growth using platinum microelectrodes is proposed. This new method is based on the electrochemical reduction of acids produced by the yeast Candida utilis in liquid medium containing glucose as carbon and energy source and ammonium ions as nitrogen source. Current intensities, from linear sweep and square wave voltammetry, were compared with the corresponding values of absorbance at 640 nm (A640nm) resulting from the light scattering of cell suspensions. The equivalence between turbidimetry and electrochemical measurements was checked in experiments performed in growing cultures and in cell suspensions prepared from the dilution of cell cultures. The growth curves defined by either current intensities (IP or IL) or A640nm measurements were similar and displayed characteristic growth phases in a closed system. Exponential growth rates estimated both from turbidimetry and voltammetric techniques were the same.POCTI/QUI/39525/2001

Evaluation of antioxidant radical scavenging activity by means of electrogenerated hydroxyl radicals

Hydroxyl radical is one of the most reactive radicals. The generation of HO• represents an important task in different areas, such as organic synthesis [1], oxidative stress studies [2] and in the detoxification of effluents [3]. The generation of hydroxyl radicals based on metal catalyzed decomposition of hydrogen peroxide, by Fenton or Fenton-like reactions, is the most spread method. In the context of the mineralization of organic compounds from aqueous media, several papers report the electrochemical generation of hydroxyl radical by the oxidation of water. Boron doped diamond electrodes (BDD) have been successfully used as anode material for the production of hydroxyl radicals. The absence of chemical precursors is a great advantage of the electrochemical generation of HO• towards the Fenton-type reactions. The possible interference of the metal ions in antioxidant activity assays is also an important issue. In this context we tested the use of electrochemical generated HO• for the evaluation of antioxidant scavenging activity. The antioxidant scavenging activity is evaluated by means of inhibition of the HO• mediated oxidation of a non-electroactive species (sodium benzoate). The experimental conditions for the antioxidant scavenging activity assay were optimized namely, the anode material (Pt and BDD), current density and medium composition. The oxidation reactions were monitored by fluorescence, cyclic voltammetry and HPLC (UV detection).Fundação para a Ciência e a Tecnologia (FCT) - SFRH/BD/64189/200