P. Sánchez Batanero

Voltammetric determination of the phenolic antioxidants 3-tert-butyl-4-hydroxyanisole and tert-butylhydroquinone at a polypyrrole electrode modified with a nickel phthalocyanine complex.

The voltammetric behaviour of the antioxidants 3-tert-butyl-4-hydroxyanisole (BHA) and tert-butylhydroquinone (TBHQ), at a polymer electrode modified with nickel phthalocyanine as electron mediator, is described, and an electroanalytical method for the determination of these antioxidants based on their electrochemical oxidation on the modified electrode is proposed. Cyclic voltammograms showed well-defined oxidation peaks slightly shifted towards less positive potentials with respect to those obtained at the platinum disk electrode for both antioxidants. The peak currents measured at the modified electrode are considerably higher than those obtained at the unmodified electrode. A scan rate of 100 mV s(-1), a minimum methanol percentage of 0.5% and a 0.1-M Britton-Robinson medium were chosen as working conditions. The obtained results suggest that the BHA oxidation produces the TBHQ reduction product. This product is oxidised during the second cyclic scan, to generate the mentioned TBHQ as final product. These studies also reveal that sensitive response for both antioxidants can be obtained by using the differential pulse voltammetry (DPV) technique. Responses observed are dependent on the nature of the control solution (% methanol and pH) and the nature of voltammetric conditions (potential amplitude, DeltaE, and scan rate, v(b)). These factors have been modified in order to find the best analytical conditions. The mutual interferences between both antioxidants and the measurement reproducibility were tested. Using cyclic voltammetry (CV) and differential pulse voltammetry, BHA and TBHQ linear calibration graphs were obtained. The detection limits were 2.1 ppm for both when CV was used, and 18.7 ppm (BHA) and 1.23 ppm (TBHQ) for DPV. The developed methods were applied to the determination of BHA in spiked glaze biscuits and TBHQ in spiked mushroom cream.

Electrochemical study of antioxidants at a polypyrrole electrode modified by a nickel phthalocyanine complex. Application to their HPLC separation and to their FIA system detections

A polypyrrole electrode modified with a tetrasulfonated nickel(II) phthalocyanine complex (GCE/PPy/NiPcTs) was used as amperometric electrochemical detector in a flow injection system, in order to determine some antioxidants in foods. The lather were separated by HPLC and detected at the modified electrode. Previous investigations carried out in our laboratory employed this electrode and cyclic voltammetry (CV), obtaining excellent results, due to an electrocatalytic effect on the oxidation of certain alimentary antioxidants, such tert-butylhydroquinone (TBHQ), 3-tert-butyl-4-hydroxyanisole (BHA) and propylgallate (PG). In order to obtain good separation, the chromatographic conditions were optimised. The geometry of the flow injection analysis (FIA) circuit and the reproducibility of the results were also optimised and, finally, the linear relationship between the signal and the concentration was verified. The proposed procedure is more precise and accurate than the conventional voltammetry methods. These results offer new analytical possibilities to this type of modified electrodes. The developed methods were applied to the determination of these antioxidants in commercial food.

Electroanalytical study of selenium(+IV) at a carbon paste electrode with electrolytic binder and electroactive compound incorporated

Abstract Electroanalytical behaviour of Na 2 SeO 3 at solid state as well as in solution with a carbon paste electrode is studied by voltammetric techniques. We have shown that Se(+IV) is reduced to grey Se and after to Se(−II) and red Se, as well as that Na 2 SeO 4 is not electroactive with this electrode. Finally we have shown that it is possible to determine Se(+IV) voltammetrically by using this type of electrode.

Preparation of a polypyrrole electrode modified with a nickel phthalocyanine complex. Application to the determination of an antioxidant (propylgallate) in foods

In this paper a study of the electrocatalytic oxidation of propylgallate (PG) by using a polypyrrole electrode modified with tetrasulfonate nickel (II) phthalocyanine complex (Pt/PPy/NiPcTs) was carried out. Several parameters of this polymeric electrode, such as: (a) its stability in hydroorganic solution, by using an indium tin oxide (ITO) electrode and spectrophotometry; and (b) its thickness using electrochemical techniques, have been studied. In addition, several parameters affecting the PG voltammetric peak, such as pH, methanol water ratio, potential scan rate and analyte concentration, were considered. Finally, the developed method was applied to the determination of PG in spiked potato flakes and cornflakes, showing its validity.

Electrochemical study of PbO2at a carbon paste electrode with electrolytic binder

The electrochemical behaviour of PbO2 at the solid state at a carbon paste electrode is studied. Several experiments with Pb3O4 and PbO are also carried out in order to elucidate the electrochemical transformation of PbO2, occuring within +0.65 to −1.5 (sce) range, into the carbon paste electrode with NaOH 0.1 M as electrolytic binder. Finally constant potential chronoamperometry is employed to determine the percentage of this transformation

Electrochemical behaviour of copper oxides at a carbon paste electrode. Application to the study of the superconductor YBaCuO

Abstract The electrochemical behaviour of Cu2O and CuO in solid-state at a carbon paste electrode with an electrolytic binder is studied. We have carried out several experiments to elucidate the electrochemical transformation of copper oxide in 10 M KOH medium occuring at +300 to −1400 mV (itsce). The voltammograms have been employed to propose several charge-transfer mechanisms. In addition, we have recorded the voltammograms of the superconductor YBaCuO at the same experimental conditions in order to elucidate the oxidation states of the copper in the solid.

Étude du comportement électrochimique de l'oxyde d'étain solide a l'aide de l'électrode à pǎte de carbone

Abstract A voltammetry study of solid state SnO 2 has been carried out with the carbon paste electrode. This study confirms the presence of two allotropic forms. Chromnoamperometry shows that the determination of SnO 2 in casiterite is possible if we use the calibration curve Q exp = f ( m o ) although electrolytic transformation is not accomplished.

Electrochemical behaviour of bismuth(V) and bismuth(III) compounds at a carbon paste electrode. Application to the study of the superconductor BiSrCaCuO

The electrochemical behaviour of NaBiO3 and Bi2O3 at solid state at a carbon paste electrode with electrolytic binder has been studied. We have carried out several experiments to elucidate the electrochemical transformation of both bismuth compounds in 10 M KOH media between +500 and −1500 mV (sce). The voltammograms have been employed to propose several mechanisms of charge transfer. Besides, we have recorded the voltammograms of the Bi2Sr2CaCu2O8−x superconductor in the same media in order to elucidate its oxidation state distribution.

Chemical and electrochemical behaviour of lead ions in the ZnCl22NaCl mixture at 450 °C

Abstract The stability of lead chloride and oxide as well as the electrochemical behaviour of Pb 2+ ions in the ZnCl 2 2NaCl mixture at 450 °C has been studied using the techniques of potentiometry, cyclic voltammetry, chronoamperometry and chronopotentiometry. The lead oxidation states 0 and II have been shown to exist in this melt, showing that Pb(IV) is a powerful oxidizing agent which oxidizes the chloride ion of the melt according to the reaction: Pb ( IV ) + 2 Cl − ↔ Pb ( II ) + Cl 2 ( g ) The standard potential of the redox couple Pb(II) Pb(0) has been determined by potentiometry ( E 0 Pb(II) Pb(0) = −1.383 ± 0.01 rmV) (vs. Cl 2 (1 atm ) Cl − ). It was shown that PbO is not stable and enters acid-base reaction with the melt according to the reaction: Zn ( II ) + PbO ( s ) ↔ ZnO ( s ) + Pb ( II ) The solubility products of lead oxide have been determined ( pKs = − 2.6 ± 0.1 on a molality scale). All these results have enabled the construction of the E - pO 2− equilibrium diagram. Using a tungsten electrode, it has been observed that the Pb(II)//Pb(0) exchange is quasi-reversible; log k s 0 and α values for this reaction are (−4.3 ± 0.1) and (0.48 ± 0.06) respectively. Mass transport towards the electrode is a simple diffusion process, the average diffusion coefficient D pb ( II ) is (7.5 ± 0.7) × 10 −6 , and the thickness of the diffusion layer δ by thermal convection is (7.5 ± 0.2) × 10 −3 cm.

Polarographic study on the evolution of the diphenylamine as stabiliser of the solid propellants.

Differential pulse polarography (DPP) and square wave voltammetry (SWV) were investigated, in order to know the stability of solid propellants which contain diphenylamine. The simultaneous determination of N-nitrousdiphenylamine (NnDPA), 2-nitrodiphenylamine (2nDPA), 4-nitrodiphenylamine (4nDPA) and 2,4-dinitrodiphenylamine (2,4dnDPA) by DPP and SWV was proposed due to these nitro-derivatives appear during the stabilisation process from degradation of diphenylamine (DPA) used as stabiliser in propellant compositions. The proposed methods were successfully applied to the simple base solid propellant (with nitrocellulose as the only active component), with its stabiliser DPA. In all cases and with both the techniques, detection limits </=0.01ppm were obtained. When the usual LC procedure was applied to the real sample, no significant differences were found between the obtained results and those given by the electroanalytical techniques. In addition, the detection limits reached by the electrochemical methods were better than those obtained by LC. Moreover, the proposed procedure can be considered an objective test that would avoid the use of the classic stability tests and would allow one to determine the stability of propellants accurately, faster and cheaper than LC methods.