M.D. Vázquez

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.

Electrochemical study of iron(II) and iron(III) compound mixtures in the solid state. Application to magnetite characterization

Abstract The electrochemical behaviour of mixtures of several iron(II) and iron(III) compounds was studied in acidic media at a carbon paste electrode. Voltammetric techniques were used to obtain electrochemical spectra of these mixtures which were interpreted in order to elucidate the nature of their electrochemical transformations. Appropriate limited potential scans were used to verify the identity of the different chemical species involved in the electrochemical reactions. Finally, the electrochemical behaviour of magnetite was investigated and a theoretical explanation of the different processes was proposed based on its crystallographic structure.

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.

Determination of aceclofenac using adsorptive stripping voltammetric techniques on conventional and surfactant chemically modified carbon paste electrodes.

An electroanalytical method for determination of the anti-inflammatory agent aceclofenac at the ppb level using adsorptive stripping voltammetric techniques on conventional and surfactant chemically modified electrodes is developed. In this electroanalytical study a process of aceclofenac adsorption on carbon particles has been identified. In order to improve the aceclofenac determination, a chemical modification of the carbon paste was carried out employing several hydrophobic substances such as phospholipid and fatty acids, as well as several surfactants such as the non-ionic Triton X-100 and Triton X-405 and anionic sodium dodecyl sulfate. The influence of parameters such as pH, preconcentration time, carbon Nujol ratio, surfactant carbon ratio, potential scan rate, etc., on sensitivity of the method were widely studied. The proposed method was applied to the determination of the active compound in tablets.

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

Voltammetric determination of piroxicam in micellar media by using conventional and surfactant chemically modified carbon paste electrodes

A method for piroxicam determination based on adsorptive stripping voltammetric techniques, using conventional and chemically modified carbon paste electrodes in micellar media, is described. The employed surfactants were sodium dodecyl sulfate (SDS), Triton X-100, Triton X-405, Tween 80 and Brij 30. However, the purpose of this paper is, at present, to research the use of surfactants as carbon paste modifier because one of the mechanisms of hydrophobic drugs ad-accumulation on the carbon paste electrode is based on the chemical affinity. Besides, because of the water piroxicam insolubility, a special aqueous medium, such as a surfactant solution above its c.m.c. was used, this micellar media being very advantageous in relation to the use of organic or aqueous-organic media, in order to dissolve the studied drug and to remove the problems derived from the organic solvents use. In addition, a piroxicam ad-accumulation increase, on surfactant modified carbon paste electrode, with the surfactant mass incorporated into the electrode, was observed.

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.

Electrochemical behaviour of copper, lead, and bismuth solid compounds and their ternary mixtures at solid state at a carbon paste electrode with electrolytic binder. Application to the study of the high transition temperature superconductor Bi0.7 · Pb0.3 · Sr · Ca · Cu1.8 · Ox

Abstract The electrochemical behaviour of solid species of different oxidation states of copper, lead, and bismuth and their ternary mixtures is studied by scan voltammetry using a carbon paste electrode with electrolytic binder and electroactive compound incorporated (CPE-EB-ECI). The electrolytic binder was a 10N KOH solution, the scan potential range was between +0.50 and −1.50 V (sce), and the scan rate was 0.5 mVs−1. The obtained electrochemical voltammograms were employed to elucidate the corresponding ion electrochemical transformations. In this way, the obtained conclusions were utilized to characterize the different peaks of the electrochemical profile of the high transition temperature superconductor Bi0.7 · Pb0.3 · Sr · Ca · Cu1.3 · Ox.

Electrochemical behaviour of droxicam: kinetic study in aqueous-organic media

A kinetic study of the hydrolytic decomposition of droxicam was carried out in order to establish the possible pharmacological action of the drug in the organism of the human being. The electrochemical technique used was scan cyclic voltammetry on a HMDE, giving a well-defined peak. Due to the insolubility of droxicam in water, the working media was a methanol-H(2)O (4:96, V/V) mixture. The decomposition product of droxicam is piroxicam, which is also reduced on a mercury electrode giving a well-defined cyclic voltammetric peak. It is proposed that the decomposition of droxicam is controlled by a kinetic equation: v=(1.93 x 10(-3)+78.9 [D]) [D]=k(D)[D].