Shirim Machado

Electrochemical and morphological studies of electrodeposited Ni–Fe–Mo–Zn alloys tailored for water electrolysis

Abstract The co-deposition of nickel, iron and molybdenum onto mild steel substrates with or without the addition and subsequent leaching of zinc rendered cathode materials very active for hydrogen generation in alkaline solutions. SEM analysis showed that adherence to the substrate can be achieved by initial deposition of the other components prior to Zn incorporation into the solution. Coatings of Ni–Fe–Mo (51.4-5.5-43.1 at%) and Ni–Fe–Mo–(Zn) (70.9-15.3-12.7-1.1 at% after leaching) were very active for hydrogen evolution at temperatures ranging from 25 to 80°C in 6 M KOH solutions. Steady-state polarisation curves furnished Tafel plots with slopes almost invariant with temperature and revealed an electrochemical activity comparable to the best materials reported so far. For the Ni–Fe–Mo–(Zn) coating the overpotential for hydrogen evolution at 80°C was as low as 83.1 mV . Long-time electrolyses at a constant current density of 135 mA cm −2 showed a considerable degradation of the surface and a decay in performance but the overpotential value after 440 h stabilised in 157 mV , a satisfactory value for industrial applications.

Electrochemical detection of the herbicide paraquat in natural water and citric fruit juices using microelectrodes

A novel electroanalytical procedure for detecting the paraquat herbicide in natural water and citric fruit juice samples using gold microelectrodes and square wave voltammetry at high frequencies is proposed. The results obtained showed two reversible peaks for the reduction of paraquat, the first peak associated with the reduction of the paraquat molecule in solution, with subsequent adsorption of the intermediate on the electrode surface. This adsorbed species was shown to undergo electroreduction in a reaction associated to the second voltammetric peak. The variation in pH and square wave parameters indicated that the best conditions under which paraquat could be reduced were a pH of 5.0, a frequency of 1000s-1, a scan increment of 2mV and a square wave amplitude of 50mV. Under these conditions, the variation of the concentrations of paraquat from 1.00×10-6 to 1.66×10-4molL-1 presented, for peak 1, detection and quantification limits of 4.51 and 15.05μgL-1 respectively in pure electrolyte with a recovery factor of 99.50%. The proposed analytical procedure was also applied to natural water samples giving recovery factors of 95.00, 89.50 and 92.50% in three water samples collected from an urban stream. The recovery factor was observed to depend on the content of organic matter which was determined by the biochemical and chemical oxygen demand. In lemon and orange juice samples that were spiked with 5.70×10-5molL-1 of paraquat, the recovery factors obtained were 94.30 and 92.70% respectively.

Electrochemical Behavior of Nicotine Studied by Voltammetric Techniques at Boron‐Doped Diamond Electrodes

Abstract The electrochemical behavior of nicotine in alkaline media was studied using a boron doped diamond (BDD) surface as the working electrode. In order to establish the pH dependence and to gain information about the mass transport of the species, cyclic voltammetry studies were carried out in a 0.1 mol L−1 BR (Britton‐Robinson) buffer in the presence of 1.0×10−3 mol L−1 nicotine. The optimum pH value was 8 and the mass transport was controlled by diffusion of the species. The square wave voltammetry technique was used to determine the electroanalytical parameters such as frequency, amplitude, and scan increment. After optimization, an analytical curve was constructed. The limits of detection and quantification were 0.50 and 1.66 mg L−1, respectively. Theoretical calculations indicate that the probable oxidation site on the nicotine molecule was the nitrogen atom denoted “11 N” and a speculation about the reaction mechanism was proposed. Finally, an experiment using a real sample (cigarette tobacco) was carried out and a recovery study revealed a value of about 4.3 mg L−1 in 0.1 g of tobacco.

Selective oxidation of pentachlorophenol on diamond electrodes

The influence of electrode potential on pentachlorophenol (PCP) oxidation on boron doped diamond (BDD) electrodes in a 0.1 mol L−1 Britton–Robinson buffer (pH 5.5) is described. Controlled potential electrolyses were carried at 0.9, 2.0 and 3.0 V vs Ag/AgCl and the solutions analysed by square wave voltammetry, high performance liquid chromatography, chloride ion selective electrode and spectroscopy in the ultraviolet–visible region. At low positive potential (0.9 V), the formation of an adherent film on the electrode surface involving the transference of 1 electron per PCP molecule was observed. The film was identified as the dimer 2,3,4,5,6-pentachloro-4-pentachlorophenoxy-2,5-cyclohexadienone and the current efficiency was as high as 90%. At potentials close to the onset of O2 evolution (2.0 V), the formation of the corresponding quinone (p-tetrachlorobenzoquinone) was detected at the beginning of the process. This was followed by further oxidation to the hydroxy-benzoquinone with a practically quantitative yield. Electrolyses carried out well into the region of oxygen evolution (3.0 V) lead to the electrochemical combustion of PCP to CO2 and H2O as well as to the release into solution of 5 Cl− ions per PCP molecule destroyed.

Carbon Surfaces for Electroanalytical Applications: A Comparative Study

Abstract Boron‐doped diamond (BDD), glassy carbon (GC), and pyrolytic graphite (PG) are distinct carbon surfaces that are widely used for electrochemical applications. The significant differences, either in morphological or electrical properties of such surfaces, result in charge‐transfer processes with totally distinct characteristics. In aqueous electrolyte, electrochemical impedance spectroscopy demonstrates that the BDD electrode presents lower capacitive currents and lower electron transfer resistance over the potential range studied compared with PG or GC. In this way, the BDD electrode is the best choice for several electroanalytical studies, as demonstrated by the electrochemical oxidation of 4‐nitrophenol in 0.1 mol L−1 BR buffer electrolyte (pH 6.0). We thank FAPESP (procs. 01/14320‐0, 03/00710‐6) and CNPq, Brazil, for the financial support. We also thank Eng. W. Haenni from CSEM, Switzerland, for providing the BDD electrode.

Electroanalytical Determination of the Herbicide Paraquat in Natural Water and Commercial Tea Samples with Gold Electrodes Obtained from Recordable Compact Disc

Abstract This work describes the application of gold electrodes constructed from recordable compact discs for the analytical determination of the herbicide paraquat in natural water samples using square wave voltammetry. The detection limit for pure water (laboratory samples) was 21 µg L−1, lower than the EPA limit for drinking water (100 µg L−1). The experimental quantification limit was determined as 73 µg L−1. In polluted creek water samples the detection limit rose to 76.4 µg L−1 and is shown to be dependent on BOD and COD values. Recovery measurements in tea and natural water samples were approximately 95%, which indicates that the methodology can be employed to analyze paraquat in such matrices. The authors acknowledge the financial support of FAPESP and CNPq.

Estudo do comportamento eletroquímico do herbicida ametrinautilizando a técnica de voltametria de onda quadrada

The determination of the herbicide ametryne in acid solution was performed using square wave voltammetry and a static drop mercury electrode. The electroredution reaction proceeds through a completely irreversible mechanism with the diffusion of the electroactive specie being the rate determining step. It was observed a previous protonation of the reagent molecule followed by the transference of two electrons. The linearity of peak current with concentration allows to calculate a detection limit of 0,1 mg L-1 (0,1 ppb). This value is adequate for the analysis of herbicide contamination (e.g., in drinking water).