Silver chloride modified carbon paste electrode was prepared as a new electrode and used to electrocatalytic oxidation of ethanol. For the first time, the catalytic oxidation of ethanol was demonstrated by cyclic voltammetry, chronoamperometry and amperometry methods at the surface of this modified carbon paste electrode. Compared to silver chloride modified carbon paste electrode, silver bromide modified carbon paste electrode and bare silver electrode catalysts, silver chloride modified carbon paste electrode exhibited markedly superior catalytic activity for the electrocatalytic oxidation of ethanol. It can be seen that the electrocatalytic efficiency of silver chloride modified carbon paste electrode is higher than the silver bromide modified carbon paste and bare silver electrodes. The catalytic oxidation peak current was linearly dependent on the ethanol concentration. The j0 for silver chloride modified carbon paste and silver bromide modified carbon paste electrodes are 11.2 and 5.4 folds respectively higher than that of the bare silver electrode. For silver chloride modified carbon paste electrode, the charge transfer coefficient (α), the number of electrons involved in the rate determining step (nα) and exchange current density (j0) were calculated as 0.46, 1 and 5.05×10-7 respectively. The modified electrode possesses high selectivity, good reproducibility and well stability.

Um novo procedimento foi desenvolvido para microextração líquido-líquido dispersiva de cádmio antes de sua determinação por espectrometria de absorção atômica com chama. A maior dificuldade na combinação da microextração líquido-líquido dispersiva com espectrometria de absorção atômica é a alta absorção de fundo dos solventes de extração e dispersores que encobrem a absorbância do analito. A absorção de fundo foi removida trocando o solvente orgânico da fase sedimentada por um solvente aquoso. No sistema proposto, tetracloroetileno (C2Cl4) e tetrahidrofurano (THF) foram usados como solventes extrator e dispersivo, respectivamente, e, como agente complexante foi usada a ditizona. Vários fatores que podem afetar o processo de extração, como solvente dispersor, solvente de extração, o volume desses solventes, tempo de extração, temperatura da solução, concentração do agente quelante e pH foram otimizados. O efeito de interferentes na recuperação de cádmio foi estudado. Os resultados mostraram que as recuperações de cádmio são quase quantitativas na presença de íons interferentes. Sob condições ótimas, os fatores de enriquecimento e de pré-concentração foram 34,5 e 26, respectivamente; as recuperações de extração e relativa foram 69% e 99%, respectivamente. O sistema mostrou-se linear no intervalo de concentração de 5 a150 µg L-1, com limite de detecção de 1,2 µg L-1 e desvio padrão relativo (n = 8) de 2,1%. O método proposto foi aplicado com sucesso na determinação de cádmio em amostras de água

A new procedure was developed for dispersive liquid-liquid micro extraction of cadmium prior to its determination by flame atomic absorption spectrometry. The major difficulty in combination of dispersive liquid-liquid microextraction with flame atomic absorption spectrometry is high background absorption of the extraction and disperser solvents that covers the absorbance of analyte. The background absorption was removed by changing the solvent of sedimented phase from organic to aqueous. In the proposed approach, tetrachloroethylene (C2Cl4) and tetrahydroforan (THF) were used as extraction and dispersive solvents respectively. Dithizone was used as complexing agent. Several factors that may be affect on the extraction process, such as, extraction solvent, disperser solvent, the volume of extraction and disperser solvent, extraction time, temperature of solution, concentration of chelating agent and pH were optimized. The effects of common coexisting ions on the recovery of cadmium were studied. Results showed that cadmium recoveries are almost quantitative in the presence of interfering ions. Under the optimal conditions the enrichment and enhancement factors were 34.5 and 26 respectively, the extraction and relative recoveries were 69% and 99% respectively, the calibration graph was linear in the range of 5-150 µg L-1, the detection limit was 1.2 µg L-1 and the relative standard deviation (n = 8) was 2.1%. The proposed method was applied successfully for determination of trace amounts of cadmium in water samples

Este estudo investigou a oxidação eletrocatalítica de frutose, sacarose, glucose, lactose, arabinose e maltose em nanopartículas de hidróxido de cobalto eletrodepositadas na superfície de eletrodo de carbono vítreo em soluções alcalinas. Voltametria cíclica (VC), cronoamperometria (CA) e também medidas de polarização de estado estacionário foram empregados. Nos estudos de VC, na presença de carboidrato, o aumento na corrente de pico da oxidação do hidróxido de cobalto é seguido pelo decréscimo na correspondente corrente catódica. Isto indica que os carboidratos foram oxidados pelo mediador redox, o qual foi imobilizado na superfície do eletrodo, via um mecanismo eletrocatalítico. Com o uso da equação de Laviron, os valores dos coeficientes de transferência de elétron, anódicos e catódicos, e a constante da razão de transferência de carga para as espécies redox imobilizadas foram determinadas como 0,71 ± 0,03, 0,33 ± 0,03 e 0,21 ± 0,02 s-1, respectivamente. A constante de velocidade, o coeficiente da transferência de elétron e os coeficientes de difusão envolvidos na oxidação eletrocatalítica dos carboidratos foram determinados.

This study investigated the electrocatalytic oxidation of fructose, sucrose, glucose, lactose, arabinose and maltose on nanoparticles of cobalt hydroxide electrodeposited on the surface of glassy carbon electrode in alkaline solutions. Cyclic voltammetry (CV), chronoamperometry (CA) techniques and also steady state polarization measurements have been employed. In CV studies, in the presence of carbohydrate the increase of the peak current of the oxidation of cobalt hydroxide is followed by a decrease in the corresponding cathodic current. This indicates that carbohydrates were oxidized by the redox mediator which was immobilized on the electrode surface via an electrocatalytic mechanism. With the use of Laviron's equation, the values of anodic and cathodic electron-transfer coefficients and charge transfer rate constant for the immobilized redox species were determinated as 0.71 ± 0.03, 0.33 ± 0.03 and 0.21 ± 0.02 s-1, respectively. The rate constant, the electron transfer coefficient and the diffusion coefficients involved in the electrocatalytic oxidation of carbohydrates were determined.