This paper presents a comparative study about the oxygen reduction reaction (ORR) catalyzed by nanoparticles of Fe2O3 and Co3O4 applied on the surface of glassy carbon electrodes (GCE). The nanoparticles were synthesized using the modified polymeric precursor method (Pechini). These two nanomaterials were characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) techniques. The estimated average particle sizes were 21 and 31 nm for Fe2O3 and Co3O4, respectively. Electrochemical impedance spectroscopy (EIS) showed that Fe2O3/GCE has lower charge transfer resistance than Co3O4/GCE. The surface electrochemistry of both Fe2O3/GCE and Co3O4/GCE was studied in the solution free of O2, and their corresponding reaction mechanisms were analyzed. The electrocatalytic ORR activities of these two catalysts were studied by cyclic voltammetry (CV) and rotating disk electrode (RDE) in acidic solution. The results obtained by RDE indicated that both Fe2O3/GCE and Co3O4/GCE can catalyze the ORR with a dominating 2-electron transfer process to produce H2O2, using a potential of -0.8 V. These kinetic results indicate that Fe2O3/GCE is more efficient than Co3O4/GCE in terms of ORR. Considering the low cost of these two non-noble metal catalysts, they may be used as viable alternatives for ORR electrocatalysts.

A novel and simple electroanalytical method for the determination of Cu2+ in biodiesel samples by stripping voltammetric analysis was developed. The method employs a carbon paste electrode (CPE) modified with Vulcan carbon, previouly functionalized with carbazone (CBZ). The CPE/Vulcan-CBZ sensor promoted a significant increase in the analytical signal obtained from copper as compared to unmodified CPE, and the CPE modified with Vulcan carbon (CPE/ Vulcan). Vulcan-CBZ, Vulcan and CBZ materials were characterized by Fourier transform infrared spectroscopy (FTIR) technique. The electrochemical behavior of the sensor was evaluated using cyclic voltammetry (CV) and square-wave anodic stripping voltammetric (SWASV) techniques. The CPE/Vulcan-CBZ modified electrode showed excellent response and was able to detect Cu2+ at nanomolar levels. The electrochemical method is based on preconcentration of Cu2+ ions on the CPE/Vulcan-CBZ at 0.35 V vs. Ag/AgCl(sat) in 0.2 mol L-1 ammonium sulfate solution ((NH4)2SO4), pH 3.5, during 120 s, followed by subsequent chemical stripping. The analytical signal showed a linear response for Cu2+ concentrations in the range from 6 to 190 nmol L-1 (r = 0.998), with a detection limit of 1.2 nmol L-1. The sensor was successfully applied for the determination of Cu2+ in biodiesel and the average recovery varied between 97.0 and 102.8% for the soybean biodiesel samples and between 109.6 and 111.0% for the babassu biodiesel samples showing a good accuracy for the proposed method.

Eletrodo de carbono vítreo quimicamente modificado por filme fino de hexacianoferrato de rutênio (RuHCF) foi preparado pelo método da evaporação da gota. O eletrodo modificado RuHCF exibiu quatro processos redox em meio ácido forte (pH 1,5) atribuídos ao íon Fe(CN)6(3-) e três formas de rutênio (Ru(II), Ru(III) e Ru(IV)), características do óxido de rutênio. O eletrodo modificado mostrou excelente atividade eletrocatalítica para oxidação de etanol na região de potenciais onde os processos eletroquímicos Ru(III)-O-Ru(IV) e Ru(IV)-O-Ru(VI) ocorrem. Dados de espectroscopia de impedância mostraram que a resistência de transferência de carga diminui com o aumento do potencial aplicado e da concentração de etanol, indicando o uso do eletrodo modificado RuHCF como sensor para etanol. Em condições otimizadas, este sensor respondeu linearmente e rapidamente para etanol na faixa de concentração entre 0,03 e 0,4 mol L-1 com um limite de detecção de 0,76 mmol L-1, indicando uma sensibilidade adequada em análises de etanol.

Ruthenium-based hexacyanoferrate (RuHCF) thin film modified glassy carbon electrode was prepared by drop evaporation method. The RuHCF modified electrode exhibited four redox couples in strong acidic solution (pH 1.5) attributed to Fe(CN)6(3-) ion and three ruthenium forms (Ru(II), Ru(III) and Ru(IV)), characteristic of ruthenium oxide compounds. The modified electrode displayed excellent electrocatalytic activity towards ethanol oxidation in the potential region where electrochemical processes Ru(III)-O-Ru(IV) and Ru(IV)-O-Ru(VI) occur. Impedance spectroscopy data indicated that the charge transfer resistance decreased with the increase of the applied potential and ethanol concentration, indicating the use of the RuHCF modified electrode as an ethanol sensor. Under optimized conditions, the sensor responded linearly and rapidly to ethanol concentration between 0.03 and 0.4 mol L-1 with a limit of detection of 0.76 mmol L-1, suggesting an adequate sensitivity in ethanol analyses.

O presente trabalho mostra a modificação da superfície de eletrodos de grafite pelos compostos 8-hidroxiquinoleina (8-HQ) e ácido 8-hidroxiquinoleina 5-sulfônico (8-HQS). Voltametria cíclica foi usada para caracterizar o comportamento da superfície. A superfície modificada exibiu uma afinidade para complexar Cu(II) presente na solução, formando um complexo na superfície o qual foi empregado para análise de Cu(II) em nível de traços. Dos metais Zn, Ni, Pb, Co e Cd, nenhum apresentou interferência até um excesso de concentração de 10 vezes. Significante interferência foi observada para os íons Co(II), Cd(II) e Fe(II), para um excesso de concentração de 100 vezes sobre o analito. Voltametria no modo pulso diferencial, combinada com um processo de pré-concentração-redissolução e o método de adição padrão foram usados para as medidas analíticas. Um limite de detecção de 5,1<FONT FACE=Symbol>´</FONT>10-9 mol L-1 foi obtido na determinação de cobre em água.

Surface modification by 8-hydroxyquinoline-5-sulfonic acid (8-HQS) or 8-hydroxyquinoline (8-HQ) on a graphite electrode through irreversible adsorption is reported in this paper. Cyclic voltammetry was used to characterize the surface behavior. The modified surface exhibited an affinity to chelating Cu(II) in the solution, forming a Cu(II) complex, which was employed for Cu(II) trace analysis. Of the metals Zn, Ni, Pb, Co, and Cd, none presented interference until excess concentration of 10 times. Significant interference could be observed from Co(II), Cd(II) and Fe(II) for an excess concentration of 100 times on the analyte. A differential pulse voltammetry, combined with a preconcentrating-stripping process and a standard addition method was used for the analysis. A detection limit for trace copper determination in water, such as 5.1<FONT FACE=Symbol>´</FONT>10-9 mol L-1, was obtained.

The alizarin red S (ARS) has been used as a spectrophotometric reagent of several metals for a long time. Now this alizarin has been used as modifier agent of electrodes, for voltammetric analyses. In this work cyclic voltammetry experiments was accomplished on closed circuit, with the objective of studying the voltammetric behavior of alizarin red S adsorbed and of its copper complex, on the surface of the pyrolytic graphite electrode. These studies showed that ARS strongly adsorbs on the surface of this electrode. This adsorption was used to immobilize ions copper(II) from the solution.