In this work, La0.6M0.4Ni0.6Cu0.4O3 (M = Ag, Ba, and Ce, denoted as LANC, LBNC, and LCNC, respectively) electrocatalysts were synthesized by the Pechini method at 1023 K for two hours in air. Rietveld refinement allowed the identification of the crystallographic phases present in all oxides. The electrocatalytic performance of these oxides towards the oxygen reduction reaction (ORR) was examined in alkaline medium by rotating disk electrode (RDE) technique and scanning electrochemical microscopy (SECM) in the redox competition mode. The results indicate that the best performance was found with the LANC electrocatalyst prepared with carbon as a conducting agent (LANC/Carbon), which showed good catalytic activity towards the ORR via a pseudo four-electron transfer pathway. The enhanced electrocatalytic activity of LANC is probably a result of the presence of a Ag phase, which improves the synergistic effect between the perovskite and carbon added to increase the conductivity, thus leading to a higher ORR performance when compared to other materials.

A nanostructured porous gold film (NPGF) was fabricated onto a gold electrode surface by an electrochemical approach involving anodization followed by electrochemical reduction in absence of binary alloys, templates and chemical reducing agents. The electrocatalytic activity of the as-fabricated electrocatalyst was examined towards hydrogen evolution reaction (HER) and very low onset potential (−0.048 V) and overpotential values of −0.070 and −0.083 V at current densities of 10 and 20 mA cm -2, respectively, were obtained in 0.5 mol L -1 sulfuric acid solution, along with exchange current density (jo) of 0.126 mA cm -2. A small Tafel slope of ca. 36 mV decade -1 was achieved, indicating the HER takes place according to a Volmer-Heyrovsky mechanism and that the electrochemical desorption of hydrogen is the rate determining step. The performance of the NPGF electrode was compared with that of bulk gold and platinum electrodes, and a platinum-like behavior was noticed for HER. Finally, the durability of the proposed electrocatalyst was examined using voltammetric and amperometric techniques and an excellent stability was achieved.

Os sistemas estuarinos desempenham um importante papel na retenção de elementos traços devido à afinidade destes com algumas partículas dissolvidas em água. Nesse trabalho, apresenta-se o uso de um sensor voltamétrico para monitorar as concentrações de Zn (II), Cd(II) e Pb (II) na região do Sistema Estuarino Lagunar de Cananéia-Iguape (estado de São Paulo). No setor sul, (Cananéia) obteve-se menores valores de concentração, e no setor norte (Iguape) os valores foram maiores devido às atividades antrópicas, com ênfase à introdução prévia de resíduos de mineração e insumos de efluentes agrícolas, industriais e domésticos. O método proposto é viável, barato e rápido, e permite a determinação dos íons metálicos simultaneamente. Além disso, o dispositivo portátil é simples e pode ser facilmente utilizado para medições de campo a bordo de navios oceanográficos.

Estuarine systems play an important role in the retention of toxic trace elements owing to the affinity of these elements with particles dissolved in water. This work presents the use of a voltammetric sensor to monitor heavy metal (Zn (II), Cd(II) and Pb (II)) concentrations in the Cananéia-Iguape Estuarine-Lagoon region (São Paulo State, Brazil). Lower concentrations were found in the Southern estuarine system (Cananéia City) and increased concentrations observed in the Northern sector (Iguape City) were promoted by anthropogenic activities, with particular influence from the historical introduction of mining wastes and inputs from agricultural, industrial and domestic effluents. The proposed method is reliable, inexpensive and fast, can simultaneously provide information on the concentration of these metallic ions and can be easily used for field measurements aboard oceanographic ships.

Alterações morfológicas em superfícies de cobre resultantes da aplicação de um protocolo definido de potencial foram examinadas por microscopia eletrônica de varredura (MEV) e microscopia de força atômica (AFM). Os resultados mostraram uma boa correlação entre o tempo empregado na ativação do eletrodo, microestrutura resultante e a atividade eletroquímica.

Morphologic changes on copper surfaces upon applying an established potential protocol were examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results showed a good correlation between the time employed in the electrode activation and the resulting microstructure and electrochemical activity.

Common student misunderstandings regarding chemical equilibrium are reported taking into account the response of freshman students to some typical questions involving acid-base reactions. Language has been shown to be one of the main issues associated with students' difficulties on this subject, such as in the case of concentration and amount of substance (mol). Another usual problem was observed in questions involving buffer solutions after addition of a reagent. A number of recommendations have emerged from the results presented in this work as an attempt to enhance the learning of this topic in general chemistry courses given to first-year students.

Analysis at microenvironments, like single cells or in minute volumes (nL), is an area of great interest for analytical and biological sciences. Measurements at these experimental conditions demand analytical tools (microelectrodes) capable of monitoring with rapid response, good resolution and minimal perturbation of the system. The major drawbacks in producing these microscopic electrodes have been largely overcome, principally due to the development of new fabrication methods. In this review, these procedures are described with emphasis to those devoted to the construction of microelectrodes for application in microenvironments. Examples of our efforts to use these devices as effective electrochemical sensors are also addressed.

No presente trabalho são apresentados resultados referentes à fabricação de um biossensor para análise de lactato. O dispositivo foi preparado imobilizando-se a enzima lactato oxidase em filme de Azul da Prússia (PB, sigla em inglês) depositado na superfície de eletrodo de carbono vítreo em soluções de CTAB. Nestas condições experimentais observou-se melhoria na sensibilidade e seletividade das determinações de lactato, as quais foram conduzidas em sistema FIA com detecção amperométrica. Nas condições otimizadas (pH = 6,9 e E = -0,1 V), obteve-se curva analítica com limite superior igual a 0,28 mmol L-1 e um limite de detecção de 0,84 µmol L-1. A repetibilidade do método foi avaliada por meio de injeções consecutivas de amostras de lactato 0,28 mmol L-1, obtendo-se desvio padrão de 2,2 % (n = 18). O método foi aplicado em amostras de cerveja e houve boa concordância entre os resultados obtidos e aqueles oriundos do uso de procedimento espectrofotométrico de referência.

An amperometric lactate biosensor with lactate oxidase immobilized into a Prussian Blue (PB) modified electrode was fabricated. The advantage of using cetyltrimethylammonium bromide (CTAB) in the electrodeposition step of PB films onto glassy carbon surfaces was confirmed taking into account both the stability and sensitivity of the measurements. The biosensor was used in the development of a FIA amperometric method for the determination of lactate. Under optimal operating conditions (pH = 6.9, E = -0.1 V), the linear response of the method was extended up to 0.28 µmol L-1 lactate with a limit of detection of 0.84 mmol L-1. The repeatability of the method for injections of a 0.28 mmol L-1 lactate solution was 2.2 % (n = 18). The usefulness of the method was demonstrated by determining lactate in beer samples and the results were in good agreement with those obtained by using a reference spectrophotometric enzyme method.

A diagnostic instrument was developed to evaluate the basic chemistry concepts held by freshmen students of the three Chemistry undergraduate courses offered by the University of São Paulo. The instrument minimizes the use of algorithms or memorization by students and values high-order cognitive skills. Analysis of the students' performances reveals systematic use of "displacement reaction" as an algorithm and a mechanical use of Le Chatelier's Principle. Failure in comprehending the chemical equation and chemical language drives students to alternative models for chemical reactions in aqueous solution. For instance, reaction would occur between "ionic pairs" and/or between species situated in separate compartments.

Este trabalho apresenta uma técnica alternativa de microfabricação para construção de microcélulas eletroquímicas descartáveis contendo os eletrodos de trabalho, auxiliar e de referência em um único dispositivo. Trata-se de um processo simples no qual a termo-transferência de máscaras de toner sobre superfícies de ouro obtidas a partir de discos compactos graváveis (CD-R) definem o formato dos eletrodos (contornos). Numa etapa seguinte, o formato é manualmente coberto com tinta usando uma caneta de retroprojetor. A superfície de ouro não protegida pela tinta é convenientemente removida (corrosão química) e a tinta, por sua vez, é facilmente removida com etanol gerando superfícies de ouro sem contaminação. A área final e reprodutível dos eletrodos é definida pela termo-transferência de uma segunda máscara de toner. O eletrodo de referência é obtido por meio de uma deposição manual de cola de prata sobre uma banda de ouro. As microcélulas foram eletroquimicamente caracterizadas usando as técnicas de voltametria cíclica, voltametria linear e voltametria de onda quadrada e várias espécies eletroativas como sistemas modelo. A reprodutibilidade de área dos eletrodos para diferentes microcélulas foi estudada e um desvio-padrão relativo melhor do que 1,0% foi obtido. As microcélulas descartáveis foram usadas com sucesso para a análise de amostras em volumes menores que 200 µL demonstrando boa estabilidade e reprodutibilidade (RSD menor que 2,0%). Essas microcélulas também foram avaliadas para a quantificação de paracetamol e dipirona em formulações farmacêuticas.

An alternative technique for the fabrication of disposable electrochemical microcells containing working, reference and auxiliary electrodes on a single device is reported. The procedure is based on thermal-transfer of toner masks onto CD-R (recordable compact discs) gold surfaces to define the layout of the electrodes (contour). In a subsequent step, the layout is manually painted with a permanent marker pen. The unprotected gold surface is conveniently etched (chemical corrosion) and the ink is then easily removed with ethanol, generating gold surfaces without contamination. The final and reproducible area of the electrodes is defined by heat transference of a second toner mask. Silver epoxy is deposited on one of the gold bands which is the satisfactorily used as reference electrode. These microcells were electrochemically characterized by cyclic, linear, and square wave voltammetry, and several electroactive species were used as model systems. The area reproducibility of the electrodes for different microcells was studied and a relative standard deviation better than 1,0% (n = 10) was obtained. Disposable electrochemical microcells were successfully used in analysis of liquid samples with volumes lower than 200 µL and good stability and reproducibility (RSD less than 2.0%) were achieved. These microcells were also evaluated for quantification of paracetamol and dipyrone in pharmaceutical formulations.

Electrochemical sensors have attracted considerable attention in recent years because they provide data about the chemical state of our surroundings and the dynamics of the chemical transformations in the form a spatially resolved image. Particular interest has been directed to measurements in restricted-volume samples as new technologies enable the fabrication of miniaturized versions of sensors with reproducible characteristics. Taking these aspects into consideration, this review focuses on the use of electrodes of micrometer dimensions to acquire chemical information in microdomains in which concentrations may not be spatially homogeneous. This is possible because microelectrodes allow fast-response measurements with micrometer resolution to be performed. On the other hand, the use of microelectrodes as amperometric sensors presents an inherent drawback owing to the insufficient specificity toward the substrate of interest. Hence, some comments on strategies to enhance the selectivity of amperometric sensors are also made. Finally, recent applications of structurally microscopic electrodes as in vivo sensors are shown, as well as a prospect of the future trend in this field.

A autoxidação dos complexos de Ni(II) e Co(II) com tetra, penta e hexaglicina, em meio de tampão borato, é acelerada por espécies de enxofre(IV) (H2SO3, HSO3- and SO3(2-)). A formação dos complexos de Ni(III) e Co(III) foi acompanhada espectrofotometricamente em 325 e 265 nm, respectivamente. Técnicas eletroquímicas também foram empregadas para caracterizar a geração destes complexos. A velocidade da reação de autoxidação aumenta com a concentração de S(IV) e é máxima em pH <FONT FACE=Symbol>@</FONT> 8,5. O processo é autocatalítico com Ni(III) ou Co(III) atuando como iniciadores, formados pela oxidação espontânea de Ni(II) ou Co(II) pelo oxigênio molecular. A dependência da constante de velocidade de pseudo-primeira ordem com a concentração de sulfito evidenciou possíveis reações paralelas com formação de um complexo com ligantes mistos antes da etapa da oxidação.

The autoxidation of Ni(II) and Co(II) complexes with tetra, penta and hexaglycine, in borate medium, is accelerated by sulfur(IV) species (H2SO3, HSO3- and SO3(2-)). The formation of Ni(III) and Co(III) complexes was followed spectrophotometrically at 325 and 265 nm, respectively. Electrochemical techniques were also employed to characterize the generation of these complexes. The autoxidation rate increases with S(IV) concentration and is maximum at pH <FONT FACE=Symbol>@</FONT> 8.5. The process is autocatalytic with Ni(III) or Co(III) acting as initiators, formed by spontaneous oxidation by oxygen. The dependence of the pseudo first order rate constant with sulfite concentration showed evidences of back or parallel reactions with formation of mixed ligand complex prior to the oxidation step.

A oxidação anódica de eletrodos de cobre em meio alcalino foi investigada utilizando-se voltametria, cronoamperometria, medidas de impedância eletroacústica e microbalança eletroquímica de cristal de quartzo (MECQ). Os experimentos foram realizados em soluções de NaOH na faixa de concentração de 0,1 a 3,0 mol L-1. A formação de espécies solúveis e insolúveis como Cu(OH)4(2-) and Cu2O durante o processo de dissolução eletroquímica foi caracterizada por estudos potenciodinâmicos e pela análise das relações entre massa e carga. Por outro lado, os resultados obtidos utilizando a MECQ não foram satisfatórios para confirmar a natureza das espécies de cobre propostas na literatura como CuO e Cu(OH)2, visto que há mudanças viscoelásticas do filme eletrodepositado sobre o cristal de quartzo. A participação de espécies solúveis de Cu(III) no processo eletrocatalítico de oxidação do etanol foi caracterizada pelas medidas obtidas com a MECQ. Com estes dados foi possível propor um modelo para explicar o processo eletródico envolvendo a oxidação do etanol na superfície de cobre.

The anodic oxidation of copper electrodes in alkaline solutions was investigated by using voltammetry, chronoamperometry, impedance measurements and the electrochemical quartz crystal microbalance (EQCM). Experiments were carried out in NaOH solutions in the 0.1 to 3.0 mol L-1 concentration range. The formation of soluble and insoluble species such as Cu(OH)4(2-) and Cu2O during the electrodic dissolution of copper anodes was characterized by analyzing potentiodynamic mass responses and the relationship between mass changes and charge. On the other hand, EQCM data were not effective to confirm the nature of other copper species proposed in literature named as CuO and Cu(OH)2 because of viscoelastic changes in the film layer electrodeposited onto the quartz crystal. The participation of a Cu(III) soluble species in the electrocatalytic oxidation of ethanol was proved by EQCM measurements, data providing valuable information on the mechanism of the electrode process and formation of a Cu(II) insoluble species from the reaction of Cu(III) with ethanol.

A oxidação de complexos de Cu(II) com tetra, penta e hexaglicina, em solução aquosa de tampão borato, pelo oxigênio dissolvido é fortemente acelerada por sulfito. A formação de complexos de Cu(III) com máximos de absorbância em 250 nm (e = 9000 mol-1 L cm-1) e 365 nm (e = 7120 mol-1 L cm-1) foi também caracterizada usando-se voltametria com eletrodo rotativo disco-anel, na qual componentes anódicos e catódicos foram observados em voltamogramas registrados em solução contendo Cu(II). Voltamogramas, obtidos com várias velocidades de rotação, mostraram que a espécie de Cu(III) gerada eletroquimicamente não é estável em toda a janela de tempo do experimento, e em solução contendo tetraglicina a corrente limite é controlada pela cinética de um equilíbrio envolvendo espécies de Cu(II). O valor calculado da constante de decomposição de primeira ordem foi 4,37x10-3 s-1. Experimentos eletroquímicos realizados em solução de Cu(II) após a adição de quantidades relativamente pequenas de sulfito demonstraram que a espécie de Cu(III), formada na reação química, é a mesma que foi coletada no eletrodo anel quando Cu(II) é oxidado no eletrodo disco. A concentração dos complexos de Cu(III) é proporcional à quantidade de sulfito adicionada e os resultados indicaram a possibilidade de desenvolvimento de um método analítico indireto para sulfito, com detecção espectrofotométrica ou amperométrica do produto quimicamente gerado.

The oxidation of Cu(II) complexes with tetra, penta and hexaglycine in borate buffer aqueous solution, by dissolved oxygen is strongly accelerated by sulfite. The formation of Cu(III) complexes with maximum absorbances at 250 nm (e = 9000 mol-1 L cm-1) and 365 nm (e = 7120 mol-1 L cm-1) was also characterized by using rotating ring-disk voltammetry, whose anodic and cathodic components were observed in voltammograms recorded in solutions containing Cu(II). Voltammograms, obtained at various rotation speeds, showed that the Cu(III) species electrochemically generated is not stable over the entire time window of the experiment and in solutions containing tetraglycine the overall limiting current is controlled by the kinetics of an equilibrium involving Cu(II) species.The calculated first order rate constant of the decomposition was 4.37x10-3 s-1. Electrochemical experiments carried out in Cu(II) solutions after the addition of relatively small amounts of sulfite demonstrated that the Cu(III) species formed in the chemical reaction is the same as the one collected at the ring electrode when Cu(II) is oxidized at the disk electrode in ring-disk voltammetry. The concentration of Cu(III) complexes is proportional to the amount of added sulfite and the results indicated that indirect analytical methods for sulfite may be developed by means of spectrophotometric or amperometric detection of the chemically generated product.

In this paper some studies concerning the electroreduction of Mo(VI) in sulphuric acid solutions are described. We have shown that at suitable experimental conditions very stable molybdenum oxide films can be electrochemically deposited at glassy carbon electrodes, the reduction of bromate occurring at less negative potentials on the modified surface. Coulometric experiments have shown that bromide is not the only product of the catalytic bromate reduction by the molybdenum film and species like BrO2 may have part in this process. Based on chronoamperometric curves recorded at -0.60 V, analytical curves have been obtained for the reduction of bromate in the 0.1 - 0.8 mM range, a limit of detection of 20 µM for bromate being determined.