Instabilities in electrochemical systems with a rotating disc electrode

Polarization curves experimentally obtained in the electro-dissolution of iron in a 1 M H2SO4 solution using a rotating disc as the working electrode present a current instability region within the range of applied voltage in which the current is controlled by mass transport in the electrolyte. According to the literature (Barcia et. al., 1992) the electro-dissolution process leads to the existence of a viscosity gradient in the interface metal-solution, which leads to a velocity field quantitatively different form the one developed in uniform viscosity conditions and may affect the stability of the hydrodynamic field. The purpose of this work is to investigate whether a steady viscosity profile, depending on the distance to the electrode surface, affects the stability properties of the classic velocity field near a rotating disc. Two classes of perturbations are considered: perturbations monotonically varying along the radial direction, and perturbations periodically modulated along the radial direction. The results show that the hydrodynamic field is always stable with respect to the first class of perturbations and that the neutral stability curves are modified by the presence of a viscosity gradient in the second case, in the sense of reducing the critical Reynolds number beyond which perturbations are amplified. This result supports the hypothesis that the current oscillations observed in the polarization curve may originate from a hydrodynamic instability.

Eletrodos quimicamente modificados aplicados à eletroanálise: uma breve abordagem

Chemically modified electrodes (CMEs) have been subject of considerable attention since its inception about 23 years ago. CMEs result of a deliberate immobilization of a modifier agent onto the electrode surface obtained through chemical reactions, chemisorption, composite formation or polymer coating. This immobilization seeks transfer the physicochemical properties of the modifier to the electrode surface and thus to dictate and control the behavior of the electrode/solution interface. In recent years the interest in CMEs has increased particularly to enhance the sensitivity and/or the selectivity of electroanalytical techniques. In general higher sensitivity and/or selectivity may be achieved by exploiting one or more of the following phenomena: electrocatalysis, preconcentration and interferents exclusion. This paper deals with the application of CMEs in electroanalysis, including a brief presentation of the more general procedures that have been employed for the modification of electrode surfaces.

Obtenção de um material adsorvente para íons de metais pesados, através da eletropolimerização de 2-mercaptobenzimidazol à superfície de carbono vítreo reticulado

Electrochemical methods applied to organic species transformation has been used as excellent synthesis tools. C-C bonds can be established, making possible polymer synthesis by both anodic and cathodic reactions of suitable monomer species at the working electrode surface. In this study, anodic procedure was used to electropolymerization of 2-mercaptobenzimidazole at reticulated glassy carbon (RGC) surface. 2-mercaptobenzimidazole presents ligand sites towards Hg2+, Ag+ and Cu2+ ions. The obtained material has been able to adsorb the above mentioned ions in aqueous solution.

Eletrodos modificados com polímeros perfluorados e sulfonados: aplicações em análises ambientais

Perfluoro and sulfonated ion-exchange polymers are recognized as a very useful material for various mechanistic studies and applications in electrochemistry. These polymers are characterized by high equivalent weights and by a low number of ion-exchange sites interposed between long organic chains. The solubility enables a preparation of stable polyelectrolyte films on the electrode surface. Examples of the determination of trace metals and organic componds in real environmental samples are presented.

Síntese e caracterização de novos compostos de coordenação de cobre (II) com ligantes não-simétricos N,O-doadores: contribuições para o sítio ativo da galactose oxidase

The reactions of four new unsymmetrical N,O-donor ligands, {H2BBPETEN= [N-(2-hydroxybenzyl) - N,N' - bis(2 methylpyridyl) -N'-(hydroxyethyl) ethylenodiamine], H3BPETEN=[N,N'- bis(2-hydroxybenzyl) -N- (2-methylpyridyl) -N'- (hydroxyethyl) ethylenodiamine], HTPETEN=[N,N,N'- tris(2-methylpyridyl) -N'- (hydroxyethyl) ethylenodiamine] and H3BIMETEN=[N,N'-(2-hydroxybenzyl)-N-(1-methylimidazol-2-il-methyl)-N'- (hydroxyethyl)ethylenodiamine]}, with Cu(II) salts afforded the following mononuclear compounds: [CuII(HBBPETEN)]ClO4, [CuII(H2BPETEN)]ClO4 , [CuII(HTPETEN)](PF6)2 and [CuII(H2BIMETEN)]ClO4 . All were characterized by EPR, electronic spectroscopy and electrochemistry. The four copper (II) compounds showed interesting electrochemistry properties. All presented an anodic wave that can be attributed to the Cu (I) oxide formation at the electrode surface, or to a Cu0 sediment at the same surface or yet, to Cu(I) -> Cu(II) oxidation process with coupled chemistry reaction, due to their irreversibility. Two of the complexes are described as interesting synthetic models for the active site of the metalloenzyme galactose oxidase.

Tendências em modificação de eletrodos amperométricos para aplicações eletroanalíticas

The most relevant advances on the analytical applications of chemically modified electrodes (CME) are presented. CME have received great attention due to the possibility of electrode surface modification including chemisorption, composite generation and polymer coating. In recent years, the interest in CME has increased overall to improve the sensitivity and selectivity of the electroanalytical probes, considering the electron mediator incorporation and the new conducting polymers development. The general procedures employed for the electrode modification and the operational characteristics of some electrochemical sensors are discussed.

Estudo eletroanalítico do herbicida paraquat em soluções aquosas por voltametria de onda quadrada utilizando ultramicroeletrodos

The electrochemical behavior of paraquat on Pt, Au and carbon fiber ultramicroelectrodes were studied in laboratory samples by square wave voltammetry at high frequencies. The results showed two reversible peaks for paraquat reduction, in agreement to the literature data. The first peak was associated to the reduction of paraquat molecule in solution, with the further adsorption of the intermediate on the electrode surface. This adsorbed species undergoes to electroreduction in a reaction associated to the second voltammetric peak. The variation in pH and square wave parameters showed the best conditions to reduce paraquat as pH 5.0, frequency as high as 1000 s-1, scan increment of 2 mV and square wave amplitude of 50 mV. At such conditions, a variation of paraquat concentrations from 4.3 x 10-6 to 1.66 x 10-4 mol L-1 presented values for the detection limit equal to 3.9, 6.2 and 20.3 ppb on Pt, Au and carbon, respectively, at 1000 s-1. These values are quite below17 the allowed limit of paraquat in drinking water.

Comportamento eletroquímico da N-nitrosotiazolidina ácido carboxílico sobre eletrodos de ouro e mercúrio

The electrochemical behavior of N-nitrosothiazolidine carboxylic acid (NTAC) on gold and hanging mercury electrodes, using the cyclic and square wave voltammetries, was studied. Whereas NTAC suffer reduction in a single step on the mercury electrode, two peaks appears on the cyclic voltammograms on the gold electrode, one anodic peak overlaying the gold oxide process at 1.2 V and one cathodic peak at -0.41 V vs Ag/AgCl, KCl 3.0 mol L-1. The cathodic peak depends on the previous oxidation of NTAC at the electrode surface, presents irreversible and adsorption controlled characteristics and it is suitable for quantitative purposes.

Construção de célula eletroquímica para observação de amostras in situ em estereomicroscópio

The electrochemical study of glass like tungsten oxide derivatives requires the construction of special electrodes due to the fact that these glasses are not conductive. Electrodes modified with WO3 change their color when submitted to some potential perturbation. The color change of the electrochromic materials was observed in situ by coupling an electrochemical cell to a stereomicroscope. The constructed cell is versatile and may represent a great contribution to the electrochemical studies of materials, especially in the systems where it is interest to observe the working electrode surface during the electrochemical experiment.

Voltametria de micropartículas imobilizadas: fundamentos e aplicações analíticas

The theoretical aspects of Voltammetry of Immobilized Microparticles (VIM) were discussed. The immobilization of microparticles on electrode surface, the electrode cleaning processes and the electrode materials were analyzed. The three-phase electrode model and the possible reactions between the immobilized particle and the electrolytic solution were discussed. In addition, this work discusses some selected applications of VIM published in the last years.

Potencialidades do uso de ultrassom na determinação do pesticida carbaril empregando eletrodos de diamante

The potentiality of the use of ultrasound radiation in association with a boron-doped diamond electrode was evaluated on the voltammetric determination of the pesticide carbaryl. Improvements in the sensitivity, limit of detection and reproducibility of the measurements were observed due to both, the enhancement of mass transport and the cleaning of the electrode surface provided by ultrasound. Satisfactory recovery levels for carbaryl in pure water (96-98%) and pineapple juice (89-92%) for quiescent and sonovoltammetric methodologies were obtained. These methodologies can be alternative tools for the analyses of pesticides in fruit samples, mainly the insonated condition that improve the analytical performance and dispense intermediary cleanings of the electrode surface.

Planejamento fatorial e superfície de resposta: otimização de um método voltamétrico para a determinação de ag(i) empregando um eletrodo de pasta de nanotubos de carbono

A factorial design applied in a voltammetric stripping method for the measurement of Ag(I) in natural water is described. The procedure is based on the effective pre-concentration of silver ions on electrode surface. The calibration graph was linear in the silver concentration range from 7.92 x 10"7 to 1.07 x 10"5 mol L"1 with a detection limit of 3.81 x 10-7 mol L-1. The determination of Ag(I) in natural water samples was carried out satisfactory with the proposed electrode.

Atividade eletrocatalítica de sistemas biomiméticos da enzima catalase

In this work, we describe the immobilization of the dinuclear compound [Cu2(apyhist)2Cl2](ClO4)2 (1) and its derived cations complexes, obtained in water solution or by deprotonation of the imidazolate moiety in the ligand leading to a cyclic tetranuclear species, in the Nafion® membrane on glass carbon electrode surface. After that, we studied the influence of the equilibrium in the electrocatalytic activity towards the reduction of H2O2 in the development of an amperometric sensor for the analytical determination of hydrogen peroxide. This strategy proved successful, and the electrochemical behaviour of the all complexes formed within the Nafion® coatings was characterized. We also provide evidence that its related cyclic tetranuclear imidazolate-bridged complex acts as a catalysts for the intramolecular, two-electron reduction of H2O2.

Análise por injeção em fluxo com detecção amperométrica de múltiplos pulsos: potencialidades e aplicações

The potentialities and applications of the Multiple Pulse Amperometric detection (MPA) coupled with Flow Injection Analysis (FIA) are evaluated. Important aspects as cleaning and activation of electrode surface, indirect and simultaneous analysis of electroactive compounds and the use of the internal standard method for quantifications utilizing FIA-MPA are presented. The main parameters concerning the detection of electroactive analytes by multiple pulse amperometric detection in flowing solutions were also discussed. In addition, aspects such as flow rate, sample volume, application time of the potential pulses and instrumentation necessary for implementing of the method were also addressed.

Degradação fotoeletroquímica do herbicida bentazona sobre eletrodos de carbono modificados por TiO2

This work involved the study of degradation of the herbicide bentazone in aqueous solution by different routes, in order to search a method that generates safe products to the environment. It was tested electrochemical polarization methods involving positive and negative potential, irradiation with UV light and deposition of TiO2 on the electrode surface, seeking a catalytic effect. After different times of degradation, aliquots were removed and the scan of molecular absorption spectrum of UV-Vis was performed. From the spectra decay of bentazone, the kinetics of different processes was accompanied and the rate constants were determined.