Portugaliae Electrochimica Acta

Computer Simulation of Solution/Electrode Interfaces

This article is based on the plenary lecture that I was invited to present in the XIV Encontro da Sociedade Portuguesa de Electroquímica and X Iberic Meeting of Electrochemistry, a meeting which was a tribute to the late Professor César Viana. The best way I found to honour his memory was to review the highlights of a recent investigation on the computer simulation of solution/electrode interfaces carried out at our molecular simulation group, which would never happen without the enthusiasm and support of Prof. César Viana. In this paper we focus on the determination, from first principles, of the interaction potentials between ethanol and gold surfaces aiming at their use in the computer simulation of adsorption and self-assembly processes on noble metal electrodes. An analytical force field has been proposed and fitted to DFT results, by means of genetic algorithms. The force field has been tested against Monte Carlo simulations. Finally, the non-trivial, or even impossible, task of finding analytical functions that accurately match ab initio/DFT data, when many molecular degrees of freedom are involved, is briefly reviewed. The alternative of using neural networks to approach potential energy surfaces (PES) is presented and applied to the specific case of the ethanol-Au interactions. The article is signed by the persons who mainly contributed to this work. They all praised very much Prof. César Viana. It is our tribute to his memory.

Wastewaters by Electrochemical Advanced Oxidation Processes Using a BDD Anode and Electrogenerated H₂O₂ with Fe(II) and UVA Light as Catalysts

This paper reports the mineralization of an aromatic pharmaceutical as clofibric acid and the dye indigo carmine in 0.05 M Na2SO4 of pH 3.0 by electrochemical advanced oxidation processes such as anodic oxidation without and with electrogenerated H2O2, electro-Fenton and photoelectro-Fenton using a boron-doped diamond (BDD) anode. These procedures produce the strong oxidant hydroxyl radical in the form of BDD(·OH) from water oxidation at BDD and/or ·OH from Fentons reaction between added Fe2+ and H2O2 generated at the cathode by two-electron O2 reduction. Comparative degradation of both compounds is performed with an undivided electrolytic cell of 100 mL with an O2-diffusion cathode. The effect of current density and pollutant concentration on the degradation rate and current efficiency of the different methods is discussed. The decay kinetics of clofibric acid and indigo carmine and the evolution of their aromatic by-products and final carboxylic acids like oxalic and/or oxamic are described to clarify their reaction sequences. Anodic oxidation is able to completely mineralize both compounds mainly with BDD(·OH). Aromatic pollutants are more rapidly destroyed in electro-Fenton due to their faster reaction with ·OH, but final Fe(III)-oxalate and Fe(III)-oxamate can only be oxidized with BDD(·OH). The most efficient method is photoelectro-Fenton due to the parallel photolysis of Fe(III)-oxalate complexes with UVA light, although Fe(III)-oxamate complexes are not photodecomposed. Under these conditions, the degradation rate can be enhanced using Cu2+ as co-catalyst because Cu(II)-oxalate and Cu(II)-oxamate complexes are quickly destroyed by ·OH. Results on the fast mineralization of 2.5 L of cresols solutions of pH 3.0 by solar photoelectro-Fenton using a flow plant coupled to a solar photoreactor are also reported. The high efficiency and very low operational cost found for this procedure make it useful for the treatment of industrial wastewaters

Concerning the Efficiency of Corrosion Inhibitors as Given by SVET

The possibility of obtaining corrosion inhibition efficiency from the Scanning Vibrating Electrode Technique (SVET) is investigated. Results are compared with those obtained by Electrochemical Impedance Spectroscopy (EIS) for the inhibition of iron and zinc with cerium nitrate and 1H-benzotriazole. Examples of good and bad estimations are given and an interpretation for the discrepancies observed is advanced. Large deviations occur when corrosion products are formed since this leads to an underestimation of the currents measured by SVET. Visual inspection of the samples may identify the cases more susceptible to deviation. The approach discussed here can be used to obtain more quantitative information from SVET measurements but should not replace the traditional ways used to determine the inhibition efficiency.

Electrosynthesis and Characterisation of Poly(3,4-Ethylenedioxythiophene) Films Incorporating Ferrocene

Poly(3,4-ethylenedioxythiophene) (PEDOT) films were potentiostatically and potentiodynamically prepared on Pt electrodes from acetonitrile monomer solutions containing TBAPF6 or FcPF6 (or both). The polymer modified electrodes obtained with different growth charges were characterized electrochemically by cyclic voltammetry in a monomer free solution. The results point to the effective immobilization of the ferrocenium ion inside the PEDOT matrix by entrapment during the polymer synthesis in potentiostatic mode. It is discussed the influence of the presence of the organometallic cation in the polymerization solution in the growth process and the efectiveness of the incorporation of the metallocene is analysed on the basis of the electrochemical data collected for the films redox conversion.

Biosensor de Acetilcolinesterase - Colina Oxidase para Determinação do Pirimicarbo

A biosensor for the determination of pirimicarb was constructed by the co-immobilization of acetylcholinesterase and choline oxidase on a derivatised polyethersulfone membrane. The membrane was applied to an Universal Sensors electrode base system and the hydrogen peroxide formation was monitored upon addition of acetylcholine to an appropriately chosen buffer where the biosensor was dipped. The hydrogen peroxide is the product of the hydrolysis of choline, catalysed by cholineoxidase. Choline was formed by the action of acetylcholinesterase on acetylcholine. Pirimicarb inhibits these reactions and that fact can be used to determine its concentration. A calibration plot for pirimicarb was obtained.

Construiu-se um biossensor para a determinação do pirimicarbo, pela co-imobilização de acetilcolinesterase (AChE) e colina oxidase (ChOx) numa membrana de poliétersulfona modificada. A membrana foi aplicada a um sistema base de eléctrodos da Universal Sensors e monitorizou-se o peróxido de hidrogénio formado quando da adição de acetilcolina (ACh) a um tampão apropriadamente escolhido onde se mergulhou o biossensor. O peróxido de hidrogénio é o produto da hidrólise da colina, catalisada pela colineoxidase. A colina formou-se pela acção da acetilcolinesterase na acetilcolina. O pirimicarbo inibe estas reacções e esse facto pode ser usado para determinar a sua concentração. Obteve-se um gráfico de calibração para o pirimicarbo.

Preliminary Studies Towards the Development of DNA Biosensors for Detection of Cylindrospermopsin - A Cyanobacterial Toxin

The suitability of polytyramine (PTy) modified electrodes for the development of DNA biosensors is analysed in this work. PTy films were synthesised potentiodynamically on Pt electrode, from acidic aqueous solution. A cylindrospermopsin coding sequence (PKSM4 probe 5-phosphate modified) was covalently bound on the polytyramine matrix using N-hydroxysuccinimide (NHS) and 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride (EDC) as coupling agents. Changes of the open circuit potential were registered as a first approach to monitorise the immobilization and hybridization processes. The modified electrodes have been characterised by cyclic voltammetry and square wave voltammetry, using methylene blue (MB), as electroactive indicator and the successful immobilization and hybridization steps are revealed by the increase of the peak currents. The hybridization has also been confirmed by measuring the fluorescence of the modified electrodes after staining with PicoGreen® DNA dye.

Degradation of Reactive Black 5 and Basic Yellow 28 on Metallic-Polymer Composites

The textile dyes Reactive Black (RB5) and Basic Yellow 28 (BY28) were individually exposed to electrochemical treatment in 0.1 M KOH and in phosphate buffer solution pH 5.1, respectively. The cathodic decolorization of the azo dye RB5 was carried out using a Ni-polyvinylchloride composite. The methine dye BY28 was anodically treated on a Co-polyvinylchloride. The electrochemical study of the dye solution was carried out by repetitive cyclic voltammetry (RCV) under potentiostatic conditions in stationary and flow regime. The effect of various experimental parameters, such as potential and electrolysis time was investigated. The studies showed that for the RB5 (20 to 80 ppm) dye solutions, around 98 % of the color was removed after 60 min treatment at -3.0 V vs. SCE. It was also demonstrated that for the BY28 (15 ppm) dye solution, 6-10 cycles or low potentials (-300 mV) during 200 min electrolysis were sufficient for the decolorization of the solution. However, COD and TOC values did not decrease.

Online Monitoring of Corrosion in a Liquid-Steam Line Pipe of the Miravalles Geothermal Field

The goal of this study was to evaluate the internal corrosion in a liquid-steam pipeline (type A515 low carbon steel) of the Geothermal Field of Miravalles (CGM) in Costa Rica, and to determine the corrosion thickness for future geothermal fields. The monitoring was carried out using the following techniques: linear polarization resistance (LPR), electrical resistance (ER), and weight loss. The results show for neutral or alkaline pH conditions and slightly free of oxygen, that the corrosion thickness should be 1.8 mm, in order to guarantee the integrity of the pipe over the field lifetime.

Biosensors for the Polyphenolic Content of Wine Determination

Laccase of different lots was used in the construction of biosensors for the determination of polyphenolic content of wine. Laccase was immobilized on modified polyethersulfone membranes and applied to a Universal Sensor electrode base system. Parameters of the biosensors thus prepared were studied with respect to caffeic acid as the model substrate. Sensitivity of 0.102 mA × M-1, RSD of 1.9%, linear range of 5 to 35 × 10-6 M, limit of detection of 8.8 × 10-7M, were determined. Different values of the applied potential near 0 V were tested in order to choose that one which gives the best performance of the present biosensors. Citrate and tartarate buffer solutions at pH 3.5 were used. Polyphenolic content of different red wines was determined at +100 mV vs. Ag/AgCl.

Modelling of the Atmospheric Corrosion of Copper in the Province of Las Palmas. Studies Using Classic and Electrochemical Techniques

Simulation of the effects of natural atmospheres on metals using electrochemical techniques together with accelerated tests presents certain characteristics that make it highly interesting from the point of view of the study and prediction of corrosion. It will enable us to forecast the behavior of a metal exposed to a particular environment without the need to perform field test, with a consequent saving in terms of time and financial expenses. The aim of this study is to establish the correlation between the atmospheric corrosion rate of copper (obtained by atmospheric exposure at stations located in the province of Las Palmas, Spain) with the different variables that intervene in the corrosion process. In the same way, for the determination of the mathematical model that best fits the results obtained experimentally and which takes into account the most influential parameters in the corrosion process, the power Law model has been used which generically reproduces the electrochemical process which occurs in atmospheric corrosion.

La simulación de la acción de atmósferas naturales sobre metales empleando técnicas electroquímicas conjuntamente con ensayos acelerados presenta unas características que lo hacen muy interesante desde el punto de vista del estudio y predicción de la corrosión, ya que nos permitirá prever el comportamiento de un metal expuesto en un ambiente determinado sin necesidad de realizar ensayos de campo, con el consiguiente ahorro económico y de tiempo. Este estudio tiene la finalidad de correlacionar la velocidad de corrosión atmosférica del cobre obtenida mediante exposición atmosférica en estaciones en la provincia de Las Palmas con las distintas variables que intervienen en el proceso corrosivo. De la misma manera para la determinación del modelo matemático que mejor se ajuste a los resultados obtenidos experimentalmente y tenga en cuenta los parámetros de mayor influencia en el proceso corrosivo se ha utilizado el modelo de la Ley Potencial que, de forma genérica, reproduce el proceso electroquímico que se desarrolla en la corrosión atmosférica.

Chemically Modified Carbon Paste Electrodes for Ascorbic Acid Determination in Soft Drinks by Flow Injection Amperometric Analysis

Simple, rapid and precise amperometric methods for quantification of ascorbic acid (AA) are presented. Glassy carbon (GC), carbon paste (CP) and modified carbon paste (MCP) electrodes are used for this purpose. MCP electrodes are of CP with 10 % vanadate (V). All electrodes are inserted in a wall-jet device with an Ag/AgCl reference electrode and a platinum auxiliary electrode. This device is coupled to a flow injection analysis (FIA) set-up. Hydrodynamic and amperometric parameters are optimized unvaryingly. GC electrodes present slopes of 4.75 ´ 10(5) nA L mol-1 (± 6.4 %) under optimum conditions. CP and MCP electrodes show higher sensitivity, with slopes of 6.37 ´ 10(5) nA L mol-1 (± 6.6 %) and 7.32 ´ 10(5) nA L mol-1 (± 4.4 %). Linear responses range (1.0-2.0) ´ 10-6 to (0.8-1.0) ´ 10-5 mol L-1. Correlation coefficients were > 0.994 and about 48 samples are analysed per hour. Application of the proposed method to the analysis of soft drinks is presented.