Portugaliae
Electrochimica Acta

New developments in the utilization of a laccase based polyphenolic biosensor were done in order to optimize its application in wines. Laccase was immobilized on polyether sulphone membranes and applied on an Universal Sensors (US) electrode base system containing a platinum working electrode and silver, silver chloride electrode. Caffeic acid was dissolved either in tartarate standard buffer solution at pH 3.5 or in 12% ethanol tartarate standard buffer solution at pH 3.5. The referred values of pH and ethanol percentage are the average values of table wines. Wines from different regions of Portugal, namely Grão Vasco (Dão), A P_Cabernet (Setúbal), C P_Castelo de Syrah (Setúbal), Ol (Alentejo), A P (Alentejo) and Quinta dos Poços (Douro), were studied. Grão Vasco was taken as a reference. It was observed that Grão Vasco (Dão) wine presented a caffeic acid equivalent concentration superior to the other studied wines, followed by a Douro wine: Quinta dos Poços (values of 17.00 and 12.18 mg/L, respectively, were found). It was also possible to verify that the two latter referred wines did not inhibit the biosensor, which could be reused after having been dipped in those wines. However, the wines from Setúbal and Alentejo, studied in this work, have a caffeic acid equivalent concentration lower than the other two mentioned before and seemed to inhibit the system, since the biosensor response decreased after being used in them. It was, thus, not possible to do several assays with the same membrane, when those wines were used. Reproducibilities were better for caffeic acid standard solutions, Grão Vasco and Quinta dos Poços wines, than for the other wines. In fact RSDs (Relative standard deviations) of the order of 1.1, 1.4 and 5.0% were obtained for caffeic acid standard solution, Grão Vasco and Quinta dos Poços wines, whereas 20% was found in the Setúbal and Alentejo studied wines. Two particular wines, A P_Cabernet and A P, were studied in more detail. Cyclic voltammograms and biosensor responses of these two wines indicate that A P has more caffeic acid equivalents than the A P_Cabernet.

Three polyvinylchloride (PVC) membrane sensors for the determination of moexipril hydrochloride were prepared and characterized. The sensors are based on the use of the ion association complexes of moexipril cation with either ammonium reineckate (sensor 1) or tetraphenyl borate (sensor 2) or phosphotungistic acid (sensor 3) counter anions as ion exchange sites in the PVC matrix. The performance characteristics of these sensors were evaluated according to IUPAC recommendations, which reveal a fast, stable and linear response for moexipril over the concentration range of 10-6 to 10-2 M for the three sensors with cationic slopes of 29.1, 30.1 and 30.2 mV per concentration decade for the three sensors, respectively. The direct potentiometric determination of moexipril hydrochloride using the proposed sensors gave recoveries % of 99.64 ± 0.34, 99.34 ± 0.56 and 99.68 ± 0.42 for the three sensors, respectively. The sensors were used for determination of moexipril hydrochloride in pharmaceutical formulations and in plasma. Validation of the method shows suitability of the proposed sensors for use in quality control assessment of moexipril hydrochloride. The obtained results were in a good agreement with those obtained using the reported spectrophotometric method.

Aluminium metal matrix composites (AMMCs) are now gaining their usage in aerospace and automotive industries. Because of their inherent nature, difficult to machine, they find very little applications in other sectors. Even non traditional processes like Laser Jet Machining and Electro Discharge Machining result in significant sub surface damage to the work. In this paper, an attempt is made to machine the A356/SiCp composite work material using Electro Chemical Machining process. Silicon carbide with an average particle size of 40 microns is tried in three different proportions, namely 5%, 10% and 15% by weight. Taguchi’s L27 orthogonal array is chosen to design the experiments and 54 trials are conducted to study the effect of various parameters like applied voltage, electrolyte concentration, feed rate and percentage reinforcement on maximizing the material removal rate. ANOVA results have shown that all the four selected factors are significant and from the S/N graph the optimum level of each factor is chosen. A mathematical model is also developed using the regression method. Confirmation experiment is conducted and found that the data obtained have close match with the data predicted using the model.

This paper reviews some recent canonical Monte Carlo simulations of the Au(210)/H2O interface using a DFT force field developed by us. New results are reported on the solvent contribution to the potential of mean force (PMF) for the phenol adsorption, from a dilute aqueous solution, onto the Au(210) surface. The Monte Carlo simulations show the common features normally observed in the simulation of water in contact with metallic surfaces, where the water molecules adsorb forming bilayers. The molecules adsorbed over the Top gold sites form hydrogen bonds between the first and second solvent layers. The PMF calculations indicate that the phenol molecule penetrates the solvent layers with the aromatic ring in a perpendicular configuration and the oxygen atom pointing to the surface. The PMF results also suggest the existence of hydrogen bonds between the phenol molecule and the first solvent layer of the water molecules adsorbed onto the Top sites.

The numerical method for the simulation of linear scan voltammetry on the rotating disk electrode is adjusted to the problem of irreversible redox reaction between the adsorbed catalyst and the dissolved reactant under transient conditions. The response consists of the wave and the maximum. The peak current depends on the scan rate in linear scan voltammetry, while the limiting current of the wave depends on the rate of rotation of the working electrode. The rate constant of catalytic reaction is determined from the kinetic current under steady-state conditions.

The corrosion inhibition of aluminium by Ipomoea involcrata (IP) in 1 M NaOH was rapidly assessed using the gasometric technique at 30 and 60 oC . Results obtained showed that Ipomoea involcrata acts as an inhibitor for aluminium corrosion in 1 M NaOH. Inhibition efficiency increased with increase in concentration of IP up to 55.0 % at 50 v/v% and decreased with increase in temperature. The adsorption of IP was in accord with Langmuir adsorption isotherm at the studied temperatures. A mechanism of physical adsorption is proposed from the calculated values of Ea, Qads and ΔGads.

Special Issue dedicated to the XVIII Congress of SIBAE (Sociedad Iberoamericana de Electroquímica)

Carbon-supported binary PtSn/C and ternary PtSnNi/C catalysts were prepared for the electro-oxidation of ethanol. The carbon-supported nanoparticles were synthesised by employing a modified polyol methodology and characterised in terms of structure, morphology and composition by using XRD, EDX and TEM techniques. Their electro-catalytic behaviour for ethanol oxidation (EO) was investigated by employing a disc-composite electrode covered by a thin layer of catalyst imbedded in a Nafion polymer electrolyte film. An enhancement of the EO through a negative shift of the onset oxidation potential and higher current density at constant potentials were observed on the PtSnNi/C electrodes as compared to those for PtSn/C electrodes. The temperature was changed in the range 40 ºC ≤ t ≤ 80 ºC, and the variation of the activation energy calculated by linear regression of the Arrhenius plots.

The electro-oxidation, over platinized titanium and ruthenium oxide anodes, of nitrogen containing molecules (urea, reactive Blue 4 dye, acetonitrile, formamide, guanidine and pyridazine) was investigated, monitoring the products distribution. The N-mineralization leads to have inorganic pollutants (NH3/NH4+ and/or NO2-/NO3-). Amidic and aminic compounds react both in homogeneous (acid hydrolysis) and in heterogeneous phase (direct electroxidation) with a rate depending on the original state of oxidation of nitrogen. Heterocyclic and multiple-bond carbon-nitrogen molecules were effectively converted with negligible mineralization of nitrogen due to the stability of their first oxidation intermediates. The obtained results (high rate of nitrate generation) evidence the need of coupling of the direct electroxidation with other process to limit the nitrate concentration to an accepted level; in accordance, dialysis (of the ammonia cation) and indirect oxidation (chlorine-mediated) were proved to be valid alternatives.

Laccase is a poliphenoloxidase enzyme that catalyzes the oxidation of catechol compounds in the corresponding quinones. The current obtained in this redox process can be used for quantitative analysis. In this work, a carbon paste biosensor modified with a crude enzymatic extract of the Pycnoporus sanguineus fungi as a lacase source is proposed for catechol determination and other phenolic compounds. The effect of carbon paste and electrolyte composition, pH from 3.0 to 8.0, scan rate from 10 to 40 mVs-1 and potential pulse amplitude from 10 to 60 mV on the differential pulse voltammetric response was investigated. The analytical curves were linear in the catechol concentration range from 2.0 x 10-5 to 7.0 x 10-4 molL-1, with detection limits of 4,5 x 10-6 molL-1. This biosensor was used for the determination of different kind of phenolic compounds, presenting better response for catechol, hydroquinone and resorcine.

Removal of acid orange 24 color was studied in an electrochemical cell composed of a dimensional stable anode (DSA), a stainless steel cathode and an electrolyte media containing sodium sulfate, sodium chloride, and two commercial surfactants named Uniperol y Leophen, corresponding to wetting and dispersant agents, both of them required to favor the color incorporation inside the fiber. DSA used in this study comprises a titanium mesh covered by a layer of iridium, and a stainless steel plate covered by a layer of iridium-tin mixture. Experimental approach allowed getting an inside of the rol played by sulfate, chloride and surfactants for both color and chemical oxygen demand (COD) removal. Obtained results have shown that the Ir-Sn electrode provides the highest color removal under chloride mediated oxidant conditions. Evaluation of the DSA performance in solutions containing surfactants has shown that obtained color removal is similar for both electrodes; otherwise the COD removal attained with the Ir DSA is about six times the one obtained with the Ir-Sn electrode.

The electrochemical and mechanical behavior were evaluated for specimens of concrete clean and contaminated with 2 and 4% of NaCl (of the weight of cement), elaborated with two types of cements: compound and waterproof. The dosage of the mixture of concrete was done on the basis of weight in two 0.45 and 0.65 water/cement ratios. The specimens under study were exposed in two media: clean water and 3% of NaCl solution. The test to evaluate the presence of corrosion of the reinforced steel consisted in the monitoring of the half cell potentials as the norm ASTM C-876-91 indicates. For the mechanical behavior the compression test was used, according to the ASTM C-39/C 39-M-99 norms. This paper presents the results obtained for the first 4 months of monitoring, identifying the influence of the attack of chlorides in the corrosion of steel and its mechanical properties. There is no a significant influence of the type of cement on the parameters of the study.

This work is part of the DURACON iberoamerican project, which deals with the characterization of the concrete durability exposed to environmental conditions at Iberoamerica, through the exposure of reinforced concrete specimens in at least two different environments, one marine and one urban, in each participating country. In this paper, results of the first eight months of the specimen exposition to environmental conditions in the Xalapa city, Mexico, are evaluated. The specimens were built with two types of concretes at 0.45 and 0.65 water/cement ratios. In all cases, the same type of cement with three thicknesses (1.5, 2.0 and 3.0 cm) of covering were used. The tests to evaluate the susceptibility to corrosion of the reinforced steel were resistance to linear polarization and monitoring of the corrosion potentials according to the ASTM C-876 norm.

The use of silane formulations for the pre-treatment of metallic substrates prior to painting or as topcoat has been increasing during the recent years. The proposed formulations are very attractive because they present environmental friendliness, enhance adhesion properties and provide a barrier layer that delays the corrosion processes. One of the most used techniques to synthesized based-SiO2 coatings is sol-gel. The preparation of sol-gel coatings with specific chemical functions offers potential advantages over traditional methods as it offers tailoring of their structure, texture and thickness and allows the fabrication of large coatings. The present work investigates the electrochemical behaviour of commercial carbon steel coated with ceria doped SiO2 by sol-gel process in basic media. To evaluate the influence of temperature on adhesion and protective properties, the coatings were sintered at three different temperatures, 623 K, 723 K and 873 K, for 3 h. The anticorrosion behaviour, of the films has been characterized in an aggressive medium by electrochemical techniques, while structural characteristics and morphology was followed by X-Ray Diffraction, Fourier Transformed Infra-Red and Scanning Electron Microscopy techniques. The results indicated that the thin films act as a protective barrier against exposure to the corrosive medium (3.5 wt-% NaCl solution) and increase the lifetime of the susbtrate.

Nickel 200 is used in the petrochemical industry in the handling of caustic solutions using a variety of components and equipments made of materials highly resistant to corrosion. However, due to their high costs and their impact on failures by corrosion, it is necessary to resort to other alternative materials that could mitigate this phenomenon. One of them is steel coated with electroless nickel (without passing flow), which in certain situations and under certain conditions offers high resistance. The aim of this study was to evaluate the corrosion resistance of the coated steel test coupons by electroless nickel (EN) in different concentrations of caustic soda and establish the optimal coating thickness for each concentration of NaOH. The chemical analysis conducted by EDX indicates that the composition of coatings in phosphorus compounds are phosphorus by 10.5% and 89.5% nickel with hardnesses average of 550 HV. The corrosion rate in steel EN was in the order between 10.0 and 13.6 mm/yr (25 °C) and 10.2 and 14.7 mm/yr (85 ° C). While for nickel 200 it was in the order between 4.7 and 5.2 mm/yr (25 °C) and 7.0 and 9.9 mm/yr (85 °C). Nickel 200 was comparatively more corrosion resistant than steel coated with electroless nickel, but its cost is higher.

In this work, the electrochemical synthesis and characterization of rhodium nanoparticles are reported. The Rh nanoparticles were electrochemically synthesized at room temperature by direct electroreduction of rhodium (III) chloride (RhCl3) salt. We have applied different values of current intensity and potential in order to control the formed size of Rh nanoparticles. We have used tetrapropyl ammonium bromide (TPAB), as the support electrolyte and at the same time, effectively stabilizing the nanoparticles formed in aqueous medium. Size and morphology of nanoparticles formed, are subsequently studied by Transmission Electronic Microscopy (TEM). Electrocatalytic activity and stability for the oxidation of methanol, were studied using cyclic voltammetry. Nanoparticles of Rh synthesized present an electrocatalytical activity toward the electrooxidation of methanol.

The adverse effects of endocrine disruptors on human health have created a need for screening systems to detect xenoestrogens. In this project, we develop a system to detect one of these xenestrogens, Bisphenol A (BPA), through the development of a biosensor based on impedance measurements. The biosensor consists of a supported lipid bilayer that incorporates a protein receptor sensitive to the presence of BPA. This setup was originally proposed by V. Granek and J. Rishpon [19]; we now present an alternative method of analysis to increase sensibility. As a preliminary evaluation of bioactivity of BPA we measured the direct interaction of this xenoestrogen with the membrane by analyzing changes in the membrane impedance as a function of BPA concentration. The results indicate that BPA inserts and disrupts the membrane, but only at very high unphysiological BPA concentrations (> 1 ppm). We then developed a high sensitivity biosensor, based on the detection of the electric signal by means of impedance spectroscopy, resulting from the interaction between BPA and an estrogen receptor reconstituted in a lipid membrane. The results show that the presence of the receptor increases by several orders of magnitude the sensitivity of the system, making it possible to detect BPA at ppb.

Cations present in the soil solution interact with soil charged surfaces by electrostatic forces. An applied electric field promotes principally, the migration of ions of the soil solution, whose interactions with charged surfaces are not so strong. If, by means of an applied electric field to the soil, the transport of a higher valence ion such as Ba2+ is promoted, it would be expected that cations of lower valence should be desorbed from the soil exchange sites. The main goal of the present work was to study the effect, not only of the applied electric field, but the Ba2+ ion migration, in the extraction of a major cation (K+), naturally founded in a volcanic soil. Experimentally, it was found that Ba2+ migration through the soil, increased in an important way the amount of K+ ions which are mobilized by effects of the applied electric field.

Amorphous and/or nanocrystalline Al-based alloys have better mechanical properties when compared with crystalline conventional Al-alloys. Different techniques can be used to obtain amorphous Al-based alloys, usually in a ribbon form (from melt-spinning processing) or powders (from gas atomization or ball milling processing). To obtain bulk samples, that is, samples with dimensions typically much larger (mm scales) than the ones obtained from the above mentioned techniques (<40 m), the ribbons or powders must be hot-consolidated. One of the most important challenges in the development of such alloys is to keep a refined microstructure after the necessary heating. The present work focuses the pitting resistance by electrochemical corrosion resistance test in solution 0.9 % NaCl and pH 7.0, for Al90Fe7Nb3 and Al90Fe7Zr3 nanocrystalline alloys, which were obtained by hot-extrusion of mechanically alloyed powders, and melt-spinning process. The results of the polarization curves indicated that the Al90Fe7Nb3 and Al90Fe7Zr3 ribbons present better corrosion properties than the extruded alloys.

The electro-synthesis of a nickel metalloporphyrin by using the 5,10,15,20-tetraquis(p-hydroxyphenyl)porphyrin (TPPOH) as binding agent is reported. Before the electrosynthesis, cyclic voltammetry technique was applied to establish the best conditions for the electrochemical reduction of the porphyrin to the di-anion radical. The electrosynthesis of the metalloporphyrin was done under the conditions found from voltammetry. The electrolysis was performed in a non-divided cell, with a nickel sacrificial anode and at controlled potential, in order to favouring the chemical formation of the metal-complex by reaction between the porphyrin dianion radical and Ni(II) ions, both of them electro-generated during the process. The effect of the use of an ultrasonic wave of 20 kHz in the metalloporphyrin electro-synthesis process was studied. The porphyrin, of free base, and the products obtained in both the absence and the presence of ultrasonic were characterized through Atomic Absorption, UV-Visible, IR and elemental analysis. The results of this characterization make available to establish that the process leads to the formation of the metalloporphyrin of Ni(II) with TPPOH, with an electrolytic efficiency greater than 80%. The application of the 20 kHz ultrasonic wave at the amplitude of the study favoured the yield of the reaction and did not lead to changes in the electrochemical mechanism.