Portugaliae
Electrochimica Acta

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The first and second dissociation constants of adipic acid have been determined by a precise emf method in water and in aqueous binary mixtures of ethanol, over a wide range of solvent composition (0-70%(w/w) ethanol). Measurements were conducted at different temperatures ranging from 30 °C to 60 °C at intervals of 10 °C, by emp measurements. The thermodynamic parameters (∆H°, ∆G° and ∆S°) for the first and second dissociation reactions have been computed, analyzed and discussed in terms of solute-solvent interactions. The pK values have been correlated with the mole fraction of the organic solvent and the effect of dielectric constant on the dissociation processes was then discussed.

Electrochemical behaviour of zinc-glycine complex has been studied in the presence of thiourea on platinum by cyclic voltammetric technique. The obtained results revealed that zinc-glycine complex undergoes successive reduction to zinc with the formation of monovalent zinc complex in a slow step. Thiourea in the plating bath solution hindered both the deposition and dissolution processes. Hydrogen evolution reaction was hindered by the adsorption of thiourea, which strengthened the M-H(ads) bond and raised the hydrogen over voltage.

The corrosion inhibition of nitrogen, oxygen and sulphur containing compounds in acidic chloride solutions was studied by chemical and electrochemical techniques. The percentage inhibition efficiency was calculated from both methods is in good agreement. The corrosion rate was found to be a function of the nature, concentration and temperature of the medium. The increase in temperature was found to increase the corrosion rate. The inhibitors are adsorbed on the C-steel surface according to the Langmuir isotherm. Some thermodynamic functions were computed and discussed. Quantum data were calculated and the results of HOMO and LUMO showed that the increase in energy of HOMO corresponds to an increase in the inhibition efficiency.

The study of the electronic structure of the passive films formed on high purity Fe-Cr, Fe-Cr-Ni and Fe-Cr-Ni-Mo stainless steels was carried out by photoelectrochemical measurements. Bandgap energy and quantum efficiency are associated with the changes promoted by the alloying elements in the bilayer structure of the passive films. The analysis of the photocurrent as a function of the applied potential reveals a Poole-Frenkel effect. The donor densities extracted from photoelectrochemical measurements are compared to those obtained in previous works from capacitance measurements.

The electropolymerization of substituted manganese and iron metalloporphyrins with two pyrrole groups bonded in lateral chains is investigated. It is shown that under potentiodynamic conditions the film formation can be successfully achieved. A methodology for the polymerization of the iron containing monomer on microelectrodes is also presented. The behaviour of the polymer modified electrodes is studied for the O2 reduction, as well as for the oxygen and hydrogen evolution reactions, revealing good electrocatalytic activity.

Al 6061 powder and F800 grade SiC particles (SiCp) were used as raw materials for the preparation of metal matrix composites (MMC), via a powder metallurgy route (PM), with 0% to 20 wt % of SiCp, the reinforcement material. Corrosion studies revealed a linear dependency of the pitting potential (Ep) with the logarithm of the concentration of NaCl with a slope of ~-90 mV/decade, but there was no significant difference in the Ep values of the composite materials as a function of the concentration of reinforcement. Corrosion protection of PM Al SiC alloys was achieved by favouring the formation of a Ce/Mo-based conversion coating on the surface without the use of external polarisation. Pitting potentials were assessed in a 0.1 M NaCl solution after treatment. No increase in current density was evident for samples treated at pH 4.4 followed by post-treatment at 60°C for one hour, when polarised up to 0 mV (SCE). The role of Ce and Mo as inhibitors is analysed and discussed in terms of the protective character of the produced cerium-molybdenum conversion coatings.

Propylene carbonate (PC) and y-butyrolactone (GBL) are important dipolar approtic solvents used to prepare non-aqueous electrolytes for high energy lithium batteries [1-4]. A few salts have been tried in these solvents because they permit to work at lower and higher temperatures than the melting and boiling points of water respectively. Actually these solvents have much larger liquid temperature intervals than water and they are generally very stable and safe as it was demonstrated by Kita and Hasegawa in recent papers [5,6]. In this paper conductivity determination of lithium perchlorate (LiC104) as well as of tetra-ethylammonium bromide (Et4NBr) solutions in those two solvents have been obtained under different pressures. The resulting volumes of activation are positive for both salts and generally decrease as pressure increases for lower concentrations. The contrary is observed for higher concentrated solutions. The association equilibrium constants were also determined and they are almost constant or slightly decrease with pressure and decrease with concentration. All the experiments were carried out at 25 ºC ± 0.01 ºC.

The pitting corrosion of Aluminium was studied in 0.1 M halide solutions in various concentrations of decyl and dodecyl sodium sulphate. The results obtained from the potentiodynamic measurement show that the pitting corrosion is inhibited by the presence of organic compounds in this medium. The percentage inhibition was found to increase with increasing the concentration of the organic compounds in the medium at constant halide concentration. For solutions of constant halide concentration, the pitting potential varies with the logarithm of the inhibitor concentration according to a linear relationship. These results are interpreted in terms of competitive adsorption between halide ions and organic anions at the passive metal-solution interface. It was found that the efficiency of inhibition for a given halide concentration decreases in the order of dodecyl sodium sulphate > decyl sodium sulphate. At constant inhibitor concentration, the pitting corrosion was found to decrease in the order: CI- > Br- > I-.

Corrosion inhibition by some 4-phenylthiosemicarbazone derivatives with respect to the dissolution of copper in 2M HNO3 was measured using electrochemical techniques. Polarization curves indicated that these compounds act as mixed-type inhibitors, i.e. both the cathodic and anodic curves are affected. The observed effect follows the Temkin adsorption isotherm. The effect of the different structural features of these compounds on their inhibition efficiency has been studied. Results indicate that the rate of corrosion of copper increases with increasing temperature over the range 30-60°C both in the absence and in the presence of inhibitors. Some thermodynamic functions were also computed and are discussed.

In this work the polymer 4-pyridilmethylchitosan/Ru(EDTA) ([Ru(EDTA)(4PMC)]) was used as modifier in the preparation of chemically modified carbon paste electrodes. The electrochemical behavior of the polymer compound is governed by the a conversion of the monossubstituted species for disubstituted species when the reduction of Ru(III) to Ru(II) occurs. The cyclic voltammogram of the electrode prepared with the [Ru(EDTA)(4PMC)] in the presence of ascorbic acid presented an increment in the anodic peak current in 0,38 V, which indicates that the catalysis of ascorbic acid oxidation is mediated by polymer.

A technique to obtain normalized current was conceived for the cases in which the electrode surface corrodes. Each polarization curve point is obtained with an untreated electrode, which is submitted to two potential jumps: the first jump was applied to measure the current-potential relation. The second one was used for area normalization. Stainless steels 316 and 316 L have been used in a concentrated chloride solution.

The basic redox properties of sulpiride are investigated by means of cyclic (CV) and square-wave voltammetry at a hanging mercury drop electrode (HMDE). Sulpiride undergoes electrochemical reduction in a basic medium. The voltammetric response strongly depends on pH of the medium. A square-wave voltammetric method for quantitative determination of sulpiride is developed. A detection limit of 1.9E-6 mol/I sulpiride was obtained at the static mercury drop electrode in 2 mol/l KOH. The correlation coefficients of the calibration curves at concentration levels of 1E-5 and 1E-6 mol/I are greater than 0.99. The results of recovery tests for added sulpiride from 5.7 to 11.1E-5 mol/l ranged from 91.3 to 106 %, with relative standard deviations from 0.3 to 2.1 %.

The electrochemical behavior of copper in 4.0M HNO3 solutions without and with different concentrations from two organic compounds were studied using both galvanostatic polarization and cyclic voltammetric techniques. These two organic compounds (namely: bis [4-amino-5-hydroxy-l,2,4-triazol-3-yl] methane "compound Dj" and bis [4-amino-5-hydroxy-l,2,4-triazol-3-yl] butane "compound D2") acted as mixed inhibitor type with predominate cathodic effectiveness. Their high inhibition efficiency (>99%) may be due to adsorption of the additive itself and its further interference with the two partial processes. The results obtained from galvanostatic polarization technique are in consistent with that obtained from cyclic voltammetric measurements.

The oxidative adsorption of 0.1 M methanol in 1 M HC104 on Pt(lll), (110), (100) and Pt(poly) electrodes was studied by cyclic voltammetry. Freshly prepared single crystals were immersed in methanol-containing solution at 0.05 V vs. rhe. The voltammetric profiles of the very first positive-going potential scan exhibit a well defined anodic current peak around 0.40 V only for the Pt(100) face. The peak current (ip) and potential (Ep) were evaluated as function of the potential sweep rate. The adsorption process was assigned to an irreversible surface reaction of methanol with Pt (100) leading to the formation of oxidative adsorbed intermediates. The interpretation of the experimental data indicates that the second electron transfer, i.e. the second deprotonation reaction of CH3OH, was the rate-determining step.

Al-Li alloy 8090 and a binary alloy with the same lithium contents (2.5 wt-%) were studied in regard to their corrosion susceptibility in chloride containing solutions using electrochemical methods. Data were compared with results obtained for pure aluminium in the same experimental conditions The effect of lithium ions on aluminium passivation has been demonstrated previously as particularly effective in alkaline solutions, with the obtained film having a protective character and an inhibitive action on the pitting nucleation process. In this work voltammetric and impedance spectroscopy studies were carried out in presence and absence of lithium ions, in alkaline solutions. Chloride ions were added in concentrations varying from 0.001M to 0.6M allowing pitting potentials to be determined and confirming the role the lithium ions in the promotion of corrosion resistance of aluminium and aluminium alloys.

Transition metal compounds like oxides , sulphides, nitrides, carbides , silicides etc are finding extensive importance in the electrical and electronic devices. LiNiC>2 has greater potential demand in advanced battery research. LiNiCh has been identified as one of the most effective cathode materials in rocking chair battery. Though these compounds could be synthesized by solid state reaction like precursor, sol- gel , co-precipitation etc, they can be conveniently deposited in a fine crystalline form by molten salt electrolysis , employing suitable melts and electrodes . LiNi02 was deposited on Ni anode from a melt of LiOH -NaOH at 1023 - 1123 K . The cell set-up, mainly the Ni cell with the Ni electrodes, melt composition and pretreatment, the favourable operating temperature and current density for the deposition of this transition metal oxides in fine crystalline form etc, have been presented in this paper. The properties of these oxides mainly the chemical composition, XRD data etc, have also been studied and reported.

This paper describes the evaluation of a flow injection system with voltammetric multi-site detection, which enables the detector to carry out the measure of the sample at different positions in the manifold. Some advantages of these systems are partially discussed. A voltammetric cell with a tubular configuration and small inner volume (105 p.1) is proposed. This configuration has a very reduced effect on the hydrodynamic characteristics of the flow which makes it most attractive for manifolds with sequential detection. The manifold optimization was carried out using potassium hexacyanoferrate (II) in order to achieve a good separation of both analytical signals, a minimal sample dispersion and a high sampling rate.

An electroanalytical method for the quantification of bentazon in the presence of MCPA has been developed. Both bentazon and MCPA are herbicides used in rice cultures and bentazon appears alone or mixed with MCPA in a proportion of 7:1 in commercial formulations. In this work the interference caused by MCPA was evaluated and it was shown that the use of Triton in the supporting electrolyte solution leads to the elimination of its interference with bentazon.