Portugaliae
Electrochimica Acta

Sex hormones like 17ß-estradiol (ßES) and progesterone have shown rapid non-genomic vasodilator effects, which could be involved in the protection of cardiovascular system. However, the precise mechanism by which this effect occurs has not been elucidated yet, even if Ca2+ influx inhibition seems to be implicated. The aim of this study was to study the influence of ßES and progesterone on the L-type Ca2+ current measured by whole cell voltage-clamp in A7r5 cells. Voltage-operated Ca2+ currents were elicited by square-step voltage pulses and pharmacologically characterized as L-type currents by (-)-Bay K8644 (BAY) and nifedipine. Both ßES and progesterone (1-100 µM), rapidly and reversibly inhibited, in a concentration dependent manner, either non-stimulated or BAY-stimulated Ca2+ currents registered in A7r5 cells. These results suggest that ßES and progesterone inhibit L-type voltage-operated Ca2+ channels through a non-genomic pathway. Consequently, these hormones inhibit the Ca2+ entry into smooth muscle cells from rat aorta, an effect that can contribute for the protection of the cardiovascular system.

The electrochemical behaviour of the new vanadium(IV) complex [VCl3(SO3Cpz3)] (pz = pyrazolyl), obtained by reaction of VCl3 with Li[SO3Cpz3], investigated by cyclic voltammetry and controlled potential electrolysis is reported and compared with those of hydrotris(pyrazolyl)borate vanadium(IV) complexes.

A FIA system with electrochemical detection was developed for the automatic determination of paracetamol in pharmaceutical formulations. The analytical difficulties caused by adsorption of matrix excipients in the electrode surface were surpassed by an on-line electrochemical regeneration of the glassy carbon tubular electrode, using the carrier solution, enabling the renewal of the electrode surface in a simple and rapid way. A single channel FIA manifold was developed, which provided reproducibility of sample transport to the detector and minimized the contact time between samples and electrode surface, thus reducing its cleaning frequency and permitting a high sampling rate to be achieved. Furthermore, the physical characteristics of the electrochemical cell permitted its easy incorporation in the FIA manifold, offering robustness to the system, and allowed it to be generalized to routine analysis applications. With the optimized parameters, a linear correlation between paracetamol concentration and peak current intensity was obtained up to 4  10-4 mol L-1, with a detection limit of 2.5  10-5 mol L-1. Paracetamol was quantified in pharmaceutical dosage forms and the results were compared to those obtained for the official spectrophotometric method (BP 1998). Correlation between the results obtained by both methods was linear and no statistical difference between methods was found at the 95% confidence level.

The electrochemical oxidation of four different azo dyes, C.I. Acid Orange 7 (AO7), C. I. Direct Red 254 (DR254), C. I. Direct Red 80 (DR80) and Yellow Gold Sandolan (YGS), was performed using as anode material an oxide with a perovskite like structure, BaPb0.9Sb0.1O3-δ. Bulk electrolysis was studied using Na2SO4 as electrolyte, at a current density of 5 mA cm-2. UV-Visible absorbance measurements, Chemical Oxygen Demand (COD), Total Organic Carbon (TOC) and High Performance Liquid Chromatography (HPLC) analysis were performed, in order to follow the colour and COD removals and the mineralization index. The obtained results show an almost complete colour removal for the solutions containing AO7, DR254 or DR80, after 24 h essay. For these dyes, after 96 h electrodegradation experiment, COD removals between 30 and 70 % and TOC removals ranging from 15 to 40 % were obtained. However, for the experimental conditions used, the electrodegradation of YGS was not possible.

In this work we tested the performance of the bismuth film electrode in the detection and simultaneous determination of toxic metals on real samples in a complex matrix, by square wave anodic stripping voltammetry. The studied samples were white poplar leaves gathered in “2ª Circular”, which is an area of major traffic intensity of the city of Lisbon. The determined metals were Pb (II) and Cd (II). After being dried, the white poplar leaves were submitted to an acid digestion process assisted by microwave technology. A complete optimization study of the voltammetric parameters was made in order to simultaneously determine the metals as well as to decrease the detection limits and improve the method’s repeatability and sensitivity. The working electrode consisted in a bismuth film deposited on a vitreous carbon disc surface. The Ag/AgCl system was used as the reference electrode and a platinum wire as the auxiliary electrode. The concentrations (in mg of metal/kg of dry matter - leaves) obtained for Pb (II) and Cd (II) were of 3.0 and 1.7, respectively. Also, detection limits of 1.80 × 10-8 mol L-1 for Pb (II) and 6.90 × 10-9 mol L-1 for Cd (II) were estimated. In addition, a well shaped voltammogram obtained in the analysis of real samples opens excellent perspectives for the use of the bismuth electrode as an alternative to toxic metals such as mercury.

Mean activity coefficients of potassium chloride were determined in aqueous solutions of potassium chloride and potassium phthalate, in the temperature range 10-50 ºC and ionic strength range 0.05-4 mol kg-1, from potentiometric measurements on a galvanic cell without liquid junction. By processing the results using the Pitzer model, interaction parameters for potassium phthalate were calculated as well as their temperature dependence.

The semiconductive properties of anodic niobium oxides formed at constant potential and constant current density to different final voltages have been examined by Mott-Schottky analysis. Thin anodic oxides were formed on sputtered niobium specimens at constant potential in the range of 2.5 to 10 VAg/AgCl in a borate buffer solution. Thicker oxides were formed, also on sputtered niobium specimens, at a constant current density of 5 mA cm-2 in 0.1 M ammonium pentaborate solution to final voltages of 10, 50 and 100 V. Capacitance measurements were performed in a borate buffer solution of pH 8.8, at a frequency range of 200 to 2000 Hz, at a sweep rate of 5 mV s-1 from +2.5 to –1 VAg/AgCl. The results obtained show n-type semiconductor behaviour with a carrier density in the range of 8 x 10^18 – 6 x 10^19 cm-3 on films formed to 10 V. Thicker films showed lower carrier densities in the range of 1 x 10^18 – 2 x 10^18 cm-3 with a calculated charge depletion layer of 33-36 nm.

The electrochemical behaviour of caffeic acid in acetate solutions with and without added ethanol was studied by cyclic voltammetry. Solutions of pH and ethanol content close to the wine values (3.5 and 12%, respectively) were studied as a first model approach, pursuing work previously done. Studies at pH 7.4 and different ionic strengths were also done. It was found that cyclic voltammograms of caffeic acid acetate ethanolic solutions had oxidation peak potential values (≈ 470 mV) at pH 3.5 irrespective of the previous excursions of potential with the same set of electrodes. However, the cathodic peaks potentials and currents strongly depended on the cyclic voltammogram of the corresponding solvent electrolyte which had previously been run. The separation of oxidation and reduction peak potentials evidenced the presence of dimmers of caffeic acid in solution, under the following conditions: 0.1 mol dm-3 acetate buffer pH 3.5 + 12% ethanol with the ionic strength increased by the addition of 0.05 mol dm-3 KCl; limits of the anodic potential: from –100 to + 700 mV and N2 bubbled through the solution for 10 minutes.

The corrosion behaviour of Zn – Al – Cu alloy was studied in pure HCl solutions and in the presence of different concentrations of inhibitors. The techniques of measurements were: weight loss, linear polarization, polarization impedance and corrosion penetration. It was found that the corrosion rate of this alloy is greatly higher than that of Zn or Al. The inhibitors used were: heterocyclic pyridine (p), 2-methyl pyridine (2 mp), 3-methyl pyridine (3 mp) and 4-methyl pyridine (4 mp). These inhibitors behave as mixed type and their inhibition efficiency increases according to the order: 2 mp < 3 mp < p <4 mp. The values of activation energy of corrosion were determined in pure acid and in the presence of inhibitors. It was found that the presence of these inhibitors increases the value of activation within the same order of their inhibitory effect. The adsorption of the investigated inhibitors on the surface of the alloy follows Frumkin’s isotherm.

The electrochemical behavior of citrazinic acid azo dye derivatives (I-V) was recorded in aqueous buffer solutions containing 10% (v/v) DMF using DC-polarography and cyclic voltammetry. The DC-polarograms displayed one or two reduction waves depending on the nature of substituent and the pH of the electrolyte solution. The reduction process takes place irreversibly and diffusion controlled at the electrode surface through the uptake of four electron/molecules for compound I and eight electron/molecules for each of compounds II-V. The recorded cyclic voltammograms displayed two irreversible cathodic peaks at different pH values. The kinetic parameters of the electrode process such as, αn, heterogeneous rate constant (ks) and activation energy (ΔG#) were evaluated and discussed from cyclic voltammetry. Also, the electrode reaction mechanism is proposed and discussed. The effect of introducing substituents on the half-wave potential of the reduction process is illustrated. The different thermodynamic parameters such as activation energy (ΔG#), enthalpy change (ΔH#) and entropy change (ΔS#) revealed the endergonic nature of the electrode reaction.

This work aimed to study the corrosion inhibition of carbon steel in 2 M HCl solutions using some aminopyrimidine derivatives. In this investigation the techniques of measurements were: weight loss, linear polarization, impedance and corrosion penetration. The inhibitory effect of the investigated compounds resulted from their adsorption onto the carbon steel. Their adsorption on the metallic surface obeyed Frumkin’s model of adsorption. The inhibition efficiency of the compound greatly depends on the electron density on the molecules, which in turn depends on its structure. The values of activation energy were determined in 2 M HCl in the absence and in the presence of inhibitors. The presence of inhibitors increases the values of the activation energy of corrosion. The adsorption of these compounds onto the metallic surface occurs via the N-atoms of amino-groups, O-atoms of hydroxy-groups, and S-atoms of mercapto group present in the compound. For this reason the inhibitory effect of the inhibitors varied according to the number and nature of adsorption centers in the molecules.

Corrosion inhibition of mild steel in 0.67 M H3PO4 by phenacyl dimethylsulfonium bromide and six of its p-substituted derivatives was studied using different chemical, electrochemical and scanning electron microscopy techniques. The order of increasing inhibition efficiency was correlated with its p-substituted through Hammett relation. Potentiodynamic polarization curves indicated that the compounds acted primarily as mixed-type inhibitors. Electrochemical impedance spectroscopy showed that the steel dissolution is controlled by charge-transfer mechanism. The kinetic-thermodynamic model of adsorption isotherm described the experimental findings. Number of active sites, binding constant and change of free energy were computed for all studied compounds. Depending on the inhibitor, it was found that each organic molecule replaced one or two adsorbed water molecules from the steel surface. The adsorption center was suggested to be the π electrons of the phenyl ring, and a flat configuration adsorption of the molecule may occur.

Differential Pulse Voltammetry (DPV) and Fourier Transformed Infrared Spectroscopy (FTIRS) techniques have been applied and compared, for the azithromycin determination in two different pharmaceutical formulations (capsules and suspension). Both methods were found to be highly precise with high recovery levels (more than 98 %) The analytical results obtained with the electrochemical method showed excellent correlation with the analytical results obtained by FTIRS. The proposed electrochemical methodology presents the advantage over FTIRS of being able to be used directly on azithromycin samples in capsules presentation as well as in suspension, since no interferences were observed from the excipients of the formulation, nevertheless the direct analysis of the suspension was not possible by FTIRS due to excipients interference.

This paper presents and discusses data collected on the electroreduction of O2 (ORR) in 1 M NaOH on a polycrystalline Pt surface that initially was pretreated by H2 evolution at -250 mV in deaerated 0.5 M H2SO4 and then evolved with time in a spontaneous and rather slow way towards a full Nekrasov (N-Pt) type behavior, both in the cathodic- and in the anodic potential scan. This fact represents a clear demonstration that a polycrystalline metal surface is a dynamic system and allowed us to carry out the thorough study of the mechanism and kinetics of the ORR on a full N-Pt surface that was not possible in the previous work reported in the Part II of this series. Essentially, we concluded that the change does not alter the ORR mechanism, which is always a series mechanism coupled to disproportionation of the HO2- produced; what varies in a given potential range is the value of the rate constants of the different mechanistic steps. It was possible to get absolute values of these constants only for some potential ranges, although the analysis of the data collected provided relative information for the remaining ranges.

Electrooxidation of carbon monoxide and formic acid was performed on Rh microparticles deposited on both, Glassy Carbon (Rh/GC) and polyaniline films (Rh/PANI), to evaluate the electrocatalytic activity of those electrodes. The deposit of Rh microparticles on those substrates provides a higher surface and exhibits a better electrocatalytic activity compared with that shown by smooth Rh for the electrochemical oxidation of formic acid in sulphuric acid. The oxidation of CO and HCOOH on Rh/GC and Rh/PANI was followed using in situ Multi-Step FTIR Spectroscopy (MS-FTIRS). Either linear or bridge bonded adsorbed CO (COl, COb), has been observed by FTIRS as the main species. In comparison to the adsorption of CO on smooth Rh surface, the IR features of CO adsorbed on Rh/GC and Rh/PANI electrodes showed an anomalous behaviour.

Liquid ammoniates, which are highly conductive electrolyte (more than 100 mS.cm-1 at 20 °C), are proposed as solvents for a redox battery working around room temperature, and up 80 °C. In the negative compartment, the anolyte is NaI • 3.3NH3, and the Na+/Na couple is proposed. It is reversible, as determined by cyclic voltammetry and galvanostatic cycling. In the positive compartment, the catholyte NaI • 3.3NH3 enriched in silver cations is proposed too. The cathodic material is merely this compound and the cathode is the silver metal. The couple Ag+/Ag is reversible too, as determined by cyclic voltammetry and galvanostatic cycling. Metallic sodium is extremely stable in this electrolyte; therefore NaI • 3.3NH3 could be used in a redox battery of high energy density and high power density. The maximum working temperature, which is proposed, 70 °C, is lower than the melting point of sodium (98 °C), and avoids high pressures of ammonia.

A conductivity technique has been developed, being highly useful for studying the association behaviour of cyclodextrins (CDs) with surfactants. In this paper, the complexation of β-cyclodextrin with alkyl sulfonates and alkyl sulfates with different carbon chains (from 10 to 16) is analysed. The effect of the ionic head group in the most hydrophobic surfactant is also analysed. The association constants for 1:1 and 1:2 complexes are calculated on the basis of a second and a third degree equation; in the latter a real solution of a Cardin-Tartagliae formula is applied to solve the third degree equation.

The corrosion and passivation behaviours of iron in ternary molten Li2CO3-Na2CO3-K2CO3 mixture at different temperatures (475- 550 oC) were studied. The techniques of measurements were: open- circuit potential, galvanostatic anodic polarization, and cyclic voltammetry. The amounts of iron dissolved in the melt were determined by atomic absorption spectroscopy after each experiment. Iron undergoes spontaneous passivation upon its immersion in the melt and the thickness of the oxide scales increases with the increase of temperature. During the passivation potential range different oxides and spinals are formed. These include a cubic solid solution of FeO and a-LiFeO2, a-LiFe5O8 and a-Fe2O3. At high anodic potentials the decomposition of carbonate takes place leading to passivity break down and CO2 and O2 gases evolution. Also, the spoliation of oxide scales in the carbonate melt occurs at the passivity breakdown. The values of corrosion parameters (Rp, io, icorr, ip) were calculated. The different values of activation energy of corrosion process were calculated. The results of cyclic voltammetric investigation indicate the formation of different oxides and spinals, and the cathodic and anodic decomposition of carbonate. The corrosion tests in 0.05 M HCl solution indicate that the oxide scales formed on iron are multilayered. The outer layer is less protective than the inner one.

The interfacial impedance (or shortly, the capacitance) of solid electrodes in the absence of faradaic reactions usually deviates from purely capacitive behaviour. On polycrystalline solid electrodes "frequency dispersion capacitance" is often observed. We present qualitative impedance experiments on pre-treated Ag-15%Cu alloy electrodes immersed in 0.1 M HClO4. The results suggest that models taking into account microscopic geometrical aspects only (i.e. roughness) are not suitable for interpreting the de facto link between roughness and capacitance dispersion. Instead, considering additional points, such as surface heterogeneities on an atomic scale (i.e. surface disorder) together with adsorption of certain ionic species, particularly the Cl-, on the surface of the metal can and do cause capacitance dispersion in the frequency range relevant in electrochemistry.

The inhibiting effect of N-cetyl N,N,N-trimethyl ammonium bromide (CTMAB) on the corrosion of mild steel in 1N H2SO4 solution has been studied by using techniques such as weight loss and electrochemical polarization, infrared (IR) and scanning electron microscopic (SEM) techniques. It is found that CTMAB is good inhibitor for corrosion of mild steel in 1N H2SO4. The maximum efficiency is about 96% for CTMAB at 10-1M concentration. The polarization curves indicate that the compound is mixed type inhibitor. The adsorption of inhibitor on mild steel surface is found to obey Langmuir adsorption isotherm. The activation energy Ea in the absence and presence of inhibitor is obtained by measuring the temperature dependence of corrosion method. The mechanisms of adsorption of CTMAB on mild steel have been investigated by IR spectra and SEM.