Portugaliae
Electrochimica Acta

In Honour of Prof. João Cabral

Advanced electrochemical oxidation processes (AEOPs) constitute promising technologies for the treatment of organic pollutants in waters. They are based on the production of oxidant hydroxyl radical (•OH) from water oxidation on the surface of a high O2-overvoltage anode and/or from Fenton’s reaction between added Fe2+ and hydrogen peroxide electrogenerated at the cathode by two-electron O2 reduction. In this paper, fundamentals of AEOPs such as anodic oxidation, electro-Fenton, photoelectro-Fenton and peroxi-coagulation are described, and comparative degradation of aqueous solutions with aromatic pollutants, such as aniline, 4-chlorophenol and several chlorophenoxyacetic and chlorobenzoic acids, in 0.05 M Na2SO4 + H2SO4 of pH 3.0 by these techniques using an undivided electrolytic cell with an O2-diffusion cathode under galvanostatic conditions is discussed. The decay kinetics of chlorophenoxyacetic acids and the evolution of their aromatic intermediates and generated carboxylic acids are also reported to clarify their mineralization processes by the different AEOPs. Anodic oxidation with a Pt anode yields poor decontamination of pollutants, while alternative anodic oxidation with a boron-doped diamond (BDD) anode leads to total mineralization of all solutions due to the greater production of •OH on the BDD surface. Electro-Fenton with a Pt anode has high oxidation ability at short electrolysis times, but the formation of stable Fe3+-oxalate complexes limits the degradation of aromatic contaminants. These products are completely oxidized in electro-Fenton with a BDD anode or photodecomposed by the action of UVA light in photoelectro-Fenton with a Pt anode. Peroxi-coagulation with an Fe anode also gives fast degradation with generation of small amounts of stable Fe3+ complexes, since organics are mainly retained in the Fe(OH)3 precipitate formed.

The presence of soluble salts (particularly sulphates and chlorides) at the metal/paint interface is known to have a detrimental effect on the integrity of most paint systems. Though this is a long-standing problem, it has recently come to receive greater attention from the protective coatings industry. International Standards Organization (ISO) has for some time been trying to develop a standard about guidance levels for water-soluble salt contamination before the application of paints and related products. In the paper the following points are reviewed: degradation mechanisms, nature and sources of soluble salts, their distribution on the metallic surface, the joint action of chlorides and sulphates, the effect of the joint presence of rust, new methodologies for sample preparation with known levels of soluble salts, measuring soluble salts, establishment of threshold levels, removal of salts and new suitable coating systems for applying on substrates contaminated with soluble salts.

This review was erected to describe briefly proteins and processes responsible for the formation of the nativ-folded thioproteins in the pro- and eucariotic cells. This issue has a significant importance due to many dieases and misfunctions caused by misfolded enzymes having low or inactive structure. On the other hand the reduction of the yield of the correctly folded enzymes in the bacterias causing health problems, we can have a needed tool for medical care. This review contains also a short list of an electrochemical methods for the detection and the quantification of the most important thioproteins.

Based on the principles of the technique of electroremediation (electrochemical decontamination), experiments were carried out with the purpose to determine the buffer capacity of two soils from the region of Antioquia, Colombia, using a new method. These studies included the analysis of variation of the conductivity and residual pH in selected soils, after application of an electric field. From the different obtained parameters a function was defined combining the values of pH in zones close to the cathodic chamber with the corresponding one to the blank. This function allows obtaining a measurement of the buffer capacity of the soils. In this case the function represents the easiness for changing the pH when the electric field is applied and therefore it would not only have agricultural utility, but also to evaluate the behaviour in the case of a electrochemical decontamination treatment. The observed variations in the function are in agreement with those obtained when acid or bases solutions are added to soils.

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.