Portugaliae
Electrochimica Acta

The effect of some mono azo dyes compounds on the dissolution of aluminium in 0.01 M NaOH solutions was studied using weight loss and galvanostatic polarization techniques. The percentage inhibition efficiency was found to increase with increasing concentration of inhibitor and with decreasing temperature. The inhibition mechanism was discussed on the basis of the formation of complex compound adsorbed on the metal surface. The adsorption process follows Frumkin isotherm. The effect of temperature on the rate of corrosion in the absence and presence of these compounds was also studied. Some activated thermodynamic parameters were computed.

A novel electrochemical sensor has been developed for the determination of nimesulide. The sensor is based on the NIM- molybdophosphoric acid (MPA) as the electroactive material in PVC matrix in presence of bis(2-ethyl hexyl) phthalate (BEP) as a plasticizer. The sensor showed a fast, stable, near Nernstian response for 1  10-2 –1  10-6 M NIM over the pH range 5 – 8 with a slope 55.6 0.5m V/decade and the response time is < 45 s. Selectivity coefficient data for some common ions show negligible interferences. The sensor was successfully applied for the determination of NIM in tablet and the results obtained are in good agreement with those obtained by the official method.

The glassy carbon electrode (GCE) modified with cobalt nitroprusside (CoNP) was prepared by electrochemical method. The CoNP modified electrode (ME) prepared electrochemically on a GCE in two-step. The electrochemical behavior of the modified electrode was studied by cyclic voltammetric technique. The cyclic voltammogram of CoNP showed a redox couple with formal potential [E0 =(Epa+Epc)/2] of 500 mV vs. SCE. The electrocatalytic oxidation of nitrite was studied on the CoNP modified glassy carbon electrode. The CoNP films on the glassy carbon electrode show an excellent electrocatalytic activity toward the oxidation of nitrite in 0.5 mol L-1 KNO3. Hydrodynamic amperometry was used to the detection of nitrite with CoNP. The linear detection limit of the CoNP elctrode for NO2- was from 20×10-6 to 220×10-4 mol L-1 and the detection limit was 10×10-6 mol L-1 (S/N=3).

The treatment of liquid organic phases to remove undesirable compounds is currently a priority in different industries. In oil refinery, it is necessary to reduce sulphur compounds, in gasolines, to below 50 ppm.The traditional form is catalytic hydrodesulphuration; however, this system is not effective for some compounds, e.g., alkyl substituted dibenzothiophenes. The processes for modulated complexation by electrochemistry are based on the capacity of a complexing agent, in a certain oxidation state, to form a bond with the molecule to be removed from a liquid phase. If the contaminant, the solute, is in an organic phase, the complexant must be soluble in aqueous phase. When the complexant is formed and in contact with both phases the contaminant is extracted. In a second stage, the complexing agent is oxidized (or reduced) to a state of less affinity to the contaminant, and the aqueous phase is placed in contact with a new organic phase where the contaminant will be discharged and concentrated. The complexant agent in aqueous phase is reduced (or oxidized) to its state of higher affinity for the contaminant and recirculated to the extraction stage. In this work, we present the conditions necessary for this process to be feasible, the criteria for the selection of the complexes, the methodology for matter balances, determination of energy consumption and thermodynamic calculations, and also three examples of an organic phase: dibenzotiophene, pyridine, and methyl thioether, using as complexing agent iron tetrakis-(p-sulphonatophenyl) porphyrin.

In the present work, the electrochemical behavior of molybdenum-silica-niobium alloy produced by arc cast technique is presented, keeping silica constant (at 24 wt %), using different niobium concentrations ( 5, 10, 15 & 20 wt %) as alloying elements. These samples were immersed in an electrochemical solution of 10 % by volume hydrochloric acid. The corrosion behavior was obtained using electrochemical techniques, namely: open circuit potential, polarization curves and electrochemical current noise. The potential behaviour as a function of time allows to determine the activity of the alloy as a function of its content, the polarization technique was used to obtain characteristic electrochemical parameters and the electrochemical current noise the type of attack while comparing them with the corrosion surface morphological features. Surface images showed different corrosion morphologies, being the most attacked the sample without niobium additions. It was found that niobium additions result in a better corrosion resistance when its concentration is increased in the alloy.

The corrosion inhibition of mild steel in one normal sulphuric acid solution by brij-30 has been studied in relation to the concentration of the inhibitor as well as the temperature using electrochemical polarization (galvanostatic and potentiostatic) techniques. The results were supplemented with scanning electron microscopy and infra-red spectroscopy. All the methods employed are in reasonable agreement. There is no particular relationship of inhibition with concentration and temperatures. The thermodynamic functions of dissolution and adsorption processes were calculated from experimental polarization data and the interpretation of the results is given. Adsorption of Brij-30 was found to follow the Langmuir’s adsorption isotherm. Brij-30 is a mixed type of inhibitor.

The corrosion inhibition of mild steel in one normal sulphuric acid solution by PEGME has been studied in relation to the concentration of the inhibitor as well as the temperature using electrochemical polarization (galvanostatic and potentiostatic) techniques. The results were supplemented with scanning electron microscopy and infra-red spectroscopy. All the methods employed are in reasonable agreement. There is no particular relationship of inhibition with concentration and temperatures. The thermodynamic functions of dissolution and adsorption processes were calculated from experimental polarization data and the interpretation of the results is given. Adsorption of PEGME was found to follow the Langmuir’s adsorption isotherm. PEGME is a mixed type of inhibitor.

The pitting corrosion of some stainless steel alloys (preoxidized at different conditions) in 3.5% NaCl solution was studied. The alloys are: one ferritic (15.05% Cr) (alloy1) and two austenitic stainless steel alloys (17.9% Cr,7.08% Ni) (alloy2) and (20.45% Cr, 8.3% Ni) (alloy3). Potentiodynamic anodic polarization and galvanic current-time measurements were used in these investigations. The susceptibility of the alloys to pitting corrosion decreases with the increase of chromium content of the alloy and with the presence of nickel in the alloy. The preoxidation of the alloys in different media improves their resistance to pitting corrosion in NaCl solution. The resistance to pitting corrosion for the investigated alloy increases according to the order: no oxidation < oxidation in air < oxidation in molten alkali nitrates < oxidation in molten alkali carbonates. This resistance to pitting corrosion may be due to the formation of a protective oxide film on the alloys’ surface. The composition of this film greatly depends on the chemical composition of the alloy, on the condition of the preoxidation process, and on the temperature.

The corrosion inhibition of mild steel in one normal sulphuric acid solution by CPB has been studied in relation to the concentration of the inhibitor as well as the temperature using electrochemical polarization (galvanostatic and potentiostatic) techniques. The results were supplemented with scanning electron microscopy and infra-red spectroscopy. All the methods employed are in reasonable agreement. There is no particular relationship of inhibition with concentration and temperatures. The thermodynamic functions of dissolution and adsorption processes were calculated from experimental polarization data and the interpretation of the results are given. Adsorption of CPB was found to follow the Langmuir’s adsorption isotherm. Brij-30 is a mixed type of inhibitor.

We present electrochemical impedance spectra made on gold alloy containing 30% silver electrodes of various roughnesses in aqueous perchlorate acid solution as supporting electrolyte in the absence and in the presence of mM of specifically adsorbed halide ions X (X = Br-, Cl-, I-), at potentials where the dominant electrode process is the adsorption of the above anions. Efforts were mainly concentrated on the importance of surface preparation technique of the electrode and its influence on impedance spectra. Atomic scale inhomogeneities are introduced by mechanical treatment and can be decreased by annealing. Due to the annealing the double layer behaves as (almost) an ideal capacitance in the absence of specific adsorption though the surface roughness remains the same. A study of the related impedance behaviour in the presence of adsorbates even at very low concentrations (10-4 M), revealed capacitance dispersion increasing with the extent of specific anion adsorption at the gold/silver surface.

Nucleation models have been tested using reported experimental nucleation data in several systems dealing with electrocrystallization from aqueous solutions, and with a wide range of supersaturations or overpotentials. The critical nucleus size has been calculated and the results obtained with the classical and atomistic models have been compared and discussed. In order to compare these values with those occurring in crystallization, the values of the critical nucleus size for several crystallization systems have also been calculated, and then compared and commented.

Conventional processes for water treatment are inefficient for the remediation of wastewaters containing toxic and biorecalcitrant organic pollutants. A large number of advanced oxidation processes (AOPs) have been successfully applied to degrade pollutants present in waters. These methods are based on the generation of a very powerful oxidizing agent such as hydroxyl radical (·OH) in solution, able to destroy organics up to their mineralization. In recent years new AOPs based on the electrochemical technology are being developed. Electrochemical advanced oxidation processes (EAOPs) are environmentally friendly emerging methods for the decontamination of wastewaters contaminated with toxic and persistent herbicides, pesticides, chlorophenols, nitrophenols, polychlorinated biphenyls, pharmaceuticals, etc. This paper reports the fundamentals, main characteristics and recent developments of EAOPs such as anodic oxidation and electro-Fenton alone and coupled with other physicochemical processes. These techniques utilize electrolytic systems such as three-electrode divided and two-electrode undivided cells with different cathodes as working electrodes (carbon-felt or O2-diffusion cathode) and auxiliary electrodes (Pt, PbO2, boron-doped diamond (BDD) or iron anode). The effect of several experimental parameters that largely influence the degradation rate of organic pollutants is discussed. Chromatographic analyses and total organic carbon (TOC) and chemical oxygen demand (COD) measurements show a quick disappearance of initial pollutants and their aromatic and aliphatic reaction products in all cases. The great capacity of oxidation and/or mineralization of all these EAOPs to decontaminate acidic aqueous solutions of common herbicides and pesticides is described.

At present chemical (AOPs) and electrochemical (EAOPs) advanced oxidation processes with ability to destroy organic pollutants in waters are being developed. These methods are based on the production of hydroxyl radical (•OH) as oxidant. In AOPs •OH radical can be obtained from Fenton’s reaction between Fe2+ and H2O2 added to the medium, photoreduction of Fe3+ species or reaction between ozone and Fe2+. In EAOPs this radical is formed from water oxidation on the surface of a high O2-overvoltage anode and/or Fenton’s reaction between Fe2+ added to the medium and H2O2 electrogenerated at the cathode by the two-electron reduction of oxygen. The present work reports the mineralization of several aromatic drugs such as paracetamol, ibuprofen, clofibric acid and salicylic acid by AOPs based on ozonation catalyzed by Fe2+, Cu2+ and/or UVA light and EAOPs like anodic oxidation, electro-Fenton and photoelectro-Fenton, which are environmentally friendly electrochemical methods. For the latter processes, the drug decay with time and the evolution of aromatic intermediates and generated carboxylic acids are studied. Anodic oxidation is only effective when a boron-doped diamond (BDD) anode is used, whereas the photoelectro-Fenton process with Fe2+, Cu2+ and UVA light is the most potent method to completely destroy the drugs. The combined use of catalysts Fe2+, Cu2+ and UVA light in catalyzed ozonation also leads to overall decontamination of drug solutions.

Biosorption is the passive sequestration of pollutants by non-metabolizing non-living biomass. The technique emerged in 1980’s as a possible alternative method for wastewater treatments in an environmentally friendly manner. In this paper it is shown that a basic research in biosorption processes is closely related to the field of Electrochemistry according to the conceptual division of this area in Ionics and Electrodics.

Multi-layered films of poly(3,4-ethylenedioxythiophene), and poly(N-methylpyrrole), have been prepared using a layer-by-layer electrodeposition technique. The electrochemical properties and the conductivity of the films formed by 3, 5 and 7 layers have been compared with their homopolymers and with the copolymers prepared from 3,4-ethylenedioxythiophene and N-methylpyrrole with different concentration ratios. The electroactivity and stability of the multi-layered systems were higher than those of PEDOT homopolymer and copolymers. Furthermore, the electrochemical properties improved when the number of layers increased. In addition, the electrical conductivity of the multi-layered films is higher than those of poly(N-methylpyrrole) and copolymers, and comparable to the PEDOT homopolymers. These results indicate that multi-layered systems show better performance than their homopolymers and/or copolymers derivatives for some applications like anticorrosive coatings or sensor materials.

It was prepared a new polyvinyl chloride (PVC)/tetrathiafulvalene–tetracyanoquinodimethane (TTF-TCNQ) composite electrode which showed good physical and electrochemical characteristics. Different PVC/TTF-TCNQ–graphite proportions were tested and the relation 1/10 (p/p) PVC/TTF-TCNQ provided the best results in terms of sensitivity, coefficients of variation and mechanical resistance. This electrode has interesting applications as a working electrode. So, employment is remarkable in a flow injection system and had been satisfactory checked to detect ascorbic acid. Also, the electrode exhibits selective voltammetric behaviour in aqueous solution that affords its use as an ion detector. The proposed electrode showed good kinetics property with a low background current and a relatively reproducible signal. Moreover, the electrode can be prepared in a simple way and its surface easily recovered.

Electrochemical characterization of a symmetric track-etched polyethylenthereftalate (PET) membrane in contact with KCl solutions at different concentrations was carried out by measuring impedance spectroscopy (IS) and membrane potential (MP), which allow the estimation of membrane electrical resistance and ion transport numbers, respectively. IS measurements permit us the characterization of membranes in “working conditions” (in contact with electrolyte solutions) and the determination of electrical parameters for the membrane and the membrane/solution interface (resistance, capacitance and Warburg impedance) by analyzing the impedance plots and using equivalent circuits as models. The non-reproducibility of membrane potential values for two series of measurements and the asymmetry of the impedance curves show the modification of the membrane/aqueous solution interface. This point was confirmed by the time evolution of the membrane system electrical resistance and the increase of nitrogen content obtained from X-ray photoelectron spectroscopy (XPS) analysis for dry and PET samples maintained in water for different periods of time, and the results indicate an increase in nitrogen content, which is attributed to bacterial presence on the membrane surfaces. These results seem to indicate modification of the PET-membrane/solution interface due to fouling and the possibility of its determination by electrical measurements.

Simple photochromic dithienylethylenes with either perfluoro or perhydro cyclopentene ring, and a variety of substituents have been prepared and their electrochemical behavior explored by cyclic voltammetry. All present two electron irreversible oxidation waves in their open form, but the radical cation of the open isomers can follow three different reaction pathways: dimerisation, ring closure, or ring reopening. Whereas the chloro derivative follows a dimerisation mechanism (EC2E mechanism), the phenylthio substituted compound displays an efficient oxidative ring closure (ECE or DISP1 mechanism). Interesting electrochromic behavior is associated with this compound, a redox process occurring in the range 0.5-1.5V is observed by monitoring the absorption species changes (colored species) in function of the applied potential. Furthermore, electrochromic properties are also found in the corresponding ring closed isomers. Depending on the substituents on the thiophene ring and the perfluro or perhydro cyclopentene ring, open isomers can be obtained from oxidation (chemical or electrochemical) of the corresponding ring closed isomers via EC mechanism. These observations should be taken into account for the potential design of three-state conjugated systems and photoelectrical molecular switching.

The study of the reversible reduction/oxidation molecules containing multiple interacting or noninteracting redox centers has been carried out using the multipotential step technique Differential Staircase Voltammetry (DSCV). Methods to obtain the formal potentials and other characteristic parameters of the process are proposed for any interaction degree between the centers, when using spherical electrodes of any size (from planar to ultramicrospherical electrodes) and, in particular, the advantages of microelectrodes are discussed. The theoretical predictions have been tested with two experimental systems: ethyl viologen in acetonitrile and pyrazine in aqueous acid media, finding an excellent agreement between the theory and experiments. Therefore, the formal potentials for these systems have been evaluated using DSCV.