Portugaliae
Electrochimica Acta

The electrochemical intramolecular cyclisation of allyl 2-bromophenyl ethers in N,N'-dimethylformamide at constant current in a diaphragmless cell has been developed using Ni(II) and Co(II) complexes as electron-transfer mediators. Cyclic compounds are obtained in good yields under appropriate experimental conditions.

Iron disulphide molten salt electrochemical cells are among the most promising technological options for batteries. The electrochemical behaviour of pyrite allies excellent cathodic characteristics to optimal performance and low operational costs. The cathodic iron disulphide mechanism involves many processes, encompassing from polysulphides formation to the reduction of iron to the metallic state. The use of X-ray diffraction together with scanning electronic microscopy analysis on cells of the Li/KCl-LiCl/FeS2 system made possible to identify intrinsic and extrinsic parameters to the electrochemical process involved in the establishment of the polysulphides stoichiometry. It is necessary to note that the augmentation of the cell's internal resistance and loss of electrical capacity are directly related with the formation of these same substances. Researches in the electrochemistry of these phenomena aim to elucidate the cathodic interphase processes and the effect of every reaction in the global mechanism.

On the basis of an ohmic model and a Tafel equation describing relations between current density and overpotentials in the film and at the metal/film interface, respectively, it is shown that a quantitative analysis of galvanostatic transients for the growth of passivating ultra-thin films on the so-called non-noble metals can be obtained. As an example, the growth of ZnO on Zn in a boric/borate buffer solution is considered. In this case, the values of the transfer coefficient and the exchange current density of the reaction at the metal/film interface were found to be 1.2 and 0.11 mA cm-2, respectively. It was shown that a single, first film occurred at low current densities and two films at high ones. The ionic resistivity inside the single, first film, during the transients, has an initial constant value region followed by a final increase indicating the aging process. For this variation the evolution of the point defect concentrations is taken into account. For the variation of the ionic resistivity with the galvanostatic current density two types of behaviors were found, depending on the current density. An interpretation of these results is advanced in terms of the concentrations, mobilities and recombination rate of point defects inside the film.

CoFe films were electrodeposited on platinum and copper from an acidic sulfate bath. The deposits surface morphology was analyzed using SEM and composition was determined by XPS. For deposition potentials lower than -0.80 V (SCE), the anodic stripping voltammogram showed a complex anodic current peak evidencing the complexity of the deposition process. Analysis of the deposition current transient curves showed that the nature of the substrate influenced the kinetic of the process: it was observed an instantaneous nucleation on Pt and a progressive nucleation on Cu. Pure cobalt and cobalt-iron films formed on Pt and on Cu presented binding energies for Co2p3/2 signal corresponding to cobalt oxide, while the Fe2p3/2 signal is related to metallic iron. The corresponding metallic cobalt was observed for the Co2p3/2 signal with binding energy of about 778.0 eV.

The degradation of a reactive blue 4 dye in acidic solution has been compared using electrochemical reduction and oxidation at reticulated vitreous carbon electrode (RVC) and Ti/SnO2/SbOx (3%mol)/RuO2 (30%mol) electrodes and photo-Fenton method under UV irradiation. The reduction of RB4 dye at -0.6 V on RVC electrode results in 50% of color removal and up to 64% of TOC removal. The direct oxidation of RB4 dye at +1 V on RVC promotes only the oxidation of amine group and there is no color removal. On SnO2 electrode, 58% TOC was removed and the decolorization is around 100 % after 1 hour of electrolysis at 2.4 V indicating good efficiency. Best performance was obtained by photo-Fenton method that indicates 80% elimination of TOC and 100% of color removal but the method requires oxidant addition and time of rest to remove the generated residues.

Cyclic voltammetry and controlled potential electrolysis, and other analytical techniques were used to study the reactions of mercury with iridium oxides formed by repetitive cyclic voltammetry (RCV) or by controlled potential electrolysis (CPE) on pure iridium substrate. The oxides were formed in a Hg(I)-containing solution. The SEM images, XPS spectra and CV obtained after the RCV treatment showed a more attacked surface with less mercury on the Ir. On the contrary, the SEM images, XPS spectra and the CV for the electrode prepared by CPE indicated a less attacked surface with more Hg. The oxides formed when the CPE condition was applied inhibited the UPD of mercury.

This will enable Pb(II) determination optimisation, regarding environmental aspects of the perpetuation of Quercus suber Montado (similar to Forest). It will also allow a more profound toxicological quality control of corks (barks). These two last referred aspects are very related ones, once accepted that the more advanced could the certification of the quality patterns of corks (barks) be, more will be the sustainability of Quercus suber ecosystem. Which we believe can be done, amongst other vectors, by studying such an important environmental issue as heavy metals. It is shown that Pb(II) quantification in the cork (bark) of Quercus suber tree (Cork Oak) is possible, using Differential Pulse Anodic Stripping Voltammetry (DPASV). The samples digestion was made in a mixture of H2O2 and HNO3 1:8, in a closed recipient at approximately 90 ºC. The measures were made in NaCl 0.1 M, using a Hanging Mercury Drop Electrode, a Glassy-Carbon Rod Counter Electrode, and an Ag/AgCl/KCl 3M reference electrode, after a 180 s deposition step. The results have shown the determination of a coherent stripping potential of -0.405 V (SD=0.0005 V), and a fine linear adjustment after the standard addition method (R2 = 0.997). They have also revealed the interest of further studies. The need to test other electroanalysis requisites was understood, and imposed by the proof of the complex nature of the matrix.

The behaviour of the neutral alkynyl complexes trans-[FeBr(L)(depe)2] (L = -C=C-C(=CH2)Ph 1a, -C=C-CPh2(H) 1b; depe = Et2PCH2CH2PEt2) is studied by cyclic voltammetry and the electrochemical PL and EL ligand parameters for the alkynyl ligands are estimated showing that they behave as very strong net electron-donors.

Erratum to “Electrochemical Behavior of Pt(IV) on Mercury Electrode in the Presence of Dimethylglyoxime” Volume 20, Issue: 4, Year: 2002, pp.179-189.

The rate of copper dissolution in presence of phosphoric acid tert-butanol electropolishing mixtures was studied by measuring the limiting current which represents the rate of electropolishing. The rate of dissolution decreases by increasing phosphoric acid concentration, electrode height and the mole fraction of alcohol. The data show that addition of tert-butanol to phosphoric acid decreases the rate of dissolution of copper by an amount ranging from 68 % to 90 % depending on the dielectric constant of mixtures. Thermodynamic parameters are given.

A solid of Fe2O3 based carbon-epoxy composite electrode was investigated for use as a potentiometric pH sensor. The electrode was constructed with a mixture of sulfated iron(III) oxide, carbon power and epoxy resin that was deposited directly onto a glass tube. The effect of composition (Fe2O3, carbon and epoxy resin) on the electrode response and its calibration curve (mV/pH) were investigated. The analytical behaviour of the electrode in acid-base titrations was compared with that of a glass electrode. A linear response from pH 1.7 to 12.2 with a slope of -39.7 ± 0.6 mV/pH (at 25 ºC) was observed.

The formation and stability of alkali metal complexes with crown ethers containing an anthraquinone unit has been investigated in methanol and acetonitrile solutions by potentiometric methods. Complexes of 1:1 stoichiometry were observed for all the studied systems; 2:1 complexes were only detected for the larger alkali cations (Rb+ and Cs+) and with the ligand with the larger macrocyclic cavity (AQ21C7). The 1:1 complexes with the highest stability are formed by K+ ion with the AQ18C6 ligand and by Cs+ ion with the AQ21C7 ligand. In the alkali cation group and in both solvents, the stability of the 1:1 complexes was found to vary in the following order, Li+ < Na+ < K+ > Rb+ > Cs+ with AQ18C6 ligand and Li+ < Na+ < K+ < Rb+ < Cs+ with AQ21C7 ligand. The stability of the complexes of AQ21C7 is smaller than those of their AQ18C6 analogues. The stability of the silver complexes of AQ18C6 and AQ21C7 was found to be smaller than that of the alkali metal complexes of comparable size. The incorporation of an anthraquinone unit in crown ethers induces some loss of complexation stability for all cations, but the cation selectivity is not changed. The results obtained are analysed and discussed regarding the effects of the relative sizes of the cations and macrocyclic cavity of the ligands, solvating ability of the solvents towards the cation and influence of the anthraquinone unit.

The electrochemical behavior of Pt(IV) ions in the presence of dimethylglyoxime has been investigated using adsorptive stripping voltammetric method. The results indicated that the platinum is reduced from its adsorbed state as bis-dimethylglyoxi-mate platinum(IV) complex. The peak current is linearly proportional to the Pt(IV) concentration in the solution, thus representing the basis for a quantitative analysis. Various experimental parameters affecting the stripping response, indicating pH, the ligand concentration or the accumulation time and potential, and the scan rate, were studied to obtain a highly linear response. From the results the scheme for the reduction process of the platinum dimethylglyoximate complex in acidic media is presented.

In liquid ammonia, the anodic behavior of n- and p-type III-V semiconductors are strongly different from those observed in aqueous media. For both types, the anodic process in liquid ammonia involves the formation of an adsorbed film, onto the semiconductor surface just before its dissolution. This film results from the chemisorption of N atoms, issued from the oxidation of the solvent.

The cathodic electrochemical behavior of InSb was studied in liquid ammonia. In neutral unbuffered media, in spite of strong negative polarization no electrochemical modification of the interface was observed. However, in acidic media, a cathodic decomposition was detected after scanning InSb in the potential range of hydrogen evolution. Hydrogen embrittlement results from a strong interaction of hydrogen radical (H.) and indium surface. A coated film of indium results from this cathodic decomposition.

The effect of some thiourea derivatives on the corrosion inhibition of copper has been investigated in 2 M HNO3 solution. Weight-loss and polarization measurements were applied to analyse the metal corrosion behaviour in the absence and presence of the inhibitor. The results show that all the studied thiourea derivatives inhibit the corrosion of copper in the solution and that the inhibiting efficiency increases with a decrease in temperature or an increase in concentration of the thiourea derivatives. The obtained results show that the adsorption of the additives used follow the Temkin adsorption isotherm. Moreover, the thermodynamic activation parameters of the copper corrosion reaction were calculated and discussed.

Studies on the initial stages of the atmospheric corrosion of copper exposed to the urban atmosphere of the city of Lisboa, at a site called Campo Grande, from November 2000 till February 2001 (winter period), are presented in this paper. Average corrosion rates of 0.139 and 0.105 g/(m2 day) were obtained of copper samples after one and three months of exposure, respectively. From SEM/EDS data it was concluded that during the first month of exposure corrosion products were mainly composed by copper oxides and minor amounts of copper chlorides, while exposures of two and three months lead to the formation of copper sulphates apart from copper oxides and copper chlorides. A uniform and adherent film was visible on the surface after three months of exposure. During the period under evaluation concentrations of NO2 were between 32 and 36 ppb and deposition rates of SO2 between 17 and 20 mg/(m2 day), without significant changes during the three month period. The degree of humidity and the average precipitation were both quite high (winter season). The SO2 present in the atmosphere with a high degree of humidity explains the formation of copper sulphates.

An ideal low temperature synthetic route to prepare vanadates such as LiMnVO4 and LiNiVO4 with high purity, better morphology and good electrochemical behaviour was attempted via, Self Propagating High Temperature (SPHT) method using an internal fuel. Conventional solid-state fusion method and acetate precursor method were also carried out at high temperatures such as 1073 K and 873 K respectively and the results were compared with that of SPHT method. Samples obtained from these three methods were examined for their FTIR spectra, XRD pattern, surface area, particle size analysis and SEM. Both the compounds viz. LiMnVO4 with an orthorhombic spinel-related structure and LiNiVO4 with an inverse spinel structure were found to be synthesized stoichiometrically at a lower temperature (573 K) through SPHT method. Charge-discharge studies carried out at a constant current drain (0.1 mA) showed that these vanadates were electrochemically active. To understand the role of fuels in SPHT method, three fuels namely Urea (U) (FCV=6) and two other fuels with a high fuel calorific value viz. Glycine (FCV=9) and Pthalyl DiHydrazide (PDH) (FCV=37) were employed in the synthesis. Among the three fuels better products were obtained for Glycine and Pthalyl DiHydrazide (PDH) rather than Urea in terms of surface homogeneity and better morphology. Advantages of the SPHT method of synthesis and the role of fuels were discussed in detail.