Portugaliae
Electrochimica Acta

Copper nitroprusside (CuNP) was generated on the graphite paste electrode using a new methodology of the preparation. The electrochemical studies were carried out by cyclic voltammetry technique. The cyclic voltammogram of the modified electrode, CuNP showed two redox couples (E0’)1=0.22 and (E0’)2=0.88 V vs. SCE attributed to Cu(I)/Cu(II) and Fe(II)(CN)5NO/Fe(III)(CN)5NO, respectively. The nature of the cation affect the (E0’)1 and (E0’)2, as current intensity, shifting the E0’ for more positive potentials, for two redox processes. The voltammograms obtained with different KCl concentrations (0.1-3.0 mol L-1) exhibit a shift in the (E0’)1 to more positive potentials; this change was linear with the supporting electrolyte concentrations change. It was verified that the (E0’)1 remained practically constant at pH between 6 and 3. However, a new process with (E0’)3 (0.48 V) appears at pH<3 and it was ascribed to formation of intermediary species. The redox couple at (E0’)2=0.88 V presents an electrocatalytic response for N-acethylcysteine. The modified graphite paste electrode gives a linear response between 5.0´10-4 to 1.0´10-2 mol L-1 of N-acethylcysteine with a detection limit of 4.5´10-4 (±5%) mol L-1 (n=3) and an amperometric sensitivity of 4.9 mA/mmol L-1. The electrocatalytic oxidation of N-acethylcysteine compounds by the mediator has been used for the determination of N-acethylcysteine in a commercially pharmaceutical available product.

The electrodeposition of tin in presence of tartrate ions has been analysed by electrochemical techniques, mainly chronoamperometry, and by scanning electron microscopy (SEM). The obtained values of nucleus density with both techniques have been compared and discussed. The electrodeposition process follows an instantaneous nucleation with 3D growth under diffusion control at the initial times of the process, but a second nucleation process occurs at higher times. The influence of tartrate and of agitation conditions is also inferred from the crystal morphology.

The electro-oxidation of toluene in 0.1 M But4NPF6 + CH3CN solutions on glassy carbon electrodes was studied using electrochemical and spectroelectrochemical techniques. Toluene electro-oxidation yields an electrochemical inactive film on the electrode surface. In situ Fourier Transformed Infrared (FTIR) studies suggest the formation of a polymeric film, as the main product, on the surface. Depending on the experimental time scale toluene transformation is a complex reaction that could involve adsorption processes, pure charge transfer reaction and couple chemical reactions. Additions of small quantities of water to the electrolyte cause a cathodic displacement of the oxidation peak potential; in other words, the electro-oxidation reaction is favoured.

Azetidinone ester is the key intermediate during the synthesis of injectable cephalosporin compounds. This intermediate is undergoing electrochemical reaction during the process of conversion from penicillin to cephalosporin. The CV and RRDE studies clearly indicate that electro-reduction of this compound is taking place in three steps under the condition. The first two peaks correspond to the two, one electron transfers and the third one is due to the proton addition from the non-aqueous solvent. And during the process, the cyclisation is also taking place in order to neutralise the charge. Further CV and RRDE studies with the addition of small amount of water to the above system clearly confirm the mechanism. This was very well confirmed by the FTIR analysis of the product obtained from the bulk electrolysis.

The purpose of optimization consists in establishing suitable values for the operational parameters of a process, in order to achieve the results of that process to the greatest possible advantages. These ideal operating conditions are known as the optimum. The process parameters are current density, temperature, concentration, electrode materials, flow rate, cell voltage, mixing and diaphragm and engineering quantities are like current efficiency, space-time yield, energy consumption, yield and conversion. There are several possible targets in optimization of process and the main aim is to get maximum yield, high current efficiency, high purity and minimum energy consumption for mediated electrochemical oxidation. Mediated electro oxidation or indirect electrosynthesis is a cyclic process involving electrochemical generation of cerium(IV) and uses that agent to effect a chemical reaction. Cerium(IV) has attracted attention in electroorganic synthesis for synthesis of aldehydes and quinones.

The characteristics of corrosion inhibitors of the tolitriazole (TTA) for the carbon steel 1008, covered by zinc or tricationic phosphate (Mn, Zn, Ni), in the means of H2SO4 0.1 mol L-1 and NaCl 0.5 mol L-1, were investigated by physical and electrochemical methods. The results obtained in the absence of the inibidor TTA showed that the corrosion protection properties of the tricationic phosphate (Ptri) are better than those observed for the zinc phosphate (PZn), probably due to the presence of vacancies in the layer of PZn. The curves of anodic polarization presented current densities (j) smaller for the steel phosphatized with Ptri, when compared with the metal substrate without phosphate and with PZn. The results of impedance electrochemistry (IE) showed a polarization resistance (Rp) larger for the electrodes phosphatized with Ptri. The electrochemistry measurements of the samples of steel phosphatized, with PZn and/or Ptri, in the presence of [TTA] = 1x10-3 mol L-1, present smaller j and larger Rp when compared with the absence of TTA. In the process of phosphatization of the carbon steel with PZn + TTA, it was observed similar j and Rp in the samples of steel phosphatized with Ptri without TTA. These results suggest that the bathing of zinc phosphate with [TTA] = 1x10-3 mol L-1 can substitute the well known industrial bathing of tricationic phosphate. Corrosion testing by Salt Spray of steel phosphatized with PZn and PZn + TTA, painted with white polyester ink, confirmed the eletrochemical results.

It was developed a very sensitive analytical method to determine molybdenum in water and food grains, using the linear sweep voltammetry technique, with adsorptive preconcentration of a Mo(VI) 8-hydroxyquinoline complex, in an acetate buffer media of pH = 3.5 and KClO3 0.5 mol/L as a support electrolyte. The linear range of the methodology goes from 1.0 hg/L to 10 mg/L, with a limit of detection of 0.3 hg/L and quantification of 1.0 hg/L. The methodology is suitable in the range of mg/L and hg/L. The method was applied to grinded samples of rice, beans, lentil seeds and chickpeas with the following result: 6.2 mg/kg in lentil seeds, 6.0 mg/kg in chickpeas, 3.2 mg/kg in red beans, and 1.7 mg/kg in rice.

Some of the aluminium complexes are electroactive at positive potentials, being necessary a carbon vitreous electrode to their quantitative determination. Nevertheless the aluminium alizarine complex could be measured at negative potential with a mercury hanging electrode, but this complex is not electroactive at negative potential; instead of that, the residual alizarin non complex could be measured. The kinetic complex is very slow, so it is necessary to heat at 80 0C, for five minutes. The study of experimental conditions of complex formation was established at pH 9.25 and is determinant in the complex formation; besides, the buffers employed should be of high regulatory capacity to obtain derivative formation. The optimization of the measurement experimental conditions was done by square wave voltammetry (SWV). Detection limit got using standard deviation of seven blank solutions was 3 mg /L, the variation coefficient was 8%, and the mean recovery for 16 mg /L was 97%. The calibration curves were done in a linear interval of 8 to 64 mg /L. The method application to water samples is possible after previous evaporation.

Inside the construction industry, in the area of sea water retention structures, the chlorides contamination is a main cause of the corrosion, because in the concrete they could form macro cells upon repairing and applying a mortar free of this ion, forming anodes in the repaired zones, and thus increasing corrosion. In the reinforcements are commonly formed corrosion products because the concrete is exposed to mechanical, physical, chemical or biological conditions that commit and shorten their performance. As a consequence, it is important to determine their useful life and that of the made repairs, using electrochemical techniques that allow determining the level of protection, the effect of the macro cells in the corrosive process of the reinforcement, and the mechanism of deterioration in the structures. The present work was carried out using six beams with preoxided reinforcements, six precracking and one beam blank. After curing, those beams were exposed to accelerated cycles of deterioration for causing the cracking of the concrete, making possible to observe their electrochemical behaviour. The tests carried out were corrosion potential and corrosion rate, through the resistance to the polarization technique.

This work describes the determination of pentachlorophenol (PCP) in pure and polluted waters by square wave voltammetry (SWV) and the influence of the electrode potential on PCP oxidation on boron doped diamond (BDD) electrodes. Measurements carried out in solutions of the Britton-Robinson buffer with pH = 5.5 revealed a single oxidation peak at 0.80 V vs. Ag/AgCl for PCP in a process that is controlled by the adsorption of the species. The detection limits obtained were 5.5 mg L-1 in pure water and 15.5 mg L-1 for water taken from a local creek, respectively. Controlled potential electrolyses were carried at 0.9, 2.0 and 3.0 V vs. Ag/AgCl and the solutions analysed by SWV, HPLC, chloride ion selective electrode and UV-vis spectroscopy. At low positive potential (0.9 V), the formation of an adherent film on the electrode surface involving the transference of 1 electron per PCP molecule was observed. At potentials close to the onset of O2 evolution (2.0 V), the formation of the corresponding quinone was detected. Electrolyses carried out well into the region of oxygen evolution (3.0 V) lead to the total combustion of PCP to CO2 and H2O as well as to the release into solution of 5 Cl- ions per PCP molecule destroyed.

LiMn2O4 is an attractive 4 V positive material in lithium rechargeable batteries owing to its favourable electrochemical characteristics besides its economic and environmental advantages. However, problems of limited cyclability, especially at elevated temperatures, have limited the utility and commercialization of this cathode material. Stabilization of the LiMn2O4 spinel structure has been sought to be realized by doping the spinel with suitable cations. In this paper, the results of an exploratory research on the capacity and cyclability of LiMn2O4 cathodes simultaneously doped with Cr3+ and Mg2+ are reported. LiMgy1Cry2Mn2-y1-y2O4 spinels with y1 = 0.00, 0.05, 0.10, 0.20, 0.25 and 0.30 and y2 (0.3 - y1) were synthesized by a solid-state fusion method. While Mg2+ bestows a positive effect on cyclability, it leads to a considerable reduction in capacity due to the oxidation of Mn3+ to the inactive Mn4+ as a result of charge compensation. Cr3+ on the other hand, leads only to half as much reduction in capacity while according added stability to the structure. Any expectation of a synergistic effect by Cr3+ and Mg2+ ions was belied by these findings.

The transfer of acetylcholine cation through a monolayer of a lipid, dipalmitoylphosphatidylcholine, formed at a polarized water | 1,2-dichloroethane interface has been studied using electrochemical impedance spectroscopy. The ion transfer process was characterized at different levels of interfacial coverage. From measurement of the cyclic voltammograms and double layer capacitance it was found that cation transfer through dipalmitoylphosphatidylcholine monolayers was cation and phospholipid concentration dependent. An increase on current intensity and capacitance values was observed at more positive potentials due to the transfer of acetylcholine across the interface in the absence or presence of lipid in the organic phase. The thermodynamic and kinetic parameters were also accessed from electrochemical impedance measurements.

Corrosion inhibition of 3-phenyl hydrazonoacetyl acetone derivatives on electrodeposited tellurium and palladium in nitric acid was measured by using electrochemical method. Polarization curves showed that these compounds are cathodic inhibitors. The inhibition appears to function through general adsorption following the Flory-Huggins adsorption isotherm. The rate of corrosion depends on the nature of the inhibitor and its concentration, mode of interaction with the metal surface, molecular size, formation of complexes and the active center in the molecules and its electron charge density. Also, ΔGoads. values were calculated. The reaction rate was found to be proportional to the absolute value of the Hammett constant (s).

Formation of complexes of alkaline-earth cations with crown ethers containing an anthraquinone unit (AQ18C6 and AQ21C7) was investigated in methanol and acetonitrile solutions. Stability constants of the resulting complexes were estimated by means of potentiometric methods. The results obtained show the formation of complexes of the LM2+ type for all alkaline-earth ions; for the larger cations, (Sr2+ and Ba2+) and in excess of ligand, L2M2+ complexes were also detected. The most stable complexes were obtained with Ba2+ cation and for both ligands. The binding strength for the cations towards AQ18C6 and AQ21C7 ligands and in both solvents, increased in the order, Mg2+ < Ca2+ < Sr2+ < Ba2+. The results of this study suggest that on the 1:1 complexes, the size of the cation and its fit into the macrocyclic internal cavity of the ligand was a dominant factor on the coordination binding. The number of donor atoms in the ring of the macrocycle does not affect the binding strength and the AQ18C6 complexes have larger stability than the corresponding AQ21C7 complexes. The presence of an anthraquinone unit on the structure of the crown ether induced a decrease of the cation binding strength. The external carbonyl group of the ligand molecules was not involved on the coordination to the cation. Considering the two solvents, the complexes were more stable on acetonitrile, the solvent of weaker solvating capacity.

The corrosion behavior of pure copper rotating cylinder electrode (RCE) exposed to 3% NaCl or artificial seawater prepared as Vätäänen nine salt solution (VNSS) has been studied using electrochemical impedance spectroscopy (EIS) and polarization techniques. EIS experiments for copper RCE were carried out at different rotation rate from 0 rpm to 1600 rpm. Polarization resistance (Rp) values were obtained from both EIS and polarization experiments. Excellent agreement between impedance and polarization data is observed. RCE experiments demonstrated that Ecorr and corrosion rate for copper depend linearly on rotation speed r0.7. The results obtained showed that EIS is a powerful electrochemical method to follow the change of corrosion mechanisms.

A few 1-aryl-2,5-dithiohydrazodicarbonamides and their triphenyltin and dibutyltin complexes have been studied as corrosion inhibitors for mild steel in dimethyl sulphoxide containing HCl at 25 ºC using electrochemical polarisation technique. Among dithiohydrazodicarbonamides, ethoxyphenyl derivative exhibited maximum inhibition efficiency (IE) followed by benzyl and phenyl analogues. The same order of IE was observed in their corresponding organotin complexes; however, IE of a complex was found to be significantly higher than that of the constituents ligand. All organotin complexes behaved predominantly as cathodic inhibitors. Triphenyltin complexes proved to be better corrosion inhibitors than their dibutyltin analogues.

The objective of the present work is the study and optimization of a voltammetric methodology to determine palladium traces and ultratraces, with the purpose of being able to establish the environmental levels of this element and to verify if a contamination takes place starting from the gases of escape of the vehicles equipped with catalytic converters. The voltammetric determination of palladium is based in the formation of a complex of palladium with dimethylglyoxime that adsorbes actively on the surface of a hanging mercury drop electrode, for then to be reduced when making the square wave potential sweep. The quantification is made by the method of standard additions. The study of interferences demonstrated that Na+, Mg2+, K+, Ca2+, Zn2+, Cu2+ and Al3+ are not even interferences at the level of 10 mg/L. The detection and quantification limits of the methodology are respectively 10 ng/L and 33 ng/L, with a sensibility of (4.3±0.8)*102 nA L/ng.