Portugaliae
Electrochimica Acta

This paper describes the use of a FIA system for the automatic determination of uric acid in urine, using a tubular amperometric detector. In this system, glassy carbon electrodes of tubular configuration were activated by successive cyclical scans from - 0.30 to 2.00 V versus Ag/AgCl reference electrode in 0.50 mol.L-1 sulphuric acid solution. Samples were diluted in supporting electrolyte before analysis and no other pre-treatment was employed. Such dilution enabled the matrix effects to be reduced and therefore improving the sensitivity provided by the activation step that allows the uric acid (UA) to be detected at lower concentration levels after dilution. Uric acid determination was performed in a single channel FIA manifold, which provided reproducibility of sample transport to the detector and enabled a sampling rate of 120 samples per hour to be achieved. The tubular configuration of the cell offered robustness to the system since it was rigidly fixed to the manifold, allowing the manifold to be generalised for routine analysis application. The results obtained with the FIA system for UA determination in urine were compared with those from the enzymatic method used in clinical analysis laboratories. No statistical difference between methods was found at the 95% confidence level. Relative deviations between both methods were within ± 4% and the proposed system showed good repeatability (about 3%, n = 10).

The electronic structure of the passive films formed on stainless steels of different chemical composition in artificial sea water is examined by capacitance measurements (Mott-Schottky approach) and photo-electrochemistry. Analytical characterization is carried out by Auger Electron Spectroscopy. The influence of the main alloying elements (Cr, Ni) on the development of space charge regions, which can be described as a depletion or accumulation layers are discussed.

The electrochemical removal of the textile dye C. I. Direct Red 80 (DR80) was carried out using three different materials as anodes: iron, polypyrrole (PPy) and boron doped diamond (BDD). Iron electrodes are consumed during the electrolysis, and promote flocculation/coagulation of the dye. Polypyrrole, a conducting polymer, was prepared by chemical/electrochemical precipitation over a cotton cloth; it enables the precipitation of the dissolved dye through a conversion reaction in a less soluble compound. Boron doped diamond electrode was used to achieve the complete mineralization of the dye. Bulk electrolysis were studied using Na2SO4 as electrolyte. In the case of the PPy electrode, NH4NO3 was also used as electrolyte. The variation of the dye concentration was followed by UV-visible absorbance measurements and chemical oxygen demand (COD) tests were also performed, to compare the rates of colour and COD removal in each case. From the COD measurements over the time of electrolysis, using the BDD anode, a mass transfer coefficient for the DR80 molecule was determined. The obtained results show an almost complete colour removal for all the electrodes tested and a COD removal between 50 and 90 %, being the best results obtained with the BDD electrode.

In the present work, the electronic structure of the passive film formed on austenitic stainless steel of the 304 type and its implications on the initiation of localised corrosion are investigated taking into account concepts developed in semiconductor physics and semiconductor electrochemistry. Capacitance measurements (Mott-Schottky approach), show that the susceptibility of AISI 304 stainless steel to stress corrosion cracking (SCC) in boiling chloride containing aqueous solutions is closely linked to the formation of a chromium rich passive oxide film with p-type semiconductivity. A small polarisation is required to drastically change the electric field at the film-electrolyte interface, as a consequence of the high doping level of the passive film. Initiation of the SCC phenomenon is described as the consequence of localised changes in the semiconductive properties of the passive film.

Iron nitroprusside, Fe(II)NP was incorporated into a carbon paste electrode and the electrochemical studies were performed by cyclic voltammetry. The cyclic voltammogram of the modified electrode, Fe(II)NP showed two redox couples ( E½ox)1 = 0.24 and (E½ox)2 = 0.85 V vs. SCE attributed to Fe(II)/Fe(III) and Fe(II)(CN)5NO / Fe(III)(CN)5NO, respectively. The redox couple at (E½ox)1 = 0.24 V presents an electrocatalytic response for ascorbic acid. The modified paste electrode exhibits a decrease of 180 mV in the ascorbic acid oxidation. The modified graphite paste electrode gives a linear range from 1.0 × 10-3 to 2.6 × 10-2 mol L-1 of ascorbic acid with a detection limit of 1.4 × 10-3 (±1.2%) mol L-1 (n =3). The electrocatalytic oxidation of ascorbic acid compounds by the mediator has been used for the determination of ascorbic acid in a commercially pharmaceutical available product.

The semiconducting properties of anodic passive films formed on polycrystalline copper in aqueous borax solutions, pH 9.2, are studied using electrochemical impedance spectroscopy (EIS) and voltammetry. The semiconducting nature of the cuprous passive layer is analysed in the potential region near de rest potential as a function of the electrode potential and the presence of CO dissolved in the electrolyte. The oxide formation is explained as a sequence of Cu2O growth, cation adsorption, Cu(II), and dissolution steps similarly to previous reported investigationsfor the metal in CO free solutions. The different growth conditions change the defect or excess of cations accumulated in the outer side of the cuprous layer/electrolyte interface leading to different semiconducting properties.

Pt-Ru/C, Pt-Dy and Pt-Ru-Mo/C electrocatalysts prepared by Bönnemann´s method have been studied as porous thin films on high surface area carbon electrodes, in order to evaluate their electroactivity on CO desorption in PEM fuel cells. For comparison electrode precursor powders with and without thermal treatment were considered. Cyclic voltammetry showed that addition of Mo in the well-established Pt/Ru system is very promising for methanol oxidation. In order to compare the electroactivity of different catalysts a normalization procedure based on the amount of Pt was used.

The rate of copper II/zinc cementation from copper sulphate solutions in the absence and in the presence of methanol (CH3OH) and dimethyl sulphoxide (DMSO) has been studied and the reaction was found to follow first-order kinetics. The influence of several parameters on the course of the reaction, such as cylinder rotation speed, initial concentration of Cu2+ ions, temperature and concentration of organic solvent, was investigated. In the case of CH3OH, rotating zinc cylinder was used, while in the presence of DMSO stationary zinc sheet and rotating zinc cylinder were used. It was found that (1) the percentage inhibition caused by methanol ranged from 10.70 to 58.38 depending on the concentration of the alcohol used; (2) the rate of cementation in the presence of DMSO using rotating zinc cylinder > the rate of cementation in the presence of DMSO using stationary zinc sheet; (3) the rate of cementation on zinc cylinder in the presence of DMSO < the rate of cementation in the presence of CH3OH. Different reaction conditions, and the physical properties of solutions are studied to obtain dimensionless correlation among all these parameters. Thermodynamic parameters ΔS*, ΔH* and ΔG* were studied.

The inhibition of mild steel corrosion in 1N sulphuric acid was studied in the presence of various concentrations (10-150 ppm) of substituted dithiobiurets and their molybdenum and tungsten complexes at 25 °C. All the compounds behaved as ambiodic corrosion inhibitors for mild steel and were considered to inhibit the corrosion by getting adsorbed on the surface. The inhibition efficiencies (IEs) of the complexes, in general, were found to be appreciably higher than those of corresponding ligands. However, the IE values of the complexes of molybdenum and tungsten of the same ligand did not differ much in their magnitude indicating thereby that the nature of central metal ions hardly influenced the inhibition efficiency. The slight difference in the IEs of the complexes at a given concentration seems to be caused by the slight difference in their size. The variation in IEs of different derivatives of ligands has been attributed to change in their size, number of active sites for adsorption and the electron density on these active sites. Scanning Electron Microscopy was used to study the surface of corroded and inhibited specimens.

The ionic conductivity of lithium based solid polymer films prepared from poly (ethylene oxide) (PEO) and lithium hexafluoarsenate (LiAsF6) with varying compositions of plasticizers likedibutyl sebacate (DBS) and ethylene carbonate (EC) was measured by AC impedance method. Polymer film composition viz. (PEO)8-LiAsF6-(DBS)0.4-(EC)0.1 has been evaluated as an optimum composition as evidenced from its high conductivity and freestanding ability. The high conductivity observed for the polymer electrolyte with this composition has been attributed to an enhanced amorphous character and a reduced energy barrier to the segmental motion of lithium ions in the matrix. The temperature dependence of conductivity on the polymer films, with and without plasticizers, appears to obey the Arrhenius law. However, the activation energy of the plasticized polymer film is 0.81 KJ/mol, a value considerably lower than 10 KJ/mol obtained for the unplasticized electrolyte, making the polymer to be a prospective candidate as lithium-ion conducting electrolyte for rechargeable lithium batteries.

Guar gum was tested as corrosion inhibitor for carbon steel in 1 M H2SO4 solution using weight loss and Tafel polarization techniques. The results showed that the inhibition efficiency increases with the increasing of the guar gum concentration, which act as an inhibitor of the mixed type. The inhibition action of guar gum was discussed in terms of its horizontal adsorption on the metal surface. The adsorption follows Langmuir adsorption isotherm. The effect of the presence of chloride ion in pitting corrosion was analyzed by the potentiodynamic anodic polarization technique. The pitting corrosion potential changes with the concentration of Cl- ion according to a sigmoid S-shaped curve. This behaviour was explained on the basis of the formation of passivatable, active and continuously propagated pits.

The electrochemical data obtained by cyclic voltammetry on several camphor-type (A, B and C) ligands and derived complexes are revised. A direct positive correlation is established between the redox potentials (anodic or cathodic) of the camphor species and the σp-Hammett parameter of the R group on the camphor skeleton. A study of the effect of the ligand on the redox properties of some Pd, Pt or Cu camphor complexes shows that the ligands influence either the anodic or cathodic processes but the characteristics of the metal drive the potential values.

This work reports on cyclic voltammetry and spectroscopic UV-Vis investigations of some pyridazine derivatives 1-8 in dimethylformamide. In the electrochemical study, monochlorinated pyridazines 2-8 exhibit two reductions but in the case of dichlorinated derivative 1 an additional wave is seen for the reduction of the second carbon-chloride bond. The electronic absorption spectra display an intramolecular charge transfer band π-π* in the UV region of which depend substantially on the nature of both donor and acceptor moieties. These results indicate the π-electron delocalization in the conjugated system.

The bis(cyanoimide) complex trans-[Mo(NCN)2(dppe)2] (dppe = Ph2PCH2CH2PPh2) is susceptible of electrophilic attack to form the acylated and aroylated derivatives trans-[MO(NCN){NCNC(O)R}(dppe)2]Cl (R = Et 2a or Ph 2b). Herein, we report a preliminary study on the electrochemical behaviour of complexes 2 and the derivative trans-[Mo(NCN)Cl(dppe)2][BF4] 3, as investigated by cyclic voltammetry (CV) and controlled-potential electrolysis (CPE), what has allowed a comparison of the electron-donor ability of the ligands.

The electrochemical properties of a series of &[M(R,R’timdt)2&] dithiolenes (M = Ni, Pd, Pt; R,R’timdt = monoanion of disubstituted imidazolidine-2,4,5-trithione) have been studied by cyclic voltammetry, controlled potential coulometry, and EPR spectroscopy. All the compounds undergo two separate one-electron reductions and one two-electron oxidation, which proceed through a single step in the case of nickel complexes, and through two separate steps in the case of palladium and platinum complexes. Inductive effects played by the R and R’ substituents influence the formal electrode potentials of the electron transfer processes. EPR spectroscopy has proved that the unpaired electron in monoanions [M(R,R’timdt)2]- is located on a SOMO, which, even if mainly ligand centred, is significantly contributed by the metal.

The complexation of nitriloacetic acid (NTA) with copper and lead was studied by square wave stripping voltammetry (SWSV) using a mercury iridium microelectrode as the working electrode. The results show that NTA interfere with the measurements, but if the mercury iridium microelectrode is coated with different concentrations of agarose (0.75%, 1.5% and 15% w/v) this interference is greatly minimized. The best results were obtained with 1.5% agarose.

A flux cell with vitreous carbon tubular electrodes, as auxiliary and working electrodes, was tested. The following procedure was used: (1) study and optimization of the mercury deposition conditions at the working electrode, (2) study the electrodes position and distance in order to minimize dispersion of the sample, (3) study of the linearity and reproducibility using different concentrations of manganese (2.10-7 to 6.10-7 M), with differential pulse anodic stripping voltammetry (DPASV) and square wave stripping voltammetry (SWSV). Good linear correlation was obtained for concentration of manganese used.

The rate of mass transfer was studied at (i) a single horizontal cylinder cathode and (ii) a vertical array of closely spaced horizontal cylinders and an array of separated cylinders. The mass transfer coefficients were obtained by measuring the limiting current of the cathodic reduction of alkaline potassium ferricyanide. Variables studied were: cylinder diameter, physical properties of the solution, and nitrogen flow rate. The mass transfer coefficient was found to increase with increasing nitrogen superficial velocity. Decreasing the diameter was found to increase the mass transfer coefficient. The mass transfer data were correlated by the following equations: For single horizontal cylinder cathode: J = 2.138 (Re. Fr)-0.24 For an array of closely spaced cylinders: J = 50.118 (Re.Fr)-0.32 For an array of separated cylinders: J = 100 (Re.Fr)-0.28 Possible practical application of gas sparged array of horizontal cylinders in building electrochemical reactors are discussed.

Four commercial non-ionic surfactant compounds, namely tween 80, 60, 40 and 20, were tested as inhibitors for corrosion of nickel in 1.0 M H2SO4 solution. Weight loss measurements, potentiostatic polarization and cyclic voltammetry techniques were used in this study. It was found that all the four used compounds act as good inhibitors for acid corrosion of nickel. The inhibition efficiencies obtained by the three techniques were almost the same, and increase with increasing the hydrocarbon chain length, the presence of a double bond in the chemical structure of the surfactant and with increasing the surfactant concentration. The polarization studies show that tween compounds act as mixed inhibitors. The inhibition action of these surfactants is interpreted in view of their adsorption on the metal surface making a barrier to mass and charge transfer. It was found that the adsorption of only tween 20 and 40 follows Langmuir adsorption isotherm. The values of free energy of adsorption for them were calculated. It was found that the adsorption process is spontaneous and increases, for different surfactants, in the same direction as inhibition efficiency. The cyclic voltammetry shows that there is only one anodic peak corresponding to the dissolution reaction of nickel electrode. The current of this dissolution peak was used also for corrosion rate measurements and in evaluation of inhibition efficiencies of the used compounds.