Portugaliae
Electrochimica Acta

Preliminary results of a study of the effect of the temperature on the response of all-solid-state ammonium ISE's prepared by casting PVC membranes with nonactin on supports of graphite/epoxy composites with up to 60% of graphite are presented. The study showed that: (i) the conductive support must contain at least 40% of graphite to produce ISE's with adequate response characteristics; and (ii) the variation of the percentage of graphite between 40 and 60% has no influence on the slope and potential temperature coefficients and on the hysteresis curves of the electrodes.

The square wave anodic stripping voltammetry of copper and zinc mixtures at wall-jet mercury thin film electrodes in continuous flow was investigated with the aim of minimizing interferences which appear in solutions containing the two cations due to Cu-Zn intermetallic compound formation in the mercury phase. The addition of gallium ions led to the preferential formation of Ga-Cu intermetallic making the determination of zinc possible. Best results were obtained in acetate buffer or acidified sodium perchlorate electrolyte at pH~3. It was shown that the method is applicable to the analysis of waters in flowing solution.

An electroanalytical procedure for total iron determination in waste water samples based on adsorptive cathodic stripping voltammetry using mercury film microelectrodes is described. Iron was determined using catechol, as the metal ligand agent, being the chelator adsorbed at the mercury film surface by imposition of a potential of -1.8 V and the reduction current measured. The results obtained by this procedure were compared to those obtained by atomic absorption spectrometry to evaluate the quality of the electrochemical method.

Potentiometric measurements in protein containing solutions are affected by the presence of protein. Overall effects on the ISE, on the reference electrode and on the electrolyte solution have been reported for BSA (Bovine Serum Albumin) containing solutions using K+ as a test cation (1-3). In this work we present studies with Human Serum Albumin (HSA) as a further step to approaching physiological conditions. Common features were found for both albumins, which are explained in terms of solution and interfacial chemistry. Measurements in HSA of particularly high concentration (100 g dm"3) show evolution in time that has not been accounted for in previous studies and do not find parallel at lower concentrations. Explanation for these latest observations may be partly associated with the formation of HSA dimers.

The effect of electron-donating substituents on the oxidation of phenol molecules was studied by cyclic voltammetry. For the considered p-substituted phenol derivatives the reactivity order observed on polyciystalline platinum electrodes was: phenol < p-cresol < hydroquinone < p-metoxyphenol. For an organic concentration of 0.1M, a linear relationship was found between the current density (log i) and the substituent constant (a). Literature values for phenol ionization were used.

Cyclic voltammetry (CV) and others voltammetric techniques, with a glassy carbon electrode, have been used to study the interaction of purines with copper in sulphuric medium and others. If the copper (II) concentration is about 10"3 mol dm"3 and the purine concentration is equal or higher, is only possible to observe one stage of reduction, but two stages of oxidation are possible. Under these conditions and pH < 2 the copper (I) is stabilized by purine. The potential variation of each peak of oxidation with increasing purine concentration show processes of electrode of one electron for the observed anodic peaks. From the shift of the anodic peak potential corresponding to Cu(0) / Cu(I) with increasing ligand concentration the stoichio-metry of the complexes have been determined. Also, the shifts of the anodic peaks potential with increasing pH show processes of one proton. The aim of this work is to describe the equilibria between purines and copper ions and to propose reactions mechanism on the complexes formation in our medium conditions.

The catalytic activity of a given substrate for the electroless metal deposition (EMD) is determined by its ability to promote the adsorption of the reductant species and to originate an anionic radical which will be involved in the metal ion reduction; it can be evaluated by measuring the open circuit potential. It is well known that the involved mechanism is highly dependent on the substrate nature, solution pH and composition1-3. Sodium borohydride is a reducing agent widely used in industrial electroless plating. It is highly unstable in acid or neutral medium, undergoing fast hydrolysis4 and if nickel ions are present, nickel boride will be precipitated with consequently bath decomposition, when the pH is maintained above 12, the hydrolysis of borohydride ions can be avoided and good quality Ni-B deposits are produced5. However, the mechanism of EMD employing this reductant is not established. In this work the electro oxidation of borohydride ion (BRf) on Ni and Cu electrodes, in the absence of traditional complexing agents and in metal ions free media, has been studied by cyclic voltammetry and potential time measurements using stationary and rotating electrodes. The results are analyzed using the "Intermediate Radical Mechanism"6 approach.

The corrosion behavior of the areas affected by welding of a steel has been performed in aqueous solution, for a synthetic sea water, by the linear polarization resistance method. Using a normalizing heat treatment at 920°C for a duration of 15 min and then air cooling, it shown to be enough to eliminate the structural heterogeneity responsible by the anodic behavior of stress material.

It is reported, for the first time in Portugal, the introduction of the electrochemical noise technique applied to the corrosion monitoring of the cooling circuit of a stator in a thermoelectric power station, allowing in real time, the detection and identification of a localized corrosion process. The circuit uses electrolytic copper in contact with low conductivity water, less than 1 uScm"1. A specially designed probe to be installed in a by-pass fashion to the cooling circuit, was developed by CML, U.K. Results obtained during a monitoring period of 3 months are discussed.

Enquadrada num trabalho mais amplo sobre a redução electroquímica indirecta de corantes, a presente comunicação teve, como objectivo, a selecção de um material de eléctrodo adequado ao complexo de ferro e trietanolamina (Fem/TEA), em meio básico, usado como mediador redox. A análise dos resultados experimentais e dos voltamogramas obtidos permitiu concluir que, dos diferentes materiais testados (aço, ferro e cobre), o cobre parece ser o que permite uma transferência electrónica de maior reversibilidade.

A procedure for quantification of nickel in cell culture medium using a mercury film microelectrode and dimethylglioxime (DMG) as the complexing ligand was developed. The results obtained by adsorptive cathodic stripping voltammetry (AdsCSV) were compared to those attained by atomic absorption spectrometry and good agreement was found.

The analytical conditions for the determination of the total iron in mice organ samples have been optimized and the suitable experimental parameters were found to be a catechol concentration of 3.0x10"4 mol/L, pH ca. 7.2 provided by PIPES buffer concentration of 8.0x103 mol/L, a deposition potential of -1.80 V, deposition time 10-30 s, frequency 50 Hz, step 4 mV and an amplitude of 20 mV. The detection limit was 13.7x10 9 mol/L after a deposition potential of 25 s. The relative standard deviation of fifteen repeated measurements of the same solution was 1.13%, indicating that the peak iron response was very reproducible.

Energy consumption in an electrolytic process is mainly governed by current efficiency and voltage efficiency. Hence an accurate calculation of the current efficiency will greatly help in evaluating energy data precisely. An attempt has been made in this study to perfect an alternate method for calculating the current efficiency (C.E.) from the chlorine gas liberated at the anode during magnesium chloride electrolysis. The anodic current efficiency (A.C.E.) thus obtained has been compared with the cathodic current efficiency (C.C.E.) calculated by metal basis. A difference between the two values has been observed and the reasons for such differences have been analyzed in this paper.

A theoretical model based upon cell configuration (Resistive Network Model) is adopted for calculating the bypass current for a stack of bipolar electrodes used in molten salt electrolysis. A simplified bypass current calculation is also made by using experimentally obtained current and potential in the magnesium cells. The results obtained from both the models are compared and discussed particularly for two different electrolyte compositions.

A definition of electrolyte presented in an international standard is critically discussed.