Portugaliae
Electrochimica Acta

Measurements of the differential capacity of a Hg electrode in contact with six aqueous solutions of urea were made using a computer controlled system which sampled 75 times during the drop life. The parameters of the adsorption isotherm are determined under appropriate thermodynamic conditions. The results indicate that urea adsorbs much less strongly than thiourea.

Ideally a lithium ion-selective electrode should detect 0.1mM lithium in the presence of about 150mH sodium as well as the usual blood components. About 40 cyclic and acyclic ionophores have been examined potentiometrically. PVC electrodes with three such ionophores have been evaluated for the analysis of real sera using a micro conduit flow injection technique. Each of the sensors suffered serious interferences from unidentified serum components.

A bromide selective electrode based on mercury salts applied to a support of electrically conductive epoxy has been prepared and tested. A 2:1 (by weight) mixture of mercurous bromide and mercuric sulphide was used as sensor. Response characteristics to bromide and mercurous ion were determined by standard procedures, showing that the electrode responds to bromide as a second kind electrode. The electrode shows a better performance than the PHI 91300 Graphic Controls commercial electrode, which uses the same type of sensor.

Mean ionic activity coefficients of tetraethyl-ammonium(TEA) Perchlorate in aqueous solutions have been calculated from the effect of the TEA salt on the perchloric acid activity coefficients. Acid activity coefficients have been determined by measuring the emf of a concentration cell. Parameters º, ^ and Cø in Pitzer's equations have also been calculated for a series of substituted ammonium salts using both previously reported data and our TEA Perchlorate values. . Keywords: activity coefficients, tetraethylammonium perchlorate, electrochemical cell measurements.

The polarographic behaviour of clobazam was investigated over a wide pH range. Different types of peaks were obtained depending on the pH. Characterization of the electroactive process included an examination of the degree of reversibility, electrocapillary curves, influence of surfactant agents and the effect on the peak current of pulse amplitude, ionic strength, drop time, temperature and clobazam concentration. A method for the determination of clobazam, with a detection limit of 46x10"7 M and a relative standard deviation of 0.97% is proposed.

The transfer of electrons across the electrode-solution interface has long been the subject of much research, not only from a fundamental point of view, but also because of its importance in electrocatalysis and other reactions of industrial interest. Electron transfer is very often coupled with preceding or following chemical reactions, so that elucidation of the full reaction mechanism and kinetics is more complex. With the advent of hydrodynamic electrodes and, more recently, greatly improved ways of analysing electrical transients, significant developments in the study of electrode kinetics have occurred. These advances will be surveyed and future prospects indicated.

The surface concentration effect of the metal ion during the stripping step of ASV, in a complexing medium, is discussed in terms of determination of stability constants, from the results for Pb(II) + TETA system. Subsequently, stability constants of Cd(II) + carboxy-phenyliminodiacetic acid, phenylmethyliminodiacetic acid, pyridine-2,6-dicarboxylic acid and pyridine-2-carboxylic acid have been determined and the values compared with those obtained by other techniques.

Potassium Ion Selective Electrode with a PVC Membrane Applied to a Conductive Epoxy Support. A new technique for the construction of "all solid state" PVC membrane electrodes which consists of direct formation of the membrane on a support of silver loaded conductive epoxy is described. The response characteristics of a potassium electrode with the Corning 47731 7 sensor prepared by this technique are presented. The electrode shows performance similar to the conventional (with internal solution) electrode with the same sensor, but longer durability. The advantages and shortcomings of the present construction technique over the procedures previously described for the assembly of PVC membrane electrodes (of conventional or "coated-wire" types) are discussed.

ABSTRACT Green solutions of [CuL(H20)] [ClO4]2.H20 (L=C30H24N6; structure below) in acetonitrile slowly become red. The process can be followed by cyclic voltammetry and differential pulse polarography. An initial reversible one-electron redox process vanishes with time; concurrently another appears, at more negative potentials. This behaviour will be described and discussed. KEYWORDS: colour change in solution; cyclic voltammetry; differential pulse polarography

A direct polarographic method for EDTA determination based on the measurement of the anodic wave (DCP) or peak height (DPP) observed at +0.100 V, vs SCE, in the oxidation of the dropping mercury electrode in the presence of EDTA at pH=6.0 is described. The conditions for a simple and reproducible method using the standard additions technique for the analysis of this chelating agent in ophthalmic products are studied. Finally, the differential pulse polarographic method is applied to the analysis of EDTA in eye drops and contact lens wetting and cleaning solutions.

Microelectrodes have recently been applied in several areas of electrochemistry. Some of their most relevant -properties will be described and an overview of the most important applications during the last few years will be given. These include kinetic studies under steady state and non-steady state conditions3 electrochemical studies without supporting electrolyte and in highly resistive media and electrochemical nucleation studies.

The ionic conductivity and the complex permittivity έ ' of a crystal of RbAg4I5 was calculated from measurements in the range of 27 to 39 GHz. The crystal was prepared from RbI and AgI, and tested by DC conductivity measurements. The conductivity is found to be constant in that microwave range, in contrast to what is observed in crystals such as a-AgI, a-CuI and 3-CuBr, where a diffusion mechanism intermediate between the jump diffusion and the single diffusion in liquids has been proposed. Therefore, elementary steps of translational diffusion occur in a time scale faster than in the above other materials: the moving Ag+ ion will go with less friction. The complex permittivity of RbAg4I5 has also been measured. Keywords: diffusion; jump-diffusion; ionic conductivity; permittivity; microwave technique; soli d electrolytes; complex conductivity; rubidium silver iodine.

This paper describes results obtained with an AISI 304 stainless steel in 1 M HCl and 1 M KCl solutions, with the aim of contributing to the understanding of the processes occurring within growing pits. The kinetics of dissolution were studied at constant potential by the scratch method. In 1 M HCl a salt film formed below the passivation peak and its rate of formation increased as the applied potential was made more noble. In 1 M KCl the rate of film formation was not affected by potential. In this solution a passivating film developed even before the current maximum in the anodic polarization curve. Since the solutions found within growing pits usually have low pH and high chloride concentrations it is concluded that, as pits develop, their surface should become covered by salts layers of increasing thickness. Key Words: dissolution kinetics, pitting and stainless steel.

Electrode kinetic parameters, p and k, the charge transfer coefficient and the conditional charge transfer rate constant, respectively, for the anodic dissolution of copper, cobalt, zinc and bismuth have been calculated from voltammetric scans carried out in solutions containing a selected supporting electrolyte and the same supporting electrolyte plus EDTA, respectively. The values found for the parameters in both media showed that they are affected by the presence of the complexing agent  was found to be related to the conditional formation constant, of the metal-EDTA complex studied and the k0/k0EDTA ratios obtained evidenced adsorption of the metal-EDTA compound on the electrode surface for all the cases but bismuth; its extent being closely related to the metal transition character of the metal ion involved in the complex. Keywords: Electrode kinetic parameters; anodic dissolution of pure metals; arbitrary current method.

There is currently a great deal of interest in solid electrolytes and mixed conductors due to their potential use in high performance battery or fuel cell systems and also because they can be employed in solid electrochemical transducers for a variety of scientific and technological purposes. Due to the unusual range of behaviour of these materials and the special requirements related to their use, particular attention must be given to the problem of appropriate measurement and evaluation methods. One of the most useful characterization techniques for these "fast ionic conductors" is the dc measurement of their electrical conductivity. The concepts behind this approach and the dc measurement techniques of the total conductivity, the ionic conductivity and the electronic conductivity are given in this talk together with examples of its use employing solid electrolyte and mixed conductor samples which have a wider applicability in the area of materials research.

The System Cu/NaOH 0.1M, pH 12,8 was studied, in the presence and absence of sulphate anions. Different sweep-rates and anodic limits, were experimented and the analysis of such results was made in terms of charge, peak currents and peak potentials as a function of sweep-rate. Results have shown that the kinetics of the film formation, in the absence of sulphate are controlled by diffusion of ions through a solid phase, while in the presence of sulphate are under mixed control, by diffusion and adsorption. The observed effects have been interpreted in terms of a decrease in the barrier of the activation energy, due to the formation of intermediates having SO^ strongly adsorved. A more quantitative model requires a better theoretical description of the adsorption of sulphate ions at the Cu/alkaline solution, and Cu20/alkaline solution interfaces.

The corrosion of mild steel in de-aerated sulphuric acid sol at ions has been studied in the pH range 0.3-1.2 at two different levels of solution agitation. Potentiodynamic polarization in the pre-Tafel region was employed throughout at various exposure times and the resistance polarization, R^, ca1cu1ated. Rp varies with pH pointing towards a maximum at pH = 1 irrespective of agitation. Agitation did not interfere with Rp vs pH curve shape but displaced it towards lower values, with a more marked effect for pH < 1. Voltammograms run after 2 h hours exposure were found to be shifted towards lower current densities for rapid agitation at a 11 pH's indicating an "inhibiting effect, due to an increase in agitation. Two anodic Tafel regions were evident, the first showing slight dependency on pH and the second-with values in the range of 0.020-0.O4O V, often found in acid solutions. Calculated b^ values found by curve fitting of the plot E vs i, using different combinations of bg and bc, gave values of the order of 0.080 V which agree with experimental values. Calculated and experimental b values were found to be 0.110 V. The surface activity of the metal by degradation brought about by hydrogen evolution as well as by high density of crystal imperfection produced by cold work is also invoked (6,7). This work is concerned with the corrosion behaviour of mild steel in sulphuric acid solutions at low pH to different levels of agitation. The tendency of the system to corrode is evaluated using the polarization resistance method and unidirectional voltammetry.

The electrochemical behaviour of 1(o-nitrobenzylidene)amino-4,6-diphenyl-2-pyridone, in ethanolic Britton-Robinson buffer, has been studied using polarographic, voltammetric and controlled-potential electrolysis methods. Reduction of -NOQ group to -NHOH is first observed in acidic and neutral media, while reduction of tine aldiminic bond (similarly as it takes place in analogous compounds), accompanied by hydroxylamine reduction, occurs later. In basic medium, reduction of hydroxylamine is easier than that of the aldiminic bond. The hydroxylamine stability is pH dependent, if is stable in neutral medium and unstable in acidic and basic media. As a consequence, the appearance of an ECE mechanism — in the reduction of -N02 group — is observed in basic medium.