Portugaliae
Electrochimica Acta

The electrochemical oxidation mechanism of a group of dimeric Vinca alkaloid type antineoplastic agents showed that homogeneous chemical reactions are coupled with a mu-ltistep electron transfer process that is also dependent on pH. The irreversibility of the process leads to final products that form an unreactive film that strongly adsorbs on the electrode surface. The differences encountered in the anodic oxidation mechanisms of the compounds studied can be explained in relation to their toxic effects.

Voltammetric studies of the octahedral Co(III) complexes with a pentadentate ligand (PICDIEN) and a ligand X (X= Cl~, Br", H20) in aqueous solution of 0.1M in KN03, pH 7 at Pt electrode were performed and the corresponding results are presented in this paper. Experimental values of E° for the complexes under study were estimated. Peaks A'/ A at potentials between -0.4 e +0.4 V vs e s c were attributed to the reduction and oxidation of Co(III), while peaks B/B' at E°= +0.90 V vs e s c are most probably due to the oxidation of Br -.

A kinetic study of the electroless autocatalitic deposition of copper-cobalt alloys from hypophosphit solutions is presented. As the cobalt is present in solution, it deposits. Its function is to promote the reducing agent oxidation which is not catalysed by copper surfaces. Degreased aluminium wires were used as substract. Several parameters were varied around standard conditions. Deposition rates were determined by gravimetric measurements and the elementar composition of the metallic deposits by X rays analysis (EDS/WDS). Also the ionic composition of each solution was calculated presupposing thermodinamic equilibrium. Distinct mechanisms were found. Activ centers only control deposition rates when cobalt molar percentages lower than 5. Hypophosphit concentration is a determinant factor if bellow 600 mol/m3. The deposition rate is sensivel to small pH variations only between 8.5 and 11. Citrate and boric acid are important to increase the reaction but should not exceed 50 mol/m3 and 400 mol/m3 respectively. It is suggested that the species resulting from the boric acid ionization are protons accepters along the metalization process.

In regard to the understanding of copper at the active site of "blue" proteins a series of open chain and macrocyclic N^S2-ligands have been studied potentiometricalfy, spectrophotometrically and voltammetrically. Cu2+ forms the species CuL2+ and sometimes CuLOH+, whereas Cu+ gives Cu(an)Lf^2+ (an — CH3CN), CuLH2+ and CuL+. From the stabilities of the Cu2+ and Cu+ species the redox potentials have been calculated and compared to the values obtained from cyclic voltammetry. The redox potentials for the open chain complexes are between 250 • 280 mV against SHE. The complexes with the macrocyclic ligands span a range of 340 mV, going from 80 to 420 mV against SHE, or six order of magnitude in relative stability reflecting the importance of subtle differences in steric requirements. This is exemplified through the X-ray structure of the Cu+ complex with one of the 16-membered macrocycles, which reveals a tetrahedral coordination geometry typical for the cuprous ion. A comparison between open chain and cyclic ligands shows that a macrocyclic effect is found for Cu2+, but not for Cu+. The ligand field strength is very different for the two types of ligands and the redox potentials are nearly independent of the chain length of the open chain ligands, but strongly depend on the ring size of the macrocycles.

After the impact produced by the introduction of anodic (and cathodic) stripping voltammetry as a very convenient method for the analysis of ultratrace compounds, especially metal ions, alternative methods for the pre-concentration step were investigated to extend the field of application of the stripping techniques to non-electroactive species. In adsorptive stripping voltammetry, species with some tendency to be adsorbed at electrode surfaces can be pre-concentrated in this way and then stripped just as in anodic or cathodic stripping voltammetry. This lecture is started with a general view of the methods used in the pre-concentration step of stripping voltammetric methods. Afterwards, focus is moved to adsorptive stripping voltammetry: first, some simple theoretical aspects are considered, related with the steps of adsorption and stripping; then, a reference is made to instrumentation, methodology and elimination of interferences; finally, examples of the use of the technique are presented.

A new electrolyte prepared from commercial poly(ethylene oxide) and neodymium triflate, in which the salt ions are solubilised by complexation with the oxygen atoms in the polymer structure, has been studied using ac conductivity measurements. The results obtained indicate that the levels of ionic conductivity registered in this system are similar to other multivalent ion based electrolytes. The nature of the charge transporting species in the electrolyte (cations, anions, charged or uncharged ion clusters) cannot be identified with the use of ac techniques and consequently only the total ionic conductivity is reported. The dependence of the ionic conductivity of the electrolyte on the concentration of added salt can however be correlated with the phase behaviour of the polymer - salt system.

The first harmonic alternating current polarography, with a selected phase angle of n/2 rad, gives structural and analytical information about the denaturation process of insulin. This method allows detecting any alteration in the composition and in the aggregational or conformational state of the protein. The reduction of the disulphide bonds and free Zn(II) gives complementary information on the capacitive contribution around the zero charge potential, for controlling the quality of the insulin samples. Kalousek K3 and K4 methods provide fast information about the structural state of proteins in solution. Proteins with disulphide bonds (as albumin and lysozyme) yield catalytic waves in Co(H)/amonia buffer that denote the different electrochemical activity of -SH lipophilic and hydrophilic residues.

Electrode surfaces modified, by monolayers of heavy metals underpotential deposited: Pt/M electrodes, electrode ads surfaces modified by over layers of their own oxides grown in-situ, i.e. Ir-oxide electrodes, and electrode surfaces with preferred orientations, namely Pt single crystals were the modified electrode surfaces selected for this lecture. The potentialities and advantages of the elec trochemical methods, namely cyclic voltammetry and potential step pulses to produce modified electrode surfaces and simultaneously to monitor changes and to test the stabili ty and the activity of the modified and non-modified electrode surface will be emphasized and illustrated with experimental data.

Porous carbon electrodes were prepared using carbon powders of different types using various catalysts. Physical properties of the prepared electrodes were evaluated using scanning electron microscopy (SEM), surface area analyser etc. Performance characteristics of these electrodes were evaluated by galvanostatic polarisation at various temperatures in 6M potassium and sodium hydroxide solutions.

Non-linearity of calibration in the determination of anions by ion-chromatography with supressed conductivity detection is re-examined. With a carbonate eluent, the curvature is demonstrated for NGg, Br" and SO*" calibrations using plots of the logarithm of the normalized peak area against the logarithm of the anion concentration. These graphs were also evaluated theoretically assuming the peak shape is Gaussian. It is demonstrated that for sufficiently low concentrations linear calibrations are obtained and that deviations are to be found in the middle of the concentration range. Several procedures are discussed to avoid having large errors in analysing samples over a wide range of concentrations.

This article presents a simple and cheap pilot plant instalation ior the Physical Chemistry Laboratory measurements (Electrochemistry). Various experiments concerning the aluminum surface treatments could be done: cleaning, polishing, anodizing and colouring. Furthermore, controling the required results as a function of time, temperature, current density, etc. can be achived by the proposed pilot plant.

The application of unequal space grid discretization to the solution of the diffusion equations in cyclic voltammetry by the line method (semi-discretization) is subjected to a preliminary analysis. It is shown that a carefully chosen exponentially expanding space grid can improve the accuracy of digital simulations by this method, although a significant improvement is only obtained for moderate values of the expanding parameter (larger values for this parameter result in performance degradation).

Solvent effects on the enthalpies of solution of t-BuCl and t-BuBr in water and 13 monoalcohols, from 1 to 5 carbon atoms, are analysed in terms of linear solvation energy relationships. The experimental data, at 25°C and infinite dilution, are well correlated through equations of the type, isH° = aQ + a1 f(c) + a2 E^ + a3 C where f(e) is the Kirkwood function of the dielectric N constant, E^, is the normalized Dimroth and Reichardt parameter and C is the solvent cohesive energy. Pertinent information on the dominant solvent-solvent-solute interactions is obtained.

The paper involves corrosion on mild steel in 0.1M HC1 solution, using harmonic analysis and faradaic rectification techniques. In the harmonic analysis a sweep of the three first harmonics of a Taylors Series expansion is carried out. The measurement of the sinusoidal currents produced allow the determination of the corrosion rate and the Tafel coefficients. In the faradaic rectification method, the corrosion rate is determined by measuring the d.c. current at the potential for which the second harmonic of a sinusoidal perturbation shows a minimum. This current is related to the corrosion rate by the equation icorr =0.6 i^.c. Values of the corrosion rate obtained for the system under study by both methods are presented which agree with the value obtained by electrochemical impedance.

The electroreduction of 02 in 0.5 M H^SO^ on platinum finely dispersed on pyrolytic graphite and coated with Nafion films ranging in thickness from 0.5 to 18 u was studied. It was found that i) the film does not alter the mechanism of the reduction reaction; ii) the film does not prevent sintering of the catalyst; HI) the film concentrates O? from the solution but, nevertheless, the current measured for the filmed electrodes only increases relatively to the uncoated electrodes near the onset of the reduction when electron transfer controls; once diffusion becomes important the current decreases because, whatever the thickness, the transport of 02 inside the film is slowed down; Iv) thick films behave like recast films whereas thin films behave like membranes.

Research over more than 150 years has made clear that many electrochemical systems exhibit highly exotic behaviour. In the past few decades detailed dynamic studies of these interesting results often reveal oscillations, which vary from periodic to quasi-periodic to chaotic. The application of linear theory of the irreversible thermodynamics to a variety of electrochemical problems demonstrates its utility, but the time evolution of certain phenomena must be modelled by non-linear equations to characterize their dynamic behaviour. The new ideas and solution methods in non-equilibrium phenomena, that are revolutionizing dynamic systems theory, provide the framework for understanding the nature of electrochemical dynamics. The extremely complex structures generated by certain systems, are analysed in terms of fractals, one of the most rewarding scientific developments of recent years. The concept of fractal, their dimensions and the relations between fractal character of the phenomena and the dynamics of the processes are object of today's fundamental research. In this work a quick overview is presented of some recent developments related to electrochemical dynamical systems. Attention is focused predominantly on the typical tools that are useful in the study of these systems. A summary of some oscillatory phenomena from different areas of electrochemistry is presented. Emphasis will be placed on those aspects that may prove beneficial for electrochemists.