Portugaliae
Electrochimica Acta

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.

Insulin-zinc suspensions are more electroactive on the hanging mercury drop electrode (hmde) when the protein is in amorphous state. Reduction and reoxidation of disulphide bridges depend on the Zn(II) present in buffer solutions. The influence of the insulin adsorption on the reduction process of zinc at the hmde, can be explained by variation oi the exchange- current, density with a change in the surface fraction covered by the protein on the electrode.

Prussian blue (PB) has been electrodeposited on ITO-glass coated electrode. Electrochromism effect, stability of the film against continued ciclation of potential and influence of nitrate and chloride ions has been studied.

A critical review of the ca.. 100 papers in which ion-selective electrodes (ISEs) are used as detectors in flow-injection analysis (FIA) systems is presented, with the purpose of identifying systematic patterns of utilization. Three points, the insertion of the ISEs in F IA systems, the characteristics of F IA systems in which ISEs are used, and their analytical applications, are considered in turn.

Modern theory of ionic solutions depends upon information on a variety of experimental methods for a realistic adjustment of parameters. Hamiltonian models yield reliable equations for various solution properties with a unique set of basic parameters resulting from fits of different appropriate experiments, thus permitting the simulation of the properties of other electrolyte solutions for which data are not available. In this way knowledge-based data bases provide the property profiles of electrolyte solutions for applied research and technology. A survey is given of the information on structure and dynamics of electrolyte solutions obtained from high frequency permittivity measurements and of chemical models at McMillan-Mayer level permitting the calculation of electrolyte solution properties. The interplay of thermodynamic, transport, spectroscopic, kinetic and computer simulation methods is stressed. Illustrative examples are quoted.

The principle of fast techniques in direct electrochemistry is presented, and the domain of their applicability is examined on the basis of classical theories. In this process two series of factors are identified which limit the method. In the one hand, ohmic drop, capacitive phenomena and coupling of diffuse and diffusion layers are intrinsic factors. They lead to built-in limitations of the time scale range of the method. These limits cannot be trespassed without the necessity of introducing severe modifications of the available theories of molecular electrochemistry. On the other hand, other limitations exist which are only due to the present technology, and are therefore expected to be broken in the future.

An electroanalytical study of the cytarabine reduction process at a dropping mercury electrode in aqueous supporting electrolyte solution using direct current polarographic technique has been carried out. The optimum parameters were found as 1000 dyne.cnr2 pressure on the mercury reservoir, Is drop time, 4 mV.s"1 scan rate and 5.50-6.60 pH range. The reversibility of the reduction on the mercury electrode was ascertained as quasi-reversible and the polarographic current was mainly diffusion controlled. The results obtained by DC, SIAP and DP polarography allowed a method developed for the determination of cytarabine in the lxlO-4 - 5xl04 mol.L"1 concentration range. Good results were obtained by applying the DC polarographic technique to the determination of cytarabine in a pharmaceutical preparation.

The experimental conclusions on the chemical-kinetic process of the chemical plating on ABS surface, one of the plastics materials more widely used, are presented, describing each step of the process

In the anodic methanol oxidation in aqueous sulphuric acid on smooth R-electrodes using the chronopotentyometric method, oscillations of potential in the V=V(t)-curves have been observed. The evolution of such potential oscillations with the value of the current impulse imposed on the electrodes has been discussed and a reaction mechanism of the methanol oxidation has been proposed in accordance with the experimental results