Portugaliae
Electrochimica Acta

Careful experiments were done with isotonic and hypertonic electrolytes in reference electrodes as well as with different forms of accurately making the liquid junction (open junction, dialysis membrane and frit restricted junctions) in cell assemblies involving ion selective electrodes. The ionic strength of test solutions was varied adding NaCI at concentrations which limit the physiological range. The experiments show that dialysis membrane restricted liquid junctions follow the same trend as open junction when NaCI concentration varies, contrary to what had been found in experiments made with modified commercial analysers by other authors. Solutions with and without added albumin were studied. Clear evidence that protein influences the liquid junction potential and thus measurements with ion selective electrodes is presented.

The pitting corrosion of mild steel was studied in 0.1 M NaCI + 0.015 M NaOH in water-glycerol, water-ethylene glycol, water-ethanol and water-dioxane solutions with various compositions (from 0.0 to 60 %v/v of the organic solvent component). The results obtained from the potentiodynamic and potentiostatic measurements show that the pitting corrosion is inhibited by the organic components in the medium. The inhibition percentage was increased with the increasing of the concentration of the organic solvent in the medium and reached about 80%. It is suggested that the organic solvents inhibited the pitting corrosion of mild steel by (i) increasing the viscosity of the medium, which leads to a decrease in the diffusion coefficient of the corrosion products. It also decreases the dielectric constant of the medium, which lowers the basicity of the solution, and (ii) adsorption of solvent molecules on steel surface.

Gasometric measurements and galvanostatic polarisation studies of different grades of magnesium in different electrolytes have been carried out at different current densities ranging from 0 - 100 mA cm-2. A relationship between current density and the rate of hydrogen evolution has been observed. The predominant role of anions in corrosion and other unusual behaviour of magnesium in aqueous solutions of electrolytes have been confirmed in this investigation. Cations are also found to play a decisive role in the anomalous behaviour of magnesium in these solutions. The present study has made it possible to arrive at a new parameter called 'Transition current density' denoted by the symbol *i^ _ * at which a transition from positive difference effect to negative difference effect or vice versa occurs. The dependence of this parameter on system variables has been explained suitably. Key words: magnesium, difference effect, transition current density, aqueous environments.

Solutions of [CuL] (CI04)2, [CuLCI] (CI04), [CuLBr] (CI04), and [CuL(NCS)2], (L=Ci7H25N3S2; structure below) in acetonitrile give well-defined cyclic voltammograms with a gold disk as working electrode. One electron metal-centered, reversible or quasi-reversible oxidation-reduction processes were observed. Differences in the cyclovoltammetric behaviour of the four complexes correspond to differences in their structures. The half-wave potentials are not unusually high, and only that of [CuL] (CI04)2 falls within the range found for Typel copper proteins. These observations will be described and discussed.

Distributions of the induction times of silver potentiostatic electrodeposition coupled with hydrogen peroxide oxidation/reduction onto carbon microelectrodes have been obtained. The nucleation process has been shown to be affected by kinetic complexities due to the disproportionation of hydrogen peroxide and therefore neither the pure birth model nor the birth and death electronucleation model have shown to be applicable to this system. The double potential step and linear sweep voltammetry measurements point to the formation of silver peroxy species during the electrodeposition process.

The concept of ion pair and the way it has been presented in the literature is critically discussed.

The concepts of absolute and relative electrode potential and their relationship to the structure of the metal/electrolyte interface are presented. The necessity of distinguishing between electrode potential and electromotive force of a half-cell reaction, particularly in teaching, is highlighted. It is shown that electrode potential should be the concept recommended for defining the electrode interface potential drop.

In vitro studies were conducted to quantify the amount of iron species released from AISI 316L stainless steel biomaterial during its implantation either in human beings or animals. By constructing calibration curves for those species using the cyclic voltammetry technique it was possible to determine the iron ions levels in physiological medium.

The electro-oxidation of D-sorbitol in aqueous perchloric medium was used as a test of the electrocatalytic activity of platinum, platinum-indium and iridium electrodes. Cyclic voltammetry was the main technique used in this study. Results have shown almost no activity for Ir electrodes and similar activities for Pt and Pt-Ir. slightly higher for Pt-Ir (90:10) electrodes.

Conductance measurements of S-n-butylisothiouronium bromide, -iodide and -picrate (S-n-Buis Br, S-n-Buis I and S-n-Buis Pi) in methanol-nitrobenzene mixtures at approximately 0.25, 0.5 and 0.75 mole fractions of methanol were studied at 25 °C, 35 °C and 45°C respectively. The data were analyzed using Fuoss' equation0^ (1980) to derive the molar conductance at infmitesirnal concentration, A, the association distance R and the association constant KA corresponding to minimum standard deviation oA. The discussion was based on the variation of anionic size, the mole fraction of methanol and with varying temperature taking into account ionic solvation and viscosity effect. Thermodynamic parameters AG/(kJ/mole), AH/(kJ/mole) and AS/(J moi1 K1) were calculated and interpreted according to the variation of temperature and the mole fraction of methanol.

In the present study, the electrochemical oxidation of 5-hydroxytryptophol, a normal metabolite of the indol amines 5-hydroxytryptophan and 5-hydroxytryptamine was investigated using various electrodes in different electrolyte solutions. It was concluded that the electrooxidation mechanism depended on the electrolyte. pH, scan rate and the nature of the electrode.

Voltammetric studies revealed that under transient conditions in neutral pH gluconate solutions, the electroactive species are Sn(GH^) . The formation of monovalent species is slow. Gluconate prevents the formation of Sn(OH)^. In high alkali solutions, the adsorbed monovalent species obey to a non activated Temkin adsorption isotherm and the first electron transfer is slow. The gluconates interact with SniOH)^ and hinder the participation of OH ions in the deposition process.

The influence of amino mercapto methyl triazole (AMMT) on the corrosion and hydrogen permeation of mild steel In 0.5 M H^SO^ and 1 M HC1 has been studied using weight loss measurements and various electrochemical techniques It is found to be more inhibitive in H2S0^. Potentiodynamic polarisation studies clearly prove that AMMT is a mixed type Inhibitor, but predominantly a cathodic inhibitor. Hydrogen permeation studies and AC impedance measurements indicate an improved performance of the compound "in H^SO^. The inhibitor of this type obeys Temkin adsorption isotherm.

In regard to the understanding of the silver electrodeposition process at carbon microelectrodes three different mechanisms have been proposed and its theoretical simulations are presented. A comparison between experimental and simulated voltammograms shows that several kinetic complexities occurs during the electrodeposition process.

Behaviour of the 1-Methyl-benzotriazole-. Benzimidazole- and l-iV//2-Benzimidazole-pentacyanoferrate (II), complexes in aqueous solution of 0.5M. NaCl04 at the platinum slectrode has been analysed by cyclic voltammetric measurements. The electrode-rate constant, k0, and the transfer coefficient, ana of the corresponding complexes have been estimated at different pH values.

Role of boric acid in the electrowinning process of nickel has been investigated through eye 1 icvo 11aminetry. Addition of boric acid to the nickel electrolyte has been found to decrease the hydrogen evolution, a side reaction at the cathode, as indicated by the features of a voltammetric peak for the oxidation of hydrogen codeposited in nickel. Control in alkalisation at the cathode vicinity has been discussed on the basis of the tendency of boric acid to release H+ ions through complex formation with nickel in the electrolyte. An attempt has also been made to fix up the amount of boric acid required on the basis of eye 1icvoltammograms generated.

The concept of the electrophoretic effect in electrolyte solutions and the way it has been presented in the literature is critically discussed.

Voltammetric studies were carried out on gold electrode to identify the oxidation products of elemental sulphur in alkali solutions. The oxidation of HS~ and S22~ is reversible followed by oxidation to sulphur which gets incorporated in the hidrous films on gold. Fitting occurs when the film break and the pitting susceptibility of gold increased with alkali concentration. On the sulphur covered gold surface, no oxygen reduction was observed and H2S evolved along with hydrogen.

Organic molecules with fragments which have a specific affinity for a charge (negative or positive) and, separately, for an electron can form ion-radicals of the special (distonic) type. Such a type is characterized by a scattering of charge and spin densities between different atoms of an organic skeleton. Reasons and results of this phenomenon constitute the aim of this report.