Portugaliae
Electrochimica Acta

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.

In this work, the various methods that have been used for the calculation of ionic strength are discussed. An iterative process is introduced where, instead of traditional approximations, calculation of ionic strengths through complete equations derived from the acidity constants is performed. Concentrations of the various species involved in the conventional assignment of pH, their distribution and activity coefficients are also obtained. The process prooves particularly useful for poliprotic acids with overlaping acidity constants Ce. g. H^Cit: k1 = 7.447 x 10"*; k2 = 1.734 x l-~5; k3 = 4.018 x 10-7 at 25° C). Finally, and as a consequence, conventionally assigned pH values of reference buffer solutions are recalculated and corrections are introduced whenever required.

The polarographic reduction of the azo dye 4-nitro-2'-hydroxy-5'-tert-butylazobenzene in Britton Robinson Buffer with pH range of 2 and 12 at room temperature has been investigated in aqueous-ethanol solution of ratio 1:1. Two straight lines were obtained from the Em -pH plots for the compound. Diffusion controlled reduction behaviour was observed in acid and alkaline media. In addition, a polarographic determination method for the analysis of 4-nitro-2'-hydroxy-5'-tert-butylazobenzene has. been developed. The detection limit was 50 pbb for this medium.

Manganese dioxide exists in several crystalline forms. Heating MnO 2 in air or, vacuum brings about changes in crystallinity. Heat-treated samples of Mn02 at 373/ 573, 723 and 1023 K were used in fabricating Li-Mn02 cells. These cells were subjected to impedance measurements at 283, 293, 303 and 313 K temperatures. The R and I values were calculated after subjecting to Cdl and R corrections. These results are discussed.

Heat treated electrolytic manganese dioxide is used as the cathode in lithium manganese dioxide primary batteries. Heat treatment at 400°C, removes the occluded water in the Mn02. Also, partial crystal change from pure gama to a mixture of gama- Beta occurs during this heat treatment. In this paper the reduction mechanism of heat treated Mn02 in 0.5 M LiC104/PC - 2 M LiC104/PC have been studied by cyclic voltammetry. The mechanism of Mn02 reduction varies with the concentration of electrolyte.

Measurements are reported of the linear sweep voltammograms of silver on the presence of mixed catalytic oxidation/reduction of hydrogen peroxide on to vitreous carbon microdisk electrode. The catalytic rate constants were determined by performing theoretical simulations of the expected responses of the current-potential profiles and good agreement was found between these calculated profiles and the experimental responses in the cathodic branch for k9°= 2.5. 10"6 mol cm'^s'. The occurrence of a nonelectrochemical pathway reaction associated to the catalytic behaviour of hydrogen peroxide at silver substrates was observed experimentally and the density flux rate constant of this reaction was found to be 3.2. 10~6 mol cm'^s'K.