Portugaliae
Electrochimica Acta

Radical cyclisation is rapidly becoming an irnporlanl method for the formation of cyclic systems. Hence, some electrochemical results obtained in the study of electroreductive intramolecular cyclisation of different types of unsaturated halides using nickel(II) complexes as mediators, are presented.

The electrochemical behaviour, as studied by cyclic voltammetry and controlled potential electrolysis, of the dinitrogen-organocyanamide complexes /ra7M-[Mo(N2)(NCNR2)(dppe)2] (1, R = Me; 2, R = Et, dppe = Ph2PCH2CH2PPh2) is reported, and the electrochemical Ph ligand parameter for the cyanamides is estimated showing that they behave as effective net electron-donors with a remarkable stabilizing influence on the coordination of dinitrogen.

Ionic conductivity measurements, X-ray diffraction analysis and thermal studies have been carried out on polymer electrolyte films prepared by the addition of zinc trifluoromethanesulphonate (triflate) to polyethylene oxide), PEO. In general this electrolyte system was found to behave in a manner similar to other divalent salt-based electrolytes with an increase in crystalline morphology as the concentration of the salt component in the electrolyte was decreased. The conductivity isotherms calculated for this system show an almost constant level of ionic conductivity over the entire range of composition studied.

Guaiacol is an undesired compound in some natural products, mainly because of its aroma. In this work, the voltammetric determination of guaiacol by oxidation at a carbon paste electrode following a cathodic accumulation was first tried, with no sucess, because there was no accumulation of the compound. On the contrary, it was found that there is accumulation of a product formed after oxidation of guaiacol, probably a dimeric product, that can be determined by reduction. Using an accumulation time of 5 minutes and a potential at which guaiacol is being oxidized, a determination limit of 2 ug/L was obtained by cathodic stripping voltammetry, a value about ten times lower than that obtained in the anodic determination. The method was applied to the determination of guaiacol in cork stoppers.

The inhibitive effect of thiourea, 2-amino [4-p-chlorophenyl] thiazole and different derivatives of their condensation products, N-2[4-p-chlorophenyl] 2-thiazolyl] thiocarbamide on the corrosion of mild steel in 20% formic acid solution has been studied using weight loss and electrochemical polarisation techniques. Different concentrations of inhibitors and temperatures have been tested. The inhibition efficiencies calculated by both the techniques are in fairly good agreement except at higher concentrations of some inhibitors. The results observed have been discussed in the light of the molecular structures of the compounds and their adsorption on the surface of the corroding metal. All the inhibitors have been found to be good inhibitors. The polarisation measurements indicate that these inhibitors influence both anodic and cathodic reactions and thereby they contribute to reduce the corrosion rate.

Voltammetric studies on copper, nickel and cupronickel alloys of various compositions in neutral chloride solutions revealed that the passivation is due to the formation of duplex film consisting of inner compact Cu2O layer with outer chloro hydroxy layer. Under transient cyclic polarisation conditions, the presence of nickel hindered the passive film formation decreasing its thickness and ohmic conductivity. The reduction of these passive films to copper was markedly favoured by chloride ions. The presence of nickel in the alloy hindered the reduction of these passive films.

In this paper we discuss the advantages of using interactive simulation programmes for electrochemical dynamical experiments to help both researchers and students in understanding the phenomena underlying these experiments, as well as in obtaining meaningful chemical and electrochemical parameters associated with the redox transformation mechanisms. A PC computer programme developed for this purpose, and which is available to anyone interested, is also presented and applied to the demonstration of virtual cyclic voltammetry.

Porous unsupported Pt-Ru single phase alloy electrodes with different compositions viz., 9:1, 7:3 and 5:5 are prepared by NaBFL reduction method at room temperature. The reactivity of these alloy electrodes towards the methanol oxidation reaction (MOR) is systematically investigated in different electrolytes of high and low pH with various quantities of methanol at room temperature by using cyclic voltammetric (CV) and steady-state galvanostatic polarisation techniques. Various electrochemical processes occurring in different potential regions are discussed. The highest activity of the MOR is obtained for a 7:3 electrode in 2.5 M H2S04 / 2.5 M CH3OH and in 6 M KOH / 6 M CH3OH mixtures, respectively. It is concluded that by choosing the proper ratio of OH ions and CH3OH in solution, it is possible to remove completely the intermediate organic species and/or poisonous species that retard the MOR.

Four complexes of 2,2'-bi-4,5-dihydrothiazine (btz) with Co(II), Ni(II), Mn(II) and Cu(II), and two complexes of 2,2'-bi-thiazoline (bt) with Co(II) were synthesized and characterized, three of them, the Mn(II) complex and the two bt complexes, for the first time. These six complexes were studied using cyclic voltammetry in acetonitrile solution.All displayed reversible or quasi-reversible behaviour, and two waves were observed in the case of [Co(btz)3] [C104]2- All the results are described and discussed.

The oxide films formed by potential cycling on commercial Ti6A14V alloy, that has been surface implanted with iridium, are studied by capacitance measurements and Auger analysis. It is observed that these films become enriched in iridium when etched in sulphuric acid solution prior to their formation, simultaneously with a change in the type of semiconductivity (n-type to p-type).

An outline of a document with a basic vocabulary used in electrochemical industries is indicated following an historical development with references to sources where it would be properly explained. The objective would be to avoid having to include "definitions" on elementary concepts in CEN and ISO standards. The inadequacy of those definitions in standards is indicated and the present approach is justified.

The quantity pH=lg aH is immeasurable, as it involves a single ion activity. The IUPAC recommendations for the pH scale for aqueous solutions are based on the Bates-Guggenheim convention for the calculation of the chloride ion activity coefficient. The idea of this convention, intended to be applied to ionic strength not higher than 0.1 mol kg"1, was to impart pH with as much fundamental meaning as possible while recognising that it is conventionally based. The Pitzer theory, involving ionic specific interactions, may be applied in the calculation of single ion activity coefficients at ionic strengths higher than 0.1 mol kg"1. This approach has the merit that all possible known buffers, with or without background electrolyte, and those yet to be developed, can be assigned pH values.

The effect of 4(2-pyridyl)-l-benzaldehyde-3-thiosemicarbazone derivatives towards the corrosion of iron in 1M HC1 solution at 303K was investigated. Potentiodynamic and weight loss measurements show that compounds under investigation act as effective inhibitors for the corrosion of iron in 1 M HC1. The results show that these compounds behave as mixed type inhibitors. The corrosion rate was found to be a function of the nature and concentration of the inhibitors. The adsorption of the compounds on the iron surface is found to obey Temkin's adsorption isotherm.

The electrokinetic behaviour of aqueous suspensions of naturally occurring clays with different mineralogical composition is reported as a function of pH and electrolyte concentration. The results indicate that the clay particles carry a net negative charge in the examined pH range. The estimated zeta (Q potential values have been interpreted in terms of the composition and structural characteristics of the clay minerals, nature and charge of the particle surfaces and properties of the electric double layers developed at the clay/electrolyte solution interface. The  potential values of the clays obtained using microelectrophoresis and electroosmosis techniques have also been compared.

Justifica-se a necessidade de compreender bem os fundamentos da electroquímica para entender e consequentemente resolver adequadamente os problemas da protecção contra a corrosão.

The potentialities and limitations of electrochemical methods in the study of trace metal speciation in natural waters will be focused, as well as the factors influencing the voltammetric signal, with a special emphasis on the diffusion coefficient of labile macromolecular systems. Some applications of voltammetric techniques to the speciation of trace metals in natural samples are presented, namely with humic matter, estuary and seawaters.

A procedure for the quantitative analysis of mixtures of acids, based in the treatment of data of potentiometric pH titrations by multivariate chemometric calibration methods, is presented. Experimentally, the procedure consists in the titration of a set of mixtures of acids similar to the sample, but with known concentration of the components, which constitute the calibration set. A two step data analysis procedure is used for the development of calibration models: (i) the raw titration curves are adjusted to a polynomial equation of the fifth degree and the inverted titration curves are obtained (volume as function of the pH); (ii) the inverted titration curves are used to develop calibration models, using multivariate chemometric methods like partial least squares and artificial neural networks which can deal with some non-linearity in the basic inverted model. The results showed that adequate predictions of the concentration of the acids of the mixture are obtained.