Portugaliae
Electrochimica Acta

Using square- wave voltammetry at a static mercury drop electrode, a fast and simple method has been developed for the assay of the water-soluble vitaniins tliiamiiie hydrochloride, riboflavin, folic acid and nicotinamide. In Britton Robinson buffer (pll 9.5) by cathodic scan thiamine hydrochloride give a peak at -1.33 V, riboflavin give two peaks at -0.61 V and -0.73 V, folic acid gives a peak at -0.91V and nicotinamide give a peak at -1.74 V vs. Ag/AgCl reference electrode at the same buffer solution. These four vitamins can be determined by the same voltammogram. The reduction peak currents are linearly dependent on the concentration of the vitamins. The method is successfully applied to determine these four vitamins contents of multivitamin preparations simultaneously. The values found agreed well with those determined by other methods.

The electrochemical behaviour of the cyanometalate complexes [Fe(CN)e]4" [W(CN)8]4\ [Os(CN)6]4-, [Mo(CN)8]4" and [Ru(CN)6]4" is investigated at high concentrations of the corresponding potassium salts (0.6 M) in alkaline solution (pH = 13) by voltammetric measurements with a platinum microdisk electrode. Under these experimental conditions the electrode reactions are reversible and the half-wave potentials are about 130 mV more positive than the standard redox potentials of these couples. The analytical characteristics of voltammetric determinations at high concentrations of electroactive species are studied using K4[W(CN)g] as a model compound.

In the present study the voltammetric behaviours of some phenothiazines namely Promazine hydrochloride, Promethazine hydrochloride and Fluphenazine hydrochloride were investigated. And it was shown that the electro oxidation of the substances was dependent on the nature of the supporting electrolyte, pH, scan rate. The results of the statistical analysis revealed that the quantitative analysis of these substances could be made using glassy carbon electrode, activated by a simple electrochemical procedure, with good accuracy and precision.

Mercury film microelectrodes, prepared by potentiostatic electrodeposition of Hg(II) from an acidic solution onto a gold microdisk electrode, in combination with adsorptive stripping voltammetry in the presence of catechol were used to study the behavior of copper, iron and nickel in aqueous solutions. The voltammetric behavior of hydrogen ion was also investigated by performing cyclic voltammetry and stripping voltammetry with mercury microelectrodes.

The use of some p-substituted benzoic acid derivatives as low cost inhibitors for the corrosion of aluminium in 60% H3PO4 solution was explored by weight loss and galvanostatic polarization measurements. The inhibition efficiency of the compounds increases in the order: p-OH > P-NH2 > benzoic acid > p-Br > p-N02- This order is not affected by the variation in temperature in the range 30-60 °C. Rate of corrosion is affected by both concentration of inhibitors and the temperature of the medium. The reaction rate was found to be proportional to the absolute value of Hammett constant (a). Some thermodynamic functions were computed and are discussed. Key words: Corrosion, aluminium, polarization, weight-loss measurements.

Basic concepts of electrochemistry are not yet properly understood in areas related with technical aspects where this science is fundamental, such as corrosion. Examples of this are given here with the objective of alerting electrochemists for the need to give their collaboration in technical bodies (e.g. ISO, CEN, OIML [1]) where it is useful to complement the very important technical knowledge of applied experts on the relevant field (e.g. corrosion) with fundamental knowledge of electrochemistry.

The effect of some 4-amino-4' -nitroazobenzene derivatives as corrosion inhibitors for copper in nitric acid solution at 30 °C was investigated using thermometric and polarization techniques. The corrosion rate was found to be a function of the nature, of inhibitor and temperature of the medium. The calculated degree of concentration surface coverage was used to evaluate the free energy of adsorption (A G°a) of the process. All the inhibitors follow the Frunkin adsorption isotherm. The mechanism of inhibitor action has been discussed on the basis of adsorption and molecular polarizahility of the inhibitors. The observed experimental data indicate that these compounds act as mixed inhibitors. Some thermodynamic function was calculated and discussed. Key words: Corrosion, inhibitors, acid solution, copper, dyes.

A recent survey of secondary school pupils' attitudes to chemical formulae show interesting similarities with the attitudes of XIX century scientists to the formulae system developed by Berzelius (the basis of the present system). Dalton's opposition to this system, Children's alterations of a Berzelius original when translating it to English, a fact praised by contemporaries, and other objections of the time are compared with today's secondary school pupil’s comments collected in a recent study. These comparisons make it easier to understand the difficulties in teaching formulae at school.

The open-ended capillary cell has been used as a reliable method to measure mutual differential diffusion coefficients of a great variety of aqueous solutions of electrolytes [1, 2]. In the last few years we have been developing, with some success, a capillary cell capable of measuring thermodynamic diffusion coefficients of electrolytes imbibed in polymeric matrices [3-5]. We have developed two different systems, called "manual" and "automatic", to measure diffusion coefficients. In this communication we will present an improvement of the "manual" system that permit us to measure a large range of concentrations, from approximately 10 M until at least 2 M, with very good precision, and accuracy.

An electrolyte local composition model is developed to establish an equation to calculate isothermal activity coefficients, in a range of concentrations until 2.5M, for solutions of strong 1-1, 1-2 and 1-3 electrolytes. The validity of the equation with two adjustable parameters has been confirmed on thirty compounds.

Mean activity coefficients of KC1 in water-ethanol solvents were determined, using the Pitzer model, from electromotive force data, at 25 °C. The molalities of KC1 varied from 0.1 mol kg"' to near saturation, in the range 0 to 20 % w/w ethanol. A modified model of the Pitzer equations was also applied to predict the mean activity coefficients of KC1 in water-ethanol and water-methanol mixtures. These results were compared with experimental values.

The electrochemical behaviour, in aprotic medium, of the 18-electron octahedral-type dinitrile complexes ira/«-[Fe(NCR)2(depe)2][BF4]2 (R = alkyl, aryl, NH2, NMe2 or NEt2; depe = Et2PCH2CH2PEt2) was studied by cyclic voltammetry and controlled potential electrolysis. They undergo one single-electron reversible oxidation, and electrochemical ligand and metal site parameters [which measure the electronic properties of the nitrile ligands and their binding iron(II) centre] were estimated from the measured half-wave oxidation potential values.

The electrochemical behaviour of some vanadium complexes with ester ligands, of the types rV2(p-Cl)2CL(L)4] [L=CH3C02Et (1) or 'MMiiCO^ih (2)], and rV4(u-0)4Clg{CH2(C02Et)2}4] (3), was investigated in aprotic media by cyclic voltammetry (CV) and controlled potential electrolysis (CPE) at platinum electrodes. They exhibit, by CV, ill-defined anodic processes which, by CPE, appear to involve one electron per vanadium atom.

The nitride ligand in the complex rra«s-[Mo(N)Cl(dppe)2] reacts with enantiomeric ICH2CONHCH(CH3)COOCH3 to form imide complexes. Sequential nitrogen-carbon and carbon-carbon bond formation, followed by electrochemical Mo-N bond cleavage, defines a pathway to optical active methyl esters of the glycyl-alanine and alanyl-alanine. The key intermediate metallo-nitrogen ylide trans-[MoCl(NCHCONHCH(CH3)COOCH3)(dppe)2] has incipient carbanion character at the a-carbon and reacts with methyl iodide to form C-C bonds.

Desulforedoxin (Dx) and Desulfoferrodoxin (Dfx) contain both a simple iron-sulfur center with a single iron atom coordinated to four S-cysteinyl residues in a distorted tetrahedral geometry. Different spectroscopic tools (UV-vis, EPR and Mossbauer) were used to characterize the metal center in these two proteins. The use of electrochemical methods to study the pH dependence of the redox potentials of these FeCys4 centers showed that there is a pH dependent process in the alkaline region. The structural origin of this effect will be discussed.

We have previously prepared several camphorimine complexes using metal sites such as PdChr, {FeCl2r, (2) or {CuCl2) « and studied their redox properties by cyclic voltammetry. We now report the electrochemical behaviour of some new platinum complexes lable 1, and compare their electronic properties with those of Pd(II), Fe(III) or Cu(II) derived from camphorimine species of type (A) or (B).

The electrochemical behaviours of the bis(cyanoimide) complex trans-[Mo(NCN)2(dppe)2] (dppe=Ph2PCH2CH2PPh2) and of the derived adducts with some transition metal [Pt(II), Re(I) or V(III)] Lewis acids have been investigated by cyclic voltammetry (CV) and, in some cases, by controlled-potential electrolysis (CPE), in aprotic media, at a Pt electrode. In general, anodic and cathodic processes are detected, by CV, being centred either at the Mo or at the other transition metal site, and the effect of the formation of the adduct on the redox potential is discussed, as well as the dependence of its stability on the electron-transfer process.

The redox properties of some Pt(0> and Pt(ll)-cinnamonitrile cyclophosphazene complexes, as well as of free cinnamonitrile and the related free substituted cyclophosphazene, have been investigated by cyclic voltammetry (CV) and controlled potential electrolysis (CPE) in aprotic media (THF, CH2CI2 or NCMe/0.2 M [NBu4][BF4]), at Pt electrodes. Cathodic processes have been detected only when the unsaturated C=C bond of the cinnamonitrile group is uncoordinated, while irreversible anodic processes are displayed only by the complexes with at least one Pt(0) site, and they are interpreted in terms of the expected redox MOs.

The application of multivariate calibration methods to stripping voltammetry is reported. The ability of Partial Least Square (PLS) regression in the resolution of the overlapped peaks is illustrated in the simultaneous determination of cadmium and indium by differential pulse anodic stripping voltammetry. The results obtained in the analysis of synthetic mixtures were satisfactory.