Portugaliae
Electrochimica Acta

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.

Preliminary results of a study of the effect of the temperature on the response of all-solid-state ammonium ISE's prepared by casting PVC membranes with nonactin on supports of graphite/epoxy composites with up to 60% of graphite are presented. The study showed that: (i) the conductive support must contain at least 40% of graphite to produce ISE's with adequate response characteristics; and (ii) the variation of the percentage of graphite between 40 and 60% has no influence on the slope and potential temperature coefficients and on the hysteresis curves of the electrodes.

The square wave anodic stripping voltammetry of copper and zinc mixtures at wall-jet mercury thin film electrodes in continuous flow was investigated with the aim of minimizing interferences which appear in solutions containing the two cations due to Cu-Zn intermetallic compound formation in the mercury phase. The addition of gallium ions led to the preferential formation of Ga-Cu intermetallic making the determination of zinc possible. Best results were obtained in acetate buffer or acidified sodium perchlorate electrolyte at pH~3. It was shown that the method is applicable to the analysis of waters in flowing solution.

An electroanalytical procedure for total iron determination in waste water samples based on adsorptive cathodic stripping voltammetry using mercury film microelectrodes is described. Iron was determined using catechol, as the metal ligand agent, being the chelator adsorbed at the mercury film surface by imposition of a potential of -1.8 V and the reduction current measured. The results obtained by this procedure were compared to those obtained by atomic absorption spectrometry to evaluate the quality of the electrochemical method.

Potentiometric measurements in protein containing solutions are affected by the presence of protein. Overall effects on the ISE, on the reference electrode and on the electrolyte solution have been reported for BSA (Bovine Serum Albumin) containing solutions using K+ as a test cation (1-3). In this work we present studies with Human Serum Albumin (HSA) as a further step to approaching physiological conditions. Common features were found for both albumins, which are explained in terms of solution and interfacial chemistry. Measurements in HSA of particularly high concentration (100 g dm"3) show evolution in time that has not been accounted for in previous studies and do not find parallel at lower concentrations. Explanation for these latest observations may be partly associated with the formation of HSA dimers.

The effect of electron-donating substituents on the oxidation of phenol molecules was studied by cyclic voltammetry. For the considered p-substituted phenol derivatives the reactivity order observed on polyciystalline platinum electrodes was: phenol < p-cresol < hydroquinone < p-metoxyphenol. For an organic concentration of 0.1M, a linear relationship was found between the current density (log i) and the substituent constant (a). Literature values for phenol ionization were used.

Cyclic voltammetry (CV) and others voltammetric techniques, with a glassy carbon electrode, have been used to study the interaction of purines with copper in sulphuric medium and others. If the copper (II) concentration is about 10"3 mol dm"3 and the purine concentration is equal or higher, is only possible to observe one stage of reduction, but two stages of oxidation are possible. Under these conditions and pH < 2 the copper (I) is stabilized by purine. The potential variation of each peak of oxidation with increasing purine concentration show processes of electrode of one electron for the observed anodic peaks. From the shift of the anodic peak potential corresponding to Cu(0) / Cu(I) with increasing ligand concentration the stoichio-metry of the complexes have been determined. Also, the shifts of the anodic peaks potential with increasing pH show processes of one proton. The aim of this work is to describe the equilibria between purines and copper ions and to propose reactions mechanism on the complexes formation in our medium conditions.

The catalytic activity of a given substrate for the electroless metal deposition (EMD) is determined by its ability to promote the adsorption of the reductant species and to originate an anionic radical which will be involved in the metal ion reduction; it can be evaluated by measuring the open circuit potential. It is well known that the involved mechanism is highly dependent on the substrate nature, solution pH and composition1-3. Sodium borohydride is a reducing agent widely used in industrial electroless plating. It is highly unstable in acid or neutral medium, undergoing fast hydrolysis4 and if nickel ions are present, nickel boride will be precipitated with consequently bath decomposition, when the pH is maintained above 12, the hydrolysis of borohydride ions can be avoided and good quality Ni-B deposits are produced5. However, the mechanism of EMD employing this reductant is not established. In this work the electro oxidation of borohydride ion (BRf) on Ni and Cu electrodes, in the absence of traditional complexing agents and in metal ions free media, has been studied by cyclic voltammetry and potential time measurements using stationary and rotating electrodes. The results are analyzed using the "Intermediate Radical Mechanism"6 approach.

The corrosion behavior of the areas affected by welding of a steel has been performed in aqueous solution, for a synthetic sea water, by the linear polarization resistance method. Using a normalizing heat treatment at 920°C for a duration of 15 min and then air cooling, it shown to be enough to eliminate the structural heterogeneity responsible by the anodic behavior of stress material.

It is reported, for the first time in Portugal, the introduction of the electrochemical noise technique applied to the corrosion monitoring of the cooling circuit of a stator in a thermoelectric power station, allowing in real time, the detection and identification of a localized corrosion process. The circuit uses electrolytic copper in contact with low conductivity water, less than 1 uScm"1. A specially designed probe to be installed in a by-pass fashion to the cooling circuit, was developed by CML, U.K. Results obtained during a monitoring period of 3 months are discussed.

Enquadrada num trabalho mais amplo sobre a redução electroquímica indirecta de corantes, a presente comunicação teve, como objectivo, a selecção de um material de eléctrodo adequado ao complexo de ferro e trietanolamina (Fem/TEA), em meio básico, usado como mediador redox. A análise dos resultados experimentais e dos voltamogramas obtidos permitiu concluir que, dos diferentes materiais testados (aço, ferro e cobre), o cobre parece ser o que permite uma transferência electrónica de maior reversibilidade.

A procedure for quantification of nickel in cell culture medium using a mercury film microelectrode and dimethylglioxime (DMG) as the complexing ligand was developed. The results obtained by adsorptive cathodic stripping voltammetry (AdsCSV) were compared to those attained by atomic absorption spectrometry and good agreement was found.

The analytical conditions for the determination of the total iron in mice organ samples have been optimized and the suitable experimental parameters were found to be a catechol concentration of 3.0x10"4 mol/L, pH ca. 7.2 provided by PIPES buffer concentration of 8.0x103 mol/L, a deposition potential of -1.80 V, deposition time 10-30 s, frequency 50 Hz, step 4 mV and an amplitude of 20 mV. The detection limit was 13.7x10 9 mol/L after a deposition potential of 25 s. The relative standard deviation of fifteen repeated measurements of the same solution was 1.13%, indicating that the peak iron response was very reproducible.