Portugaliae
Electrochimica Acta

From the polarographic studies of (E-i)t and (i-t)E curves the reduction of chromate ion in the DME has been analysed in order to verify both the irreversibility and the kinetic parameters of the processes. The study has been followed in aqueous and mixed H20-EtOH (15, 30% weight EtOH) media, with 0.1M NaOH at 25°C. A modified version of the OLDHAM-PARRY method is proposed and the possibilities of "sampled current-voltammetry" through instantaneous current, are also emphasized.

Voltammetric studies were carried out using a glassy carbon working electrode to determine the concentration of oxide ion in a CsCI-KCI-NaCI eutectic melt. The mechanism of electrode process and some kinetic parameters were established. The oxidation of O2 is irreversible two-stage process complicated by adsorption. The diffusion coefficient of oxide ions changes from 5.62-106 to 1.99-TO6cm2 s1 at 750-59CPC. The equation for the calculation of oxide ion concentration was derived.

The values of the hydration equilibrium constants of carbonyl and a-dicarbonyl compounds given in the literature are reviewed; the experimental methods used to the calculation of these constants are discussed as well. The values of the correspond mg constants of glyoxal, methylglyoxal and phenylglyoxal are discussed, the last by using a Taft-type correlation.

The redox properties of some tetrakis(pyrazolyl)borate rhenium(V) complexes of the types [ReO{ri3-B(pz)4}L](L= "0^0" type ligand, 1/2L= MeO") and [ReO{r|2-B(pz)4}(OMe)L'] (L'= 0°0" , >To", type ligands) were studied by cyclicvoltammetry. These Re(V) complexes display generally one anodic and one cathodic waves, at values of potential which are discussed in terms of the hapticity of the pyrazolylborate ligand and of the electronic properties of the chelating co-ligands.

The redox properties of complexes of general formulae [{FeCl3L}2], [{CuCl2L}2] and [PdCl2L2] (L=camphor-type ligand) were studied by cyclic voltammetry and controlled potential electrolysis. As a general behaviour, the complexes display at least one cathodic wave which has a marked metal character. The potentials are discussed on the basis of the electronic properties of the camphor-type ligands. Extensive reductive controlled potential electrolysis of one of the diiron complexes allowed the electrosynthesis of a mononuclear product. The electrochemical behaviour of the L ligands [L=A (R=Ph or Pr1) and L=B (R=NH2)] is also reported.

By cyclic voltammetry (CV) in aqueous solutions of KC1, the Amavadine models [NBu4]2[VL2] (L = HIDPA3' - N-hydroxy-a,a'-iminodipropionate - or HIDAV - N-hydroxy-a,a'-iminodiacetate) undergo a single-electron reversible oxidation. The anodically generated Vv species acts as an electron-transfer mediator in the electrocatalytic oxidation of some thiols and evidence is presented for the involvement of an interaction between the substrate (mercaptopropionic acid) and the vanadium complex. The mechanism has been investigated by CV simulation and a preliminary account is now being discussed.

The coordination chemistry of cyanamide (N=CNH-2 ) or derived compounds remains virtually unexplored and, within our interest on the activation of such species by transition metal centres [1], we have prepared some complexes of Pd(II) or Pt(II) of the following types: irara^MC^L^] ( 1, M = Pd or Pt; L = NCNMe2 or NCNEt2 ), trans-[Pt(CF3)L(PPh3)2][BF4] [2, L = NCNMe2, NCNEt2 or NCNC(NH2)2 (cyanoguanidine)], trans- or cw-[PtClL(PPh3)2][BPh4] [3, L = NCNH2, NCNMe2, NCNEt2 or NCNC(NH2)2]-. Although their syntheses and spectroscopic properties are reported separately, we now present their electrochemical behaviour, as well as that of the free cyanamides, as investigated by cyclic voltammetry (CV) and controlled potential electrolysis ( CPE ) in 0.2 M [NBU4KBF4] / NCMe at a Pt-disc or -gauze electrode, respectively.

The electrochemical behaviour, in aprotic medium, of the 18-electron octahedral-type nitrile complexes fr

In the present work we have studied the behaviour of sodium nitroprusside in the presence of Cu(II) using differential pulse voltammetry. For a nitroprusside solution at pH 7.7 three polarographic waves were observed. The cathodic stripping peak observed at - 0.60 V vs. Ag/AgCl for the nitroprusside-copper species, which was formed after accumulation at 0.00 V, can be used for a quantitative determination of copper in the concentration range of 10-7 - 10-6 mol/dm3.

Batch injection analysis in conjunction with preconcentration by anodic stripping voltammetry and adsorptive stripping voltammetry has been investigated and optimised. Mercury films are pre-formed in situ within the cell on glassy carbon substrates by injection of l00 µl electrolyte containing mercury ions. Increased electrolysis efficiency in the stripping experiment is obtained by continuing preconcentration after injection of <100 µl of the metal-ion-containing analyte. Detection limits are in the nanomolar range. This procedure should find widespread application to all types of stripping voltammetry.

The electrochemical reduction of the pentacyanonitrosylferrate(II) ion ([Fe(CN)5NO]2", nitroprusside ion) has been studied in aqueous solutions of low pH by polarography and cyclic voltammetry. There are consistent evidences for the existence of the protonated ion, [Fe(CN)5NOH]2-, electrochemically generated at the potential of the first reduction of the nitroprusside ion. Moreover, a fast regeneration of the reactant(s) of the second reduction process must be assumed, which, at high nitroprusside concentration, can be viewed as a comproportionation reaction between nitroprusside and the product of the second reduction in acidic media. The reduction products adsorb at the mercury electrode.

The influence of anions on the electro-oxidation of D-sorbitol, at a platinum polycrystalline electrode, was studied in HCIO4 and H2SO4 solutions. Results show that Pt has a higher activity in perchloric than in sulfuric medium for the oxidation of D-sorbitol. Based on the rates of oxidation of D-sorbitol, the following sequence has been found for D-sorbitol oxidation in presence of adsorbed anions: Cl04- ~ F- > HS04- (S042-) » CI- » Br-.

Rubredoxin (Rd), desulforedoxin (Dx), rubrerythrin (Rr) and desulfoferrodoxin (Dfx) are bacterial proteins containing the simplest iron-sulfur cluster found in biology: a mononuclear iron center coordinated by four cysteinyl sulfur atoms (FeCys4 or Rd type centers). Although believed to be involved in electron transfer, their function in anaerobic bacteria is still unknown. The reversibility of redox reactions of these metalloproteins systems was studied by cyclic voltammetry, differential pulse voltammetry and square-wave voltammetry. Rd and Dx have been found to exhibit quasireversibility with E°'s in the range -50 to +20 mV versus S.H.E.

Electrochemical studies on Desulfovibrio desulfuricans ATCC 27774 Aldehyde Oxido-Reductase (AOR) were carried out in order to optimize the conditions for the development of an enzymatic sensor for aldehydes. In presence of both aliphatic and aromatic aldehydes, AOR develops a catalytic activity with a characteristic Michaelis-Menten type behavior.

Studies concerning to adsorption and oxidation of amino acids have been the subject of some publication [1, 2]. In the present work, irreversible adsorption of alanine, serine and glycine has been carried out on Pt(lll) electrodes in 0.1M HC104 solution.

It is demonstrated that the electrochemical oxidation of zerovalent palladium complexes involves a two electron process at low scan rates, affording cationic palladium(II) complexes, but at high scan rates, one electron oxidation affords palladium(I) complexes. This may be interpreted by a geometrical rearrangement induced by the solvent.

The present work is a voltammetric study on the behavior followed by gold and platinum electrodes covered by copper or oxide monolayers, in aqueous 0.5 M sulphuric acid solution, and in the presence of organic adsorbates containing a sulphur heteroatom The adsorbates employed were 2-Mercaptopyridine (2-MP) and 4-Mercaptopyridine (4¬MP). All experiments were made with both electrodes.

The electropolymerization of acrylamide in the presence of NaN03 in aqueous solution has been investigated on platinum electrodes. The polymer (polyacrylamide) is obtained in the bosom of the solution. The dependence of yields and molecular weights on parameters such as current density, monomer concentration, electrolyte concentration and temperature was determined.

Conducting polymer composite, polypyrrole/polyvinylsulphonate film prepared on a Pt electrode was degradated by applying anodic potentials at which the irreversible oxidation occurs. The effect of the "overoxidation" potential and polarization time on the electroactivity degradation was determined through the percentage of loss of stored charge. The degradation kinetic promoted by anodic polarization was obtained.

Swelling process related to electrochemical switching of conducting polymers between their reduced and oxidized states has been modelled, by using a thermodynamic treatment based on polymer gels swelling theories. A free energy function is assigned to both reduced and oxidized states, depending upon chemical and physical features of the polymer. The model is able to explain the changes in polymer volume observed when oxidation charge or electrolyte concentration are varied.

A sulphate-reducing bacteria (SRB) that is able to use nitrate as a final electron acceptor (Desulfovibrio desulfuricans ATCC 27774) was grown in lactate-nitrate medium. Weight loss and open circuit potential measurements, cyclic voltammetry and unidirectional polarisation were performed, both in sterile media as well as in the culture, using a mild steel electrode. These results were compared with previous ones obtained with the same bacteria grown in lactate-sulphate medium, thus allowing testing biocorrosion mechanism where either sulphide acumulates and cathodic depolarisation occurs or when only the last mechanism occurs.

The present work is a study on the electrodeposition of Copper on Zamak 5 from alkaline copper solution. The effects of time and current density on thick and cathodic current efficiency have been studied by gravimetric methods.

The corrosion of M2 high speed steel in aqueous media containing chloride, bromide, iodide, nitrate, sulphate and perchlorate salts of sodium and potassium of varying concentration has been studied using corrosion potential measurements, voltammetric and electrochemical impedance techniques. The influence of the cation was found to be small and an order of anion aggressiveness was established as: sulphate > chloride > bromide > perchlorate > iodide > nitrate.

Influence of the composition of molten mixtures Me2R04-Me2R207- U02R04 (Me = Li, Na, K, Cs; R = S, Mo, W) and of the main potentiostatic electrolysis parameters on the chemical, phase composition of cathode deposits and on their current efficiencies have been studied. Experiments were carried out at temperatures of up to 900°C in the air. It has been showed that uranyl-containing oxide salt melts with various cationic and anionic composition can be used for electrochemical obtaining of granular uranium oxides with the 0 / U atomic ratio ranging from 2.014 to 2.667.

Potentiometric measurements have been performed, at 25 °C, in cells of the type: Reference electrode 10mol/dm-3 KCl x g d m albumin (20

The functioning of a DNA-modified glassy carbon electrode was evaluated using the electrochemical system ferricyanide/ferricyanide, after different electrode conditioning in the absence or presence of the free bases adenine and guanine, and at different applied potentials. The results were compared with those obtained at the bare glassy carbon electrode

Square wave voltammetry at microdisk gold electrodes modified with a mercury film was used to quantify nickel levels in kidney samples of mice previously treated with a solution containing those species. Comparison of the results obtained by electrochemical techniques and atomic absorption spectrometry indicates that microelectrodes are a reliable quantitative technique and greatly enhances the working conditions.

Conventional gold electrode, gold, and mercury coated microelectrodes were used to study the electrochemical behaviour of iron species in osteoblast-like cell cultures medium. Gold microelectrodes, with or without surface modification, have shown to be useful tools for in vitro qualitative and quantitative analysis of iron ions released from stainless steel in biological systems.

The influence of the temperature and the electrolyte concentration on an alkaline Fuel Cell has been studied through the variation of the current density and the potential of the cell. As anode has been utilized a palladium membrane and as cathode, a carbon-tetrafluoroethylene-perovskit porous electrode. The electrolyte used is aqueous KOH.

In this work, an UV-vis spectrophotometer technique adapted to "in-situ" absorption-reflection experiments is presented. Visible spectra at different oxidation potentials for Polypyrrole/Polyvinylsulphonate composite controlled in 0.1 M LÍCIO4 water solution were performed. The evolution of the visible-absorption spectra in the oxidized and reduced states as a function of the number of cycles of the coloration/decoloration was followed.

A bilayer structure polypyrrole-flexible and inactive polymer has been constructed to transform molecular strain in macroscopic movement during electrochemical oxidation/reduction process of polypyrrole film. With the aim of optimizing bilayer movement, the rate (rad.s-1) of those electrochemomechanical movements for a same described angle (90°), the consumed electrical charge (mC) and electrical energy (mJ) under different electrical conditions and with different bilayer dimensions has been studied. The obtained results shown an increasing of both movement rate and consumed electrical energy per mass unit at increasing applied current per mass unit (mA.mg-1). Hower the electrical charge consumed remain constant. Also the bilayer dimensions have not influence on movement rate and on consumed electrical charge or energy per mass unit. However, for a constant consumed electrical charge per mass unit the angle traveled remains constant varying bilayer dimensions.

Square wave stripping voltammetry at wall-jet mercury thin film electrodes in continuous flow has been investigated, with particular attention to the anodic stripping voltammetry of zinc, cadmium, lead and copper, and the adsorptive stripping voltammetry of cobalt and nickel via the formation of complexes with nioxime ligand. Advantages of square wave voltammetry with respect to differential pulse voltammetry in the determination step and to batch determination without continuous flow are demonstrated. Nanomolar detection limits are found. Optimisation of experimental procedures is discussed.

The electrochemical reduction of fluoxetine was studied using a hanging mercury drop electrode in different buffer solutions up to pH 13 and with concentrations of fluoxetine varying from 1.0xl0"6M to 5.0x10"5 M_- A very strong adsorption of fluoxetine on the electrode was observed and the shape of the cyclic voltammograms suggests that in these conditions it corresponds to a quasi-reversible system for adsorbed species. The results obtained for the electrochemical quantification of fluoxetine in five pharmacological formulations existing in the Portuguese market were compared.

Thiourea is a promising non-cyanide lixiviant for gold hydrometallurgy. However, its high consumption due to degradation has hindered thiourea from commercial application. Thiourea oxidation is discussed and the role of sodium sulphite as a reductant in controlling the degradation of thiourea is investigated.

The electrochemical reactions of 0.01 M CH3CH2OH and 0.01 M HCOOH on platinum in 0.1 M HC104 were studied by means of Differential Electrochemical Mass Spectrometry (DEMS). Ethanol can be oxidized to C02 and ethanol, whereas formic acid produces only C02. Electroreduction products (ethane and methane) were observed only for ethanol. The competition reactions between ethanol and formic acid were studied when both compounds are present simultaneously in the bulk of the solution (0.01 M CH3CH2OH + 0.01 M HCOOH). Using isotope labelled H13COOH it was possible to distinguish the contribution of ethanol and formic acid: formic acid oxidizes to i3C02 (m/z = 45) whereas ethanol produces 12C02 (m/z = 44). It was observed that the reactivity of ethanol is similar to that found without formic acid in the solution, whereas the production of C02 from formic acid was diminished about 50%.

Electro-oxidation and electroreduction reactions of acetylene adsorbates on gold were studied in 0.1 M HC104. It was found that acetylene forms chemisorbates in a wide potential range. On-line electrochemical mass spectrometry (DEMS) was used for the detection of the reaction products on porous polycrystalline gold electrodes. Only C02 was detected during the oxidation of the adsorbates, whereas acetylene and ethylene were identified as products of adsorbates reduction. The oxidation and reduction of the chemisorbates on single crystal faces display distinctive differences. The oxidation charge (Qox) required to oxidize acetylene adsorbed species completely to C02 depends on the adsorption potential (Uad). The shape of the curve Qox vs Uad is also affected by the structure of the gold surface.

In this work, Ni-P and Ni-P-Teflon electroless thin films were optically characterized by ellipsometry. The measured ellipsometric angles A and ¥ are used to calculate the dielectric constant spectra of these materials. Effective medium theories (Bruggeman and Maxwell-Garnett) have been employed to determine the quantity of Teflon particles in Ni-P-Teflon systems. To complement ellipsometric data, SEM/EDS and SIMS analysis have been used as well as AFM imaging.

A study of the chemistry of chromium in the passive film on Aluminium substrates after ion implantation, electrochemically grown in aqueous solutions of carbonate/bicarbonate with added chloride ions, was done by X-Ray Photoelectron Spectroscopy (XPS). The results confirmed that Cr^+ is responsible for passivation and also that pitting of Al-Cr alloys occurs when the concentration of Cr*+ in the film is reduced due to oxidization to the a more soluble species, Cr^+. Clearly, enrichment of the passive film in the solute element of the alloy is beneficial, but the pitting potential is chromium concentration dependent, exhibiting a maximum. Results will be discussed in terms of the ability of Cr to produce a more effective hydrolysis when compared to Aluminium ions.

It is proposed that the passivation region for Aluminium could be extended, outside the 4 to 9 pH range, by surface alloying with W and Ta, using ion implantation as a non-equilibrium alloying method. Extension into the acid region was achieved by surface alloying with W. This is justified in virtue of the thermodynamic stability of the solute oxide. As Tantalum exhibits passivation all across the pH range, it is reasonable to believe that passivity of Aluminium could be induced or at least greatly improved by alloying with W and Ta [1]. A fluence of 1.3E16 (W +Ta).cm"2 at different ion beam energies, (40 KeV and 150 KeV was used for surface alloying 99.999% purity Aluminium from Good Fellows Metals. The passivation of the produced implanted alloys was studied by voltammetry and impedance spectroscopy. Pre-passivated samples, in acid sulphate solutions were used in neutral 0.1M NaCl solutions. The pitting potential values are more active when polarizing without pre-exposure to acid solutions; this seems to support the idea that some Ta is lost in acid media by dissolution through the film, which is probably a mixture of Ta205 and AI2O3 in proportions that would depend on potential amongst other factors. A comparison with the values obtained using the same experimental conditions, but without pre-passivation of the sample, is made. According to the dc and ac electrochemical data the protective character of the Al implanted alloys is presumably due to the formation of a passive film containing probably a mixed oxide of Ta205 and AI2O3. The passivation model seems to agree with the solute rich interfase model [2-3], which requires that the solute forms a more protective passivating species than the Aluminium under occluded cells conditions.

The effect of perchlorate ions on the electrochemical behaviour of copper in near neutral solutions was studied using cyclic voltammetry. Results showed that perchlorate ions do not interfere with the formation of the duplex oxide film on copper in the potential range before breakdown. In fact, results seem to be in agreement with the passivation model for Cu in mild alkaline solutions. There is a solubilization of the film that gives place to Cu (I ) soluble species, that are collected at -0.875 V (SCE). Solubilization of the film acidifies the interface and precedes breakdown with nucleation and growth of pits. Soluble species of Cu (I) and Cu (n), related to breakdown, were detected in the cathodic cycle.