Portugaliae
Electrochimica Acta

Platinized carbon micro electrodes (~ 10 um diameter), positioned close (~ 5 um) to the cell membrane of a human fibroblast, the ensemble constituting a semi-artificial synapse, are used to monitor events at the cellular level. A few tens of femlomoles of reactive oxygen species produced and emitted by the cell upon mechanical pricking with a glass micropipette (~ 1 um diameter) are released into the liquid film of some hundred femtoliires comprised between the cytoplasmic membrane and the electrode surface, leading to a sudden and drastic rise in their concentrations (in the order of several micromoles). This oxidative stress-type response aims at disarming the aggressor and is thought to be shared by many (if not all) eukaryotic cells. This method allows to detect in real time and quantify the species constituting the oxidative burst cocktail: hydrogen peroxide, H2O2, peroxynitrite, ONO2-, nitrogen monoxide, N0°, and nitrite, NO2-. They are likely to derive ultimately from superoxide anion, O2°-, and nitrogen monoxide, NO*, synthesised by NADPH oxidase and NO synthase enzyme systems, respectively. By placing the microelectrode at different positions about the injured area of the cell membrane, it was concluded that the signals correspond to a spherical diffusion of the emitted electroactive species from a point-source.

There are many examples in the literature, and a few in commerce, of ihe employment of the electrochemistry of redox enzymes for analytical purposes. Though attention to the direct electrochemistry of redox enzymes is growing, so far, there have been no commercial applications of redox enzymes. That might be about to change.

Electrochemistry offers many methods for decreasing the contamination produced by different industrial processes. Among these, Electrochemical Synthesis and Electrochemical Degradation of Organic Compounds in wastewater are two of the more promising ones. There is no doubt that the best way to avoid pollution is to eliminate the process that produces it and in this way very polluting chemical syntheses can be substituted by more cleaner electrochemical processes. As an example of this, the electrochemical synthesis of L-cysteine derivatives from L-cystine will be described. As far as the second method is concerned, the electrochemical degradation of wastewater containing organics appears to be very effective when the wastewater can not be biologically treated and when the oxidation is only carried out until the wastewater can be treated by biological methods. Details of the procedure followed for the development of a pre-industrial process will be shown for the treatment of industrial wastewater containing phenol.

Advances in research and development of materials for eleclrocatalysis are reviewed. After a scrutiny of the factors governing the electrocatalytic activity of materials and an analysis of the emerging trends. advances are illustrated by presenting recent unpublished results from the author's laboratory: H2 evolulion on Ni + RuO2 composites, and on RhOx + RuO2 anodes and cathodes for intermittent electrolysis. O2 evolution on IrO2 + SnO2, O2 reduction on Co spinels prepared from different precursors.

In this research, lhe efficiency of the "ferrite process" for the purification of wastewater heavily contaminated with Cobalt (99.99% in the optimal conditions) was verified, and the three cobalt-bearing ferrites produced using three different Fe2+/Co2+ molar ratios (15/1, 7/1 and 3/1] were characterized by chemical analysis (ICP-AES and potentiometric titration), XRF, XRD and DSC, indicating CoxFeII1-xFeIII2O4, (x = 0.16, 0.35, and 0.65 respectively) as the most probable structure of the solids. Electrochemical analysis of the solid cobalt ferrites was performed using a carbon paste electrode in HClO4 and HCl media. In each case, the first cyclic-voltammogram showed the participation of solid species in the electrochemical transformation process. In second and successive scans, the voitammograms indicated the redox couples Fe3+ads + 1e- <=> Fe2+ads (E = 0.525 V vs AgCl/Ag) and Co2+ + 2e <=> Co(s) (E = -0.230 V) in HClO4, and FeCl2+ads + 1e- <=> FeCl+ads + Cl- ( E = 0.475 V) and CoClx(x-2)- + 2e- <=> Co(s) + x Cl- (E = -0.320 V) in HCl.

The oxidation of benzoic acid has been performed on boron doped diamond electrodes supported on silicon substrate. The voltammetric behaviour has shown that during the oxidation of benzoic acid at 2.7V (RHE), a polymeric film is formed on the electrode surface that is oxidized at higher potential values. Bulk electrolysis of benzoic acid in HClO4, under galvanostatic conditions results in an instantaneous current efficiency (ICE) near 100% at low current densities and high benzoic acid concentrations. Electrolysis at high current densities and low benzoic acid concentrations results in a decrease of ICE due to mass transport limitations.

The development of a FIA procedure with amperometric detection for nimesulide determination is described. The method is based on the oxidation of nimesulide at a glassy carbon electrode and was applied to the quantification of this drug in pharmaceutical preparations. The calibration graph, linear within the range 5.0 x l 0-5 – 3.0x10-4 M was possible to obtain by using a FIA manifold with a leaping detector with two parallel channels: one used for the nimesulide measurement and the other for conditioning the working electrode surface. The determinations were performed in an ethanol/ pH=6.1 Britton-Robinson buffer mixture 30/70 (%, v/v), at a potential of 1.2 V versus an AgCl / Ag (KCl, 3 M) reference electrode. The sampling rate was 60 samples h-1 and the percentage recovery of nimesulide from the tablets was within 99.1 - 102.8%.

The electrodeposition of cobalt from sulphate baths has been studied during its initial stages, due to the recent interest on cobalt and cobalt alloy electrodeposits. Potentiostatic and potentiodynamic electrochemical techniques have been applied using a three-electrode cell and a vitreous carbon electrode. The influence of pH and potential in the voltammetric and chronoamperometric response has been analysed, and nucleation and growth models have been tested under the experimental conditions of the work and in the absence of hydrogen evolution. An instantaneous nucleation under diffusion control occurs at pH=4 while at pH=6.5 progressive nucleation under diffusion control has been obtained.

Due to the recent interest on tin and tin alloy electroplating, the electrodeposition of tin from sulphate-tartrate baths has been studied in its initial stages. Potentiostatic and potentiodynamic electrochemical techniques have been applied using a tree electrode cell and a vitreous carbon electrode The influence of tartrate, pH and potential in the voltammetric and chronoamperometric response has been analysed and compared with other systems, and the model of nucleation and growth has been obtained. In the experimental conditions of the work, an instantaneous nucleation under diffusion control occurs.

Conducting polypyrrole actuators as a triple layer: (polypyrrole/adhesive tape/polypyrrole) can be operated in aqueous media. Is known that the actuator capacity in this device depends on variables that control the oxidation-reduction reaction in the conducting polymer[1] but it has not been studied the influence of the quantity of polymer in the movement (dimensions). This study reports on investigations into responses in the triple layer, with specific reference to the influence of the mass, thickness and area of polypyrrole. Chronopotenciometry is used to study the movement of the triple layer. Parameters as consumed charge, consumed energy and movement rate are used to evaluate the behaviour of the triple layer under different dimensional and electrical conditions. According to the results is demonstrated that the movement of the triple layer can be electrochemically controlled by current flow since the volume change depend on the consumed charge per mg of polypyrrole. The relations between the amount of active polymer and the movement rate, the consumed energy and the consumed charge under galvanostatic conditions have been established.

A transductor is a device that transforms the effect of a physical change like pressure, temperature, etc. in another kind of signal, normally electric. Conducting polymers could be used for the development of ionic transductors. Because they are able to interchange ions with electrolytic solutions during their redox processes to maintain the electroneutrality required in each moment. An electrical signal is then transformed in an ionical signal. In this work a quantitative study, combining Chronoamperometry (CA) and Atomic Absorption Spectroscopy (AAS), of the behavior of PPy/PSS composite films for release cations in a controllable way is done. If the release of ions from conducting polymers could be electrically controllable, potential applications like intelligent interfases for the release of drugs or artificial nerves could be developed.

A system of equations for the calculation of ionic activity coefficients in electrolyte mixtures is reported. The theoretical method used is the one from Robinson and Bates, already published. Our results are a generalisation of their equations. Mixtures with a common ion of two electrolytes with all ions hydrated are involved. A variation of the hydration number with molality is allowed. Equations for the calculation of upper absolute errors in ln(gamma)j are derived.

The reduction of maleic acid at a spherical mercury electrode from solutions containing 1 mol.m-3 acid has been studied at 25°C. The pH of solutions was controlled to 2.2 with Briton-Robinson buffer and ionic strength was adjusted with 500 mol.m-3 NaCl. The electrochemical semi-integral methods have been employed for the data processing. The derivation of i-q-m-t equations was made following the procedure used by Oldham in the case of planar electrodes. The diffusion coefficients of each components of O/R couple have been considered to share a common value D. The mathematical methods used were those of the semicalculus.

The present work describes the development of a new experimental set-up to allow determination of water in solid materials that can not be analysed by the traditional Karl Fischer's coulometric titration, as they are not soluble in Karl Fischer's solution and they only release water at high temperatures. For this purpose, a new tubular oven was designed and tested, where the temperature can be raised up to 1000 °C, which can be coupled to a Karl Fischer Coulometer. Different geological samples were tested, and the results were crossed with other traditionally used methods for this type of determinations.

Metallothioneins are a group of low molecular weight proteins, existing virtually in all living organisms. These proteins are rich in cysteine clusters, which are known to have the ability to complex heavy metals through their thiol groups. Metallothionein production and accumulation is induced by the presence of heavy metals; in the cell, metallothioneins act as protecting molecules by complexing the cations (avoiding them to be free). This (particular) characteristic allows these proteins to be used as biomarkers for heavy metal exposure of the living organisms in their biota. In order to overcome some of the actual limitations of the existing methods for metallothionein determination, a square wave adsorptive stripping voltammetric method is proposed in the present work using a hanging mercury drop electrode. The method development involved optimisation studies of instrumental and analytical conditions, and influence of interferences. The method was applied to samples extracted from fish liver, which was previously submitted to a controlled heavy metal exposure. The method presents a detection limit of 10-9 M of metallothionein and has a good linear range. The results obtained show that the method can be easily applied to metallothionein determination in trout liver samples submitted only to simple separation procedures.

An analytical equation corresponding to a reversible EE mechanism in differential pulse voltammetry is deduced. This technique is more suitable than d.c. voltammetry in order to distinguish an EE process from a single E mechanism when the formal potential values of both steps are close.

Theoretical expressions are presented which correspond to the response of adsorbed molecules which exhibit a reversible or totally irreversible charge transfer in Reciprocal Derivative Chronopotentiometry with constant current (RDC) and Reciprocal Derivative Chronopotentiometry with exponential current (RDCE). In spite of the fact that RDC enjoys an important role in the elucidation of electrode processes, when this technique is applied to adsorbed molecules exhibiting an irreversible charge transfer, peaks are not observed. Under these conditions RDCE turns out to be more suitable than RDC . Furthermore, the use of programmed currents makes the selection of an appropriate range of transition times easier than does the use of constant currents. Equations for the peak currents and peak potentials of the (dt/dE)/E and (de(wt)/dE)/E curves for reversible and totally irreversible process are given in order to show the advantages of the use of the (de(wt)/dE)/E curve when an exponential current time function is used.

The influence of the tetrabutyl ammonium cation (TBuA+) on the redox behaviour of the Cd(Hg)/Cd(II) couple in 1M chloride media was examined using differential pulse and cyclic voltammetry techniques. It was found that at low scan rates (tens to hundreds of mV/s) and with an increase in tetra butylammonium cation concentration, some results from both techniques are consistent with observations made in classic polarographic studies (e.g. the increase in tetrabutylammonium ion concentration in a solution containing the cadmium(II) ion, in chloride media, promotes a similar decrease in the cathodic current intensity of the metallic cation). At higher scan rates, the results of cyclic voltammetry experiments are suprisingly different. Among various effects observed, the presence of the tetrabutylammonium cation, below a certain concentration, seems to have no effect on the electroreduction of cadmium(ll) ions.

The cyclic voltametric behaviour of the biscyanamide-dihydride Mo complex [MoH2(NCNH2)2(dppe)2] [BF4]2 (dppe = Ph2PCH2CH2PPh2) is reported and interpreted by considering the occurrence of a cathodically-induced monodehydrogenation process of the cyanamide ligand.

The cyclic voltammetric behaviour of the allenylidene complexes trans- [FeBr(L)(depe)2][BF4] (L = CCC(Me)Ph 1a, CCCPh2 1b, CCCEt2 lc) is reported and the electrochemical PL and EL ligand parameters for the allenylidene ligands are estimated showing that they behave as slightly weaker net electron-donors than organonitriles.

By cyclic voltammetry in a methanolic solution, the amavadin model [Bu4N]2[V(HIDA)2] (HIDA= tribasic form of the N-hydroxy-,’ iminodiacetic acid) acts as an electron-transfer mediator in the electrocatalytic oxidation of pyrogallol (1,2,3-trihydroxybenzene). The mechanism of this anodic process is being investigated by digital simulation of cyclic voltammograms and a preliminary result is presented.

The cyclic voltarnmetric study of the following isocyanide complexes derived from trans-[FeH(CN)(dppe)2] 1 (dppe = Ph2PCH2CH2PPh2) enabled the estimate of the values of the electrochemical Pickett PL and Lever EL ligand parameters for the corresponding cyano-derived ligands: trans-[FeH(CNR)(dppe)2]A 2 [R = H(2a) or Et(2c), A- = BF4-; R = Me(2b), A- = I-; R = C(O)Ph(2d), A- = Cl-] or trans[FeH(CNX)(dppe)2] 3 [X = BPh3 (3a), VCl3(thf)2 (3b) or BF3 (3c)].

The reactions of 5 mM benzaldehyde at platinum in 0.1 M HClO4 were studied by means of differential electrochemical mass spectrometry (DEMS) combined with cyclic voltammetry. It was observed that this compound oxidises completely to CO2 at E > 0.60 V vs. RHE (reference hydrogen electrode), but also produces toluene, benzene and cyclohexane in the potential region for hydrogen adsorption and H2 evolution. No partial or total hydrogenation of toluene was observed. These results are compared and discussed with those previously obtained for benzyl alcohol and benzoic acid.

Annual Index