Portugaliae
Electrochimica Acta

The acidic dissociation constants of the protonated form of 2-amino-(2-hydroxymethyl)-propane-1,3-diol (tris) and 2-amino-2-methylpropane-1,3-diol (bis) in a series of water/1,2 dimethoxy ethane mixtures at 5 ºC temperature intervals from 25 to 45 ºC are determined using a cell without liquid junction potential. The corresponding thermodynamic parameters (ΔGº, ΔHº, and ΔSº) of the protonated form of these buffer indicators in the various solvent mixtures are evaluated. The variation of the standard free energy change of transfer with solvent compositions for both indicators indicates a characteristic minimum which is attributed to selective solvation of the individual species participating in the equilibrium process. Solubility measurements of these indicators at different temperatures are determined in order to evaluate the thermodynamic functions of the transfer of the free bases which may provide a better insight on the nature of the ion-solvent interactions and the relative basicity of the solvent.

This investigation deals with the stress corrosion cracking of AISI 304 stainless steel in concentrated boiling H3BO3 solution with NaCl additions. The characterization of the passive film was carried out using samples submitted to the corrosive environment, in both the annealed and cold worked states. The study shows that plastic deformation of the metallic material reduces polarization resistance and film stability (in terms of cathodic reduction). Furthermore, from the capacitance measurements, using the Mott-Schottky approach, it can be concluded that the influence of plastic deformation is higher when the passive film reveals p-type conductivity. Based on the electronic band structure of n-type and p-type semi-conductors, a representation for a possible correlation of charges of opposite sign between space charges and near surface dislocation is given.

Extensive quantities, like electrode reaction rates and double layer parameters, have to be referred to the unit real surface of the electrode, in order to compare the activity of different electrode materials and / or different preparation methods for the same material. The purpose of this paper is to review three methods usually used for the estimation of the real surface of CO3O4 and NiCO2O4 thin film electrodes. Two of them, capacitance ratio and voltammetric charge, are based on the voltammetric technique whereas the third one, Zn2+ ion adsorption (ZIA), is based on the ionic interaction at the solid-liquid interface. The applicability, advantages and limitations of the three methods are compared.

The influence of some chalcones on the corrosion of nickel in 1 N nitric acid solution has been studied using weight loss and galvanostatic polarization techniques. In general, at constant acid concentration, inhibitor efficiency increases with increase of concentration of inhibitor and decreases with rise in temperature. Polarization studies reveal that the compounds behave as cathodic inhibitors. The effect of temperature on corrosion inhibition has been studied and activation energy has been evaluated. The adsorption of the inhibitor on the nickel surface is found to obey the Frumkin adsorption isotherm. Some thermodynamic parameters are calculated and discussed.

Propylene carbonate (PC) and -butyrolactone (GBL) are important dipolar aprotic solvents good to prepare electrolyte solutions of lithium perchlorate applicable to rechargeable high energy batteries, as we have shown before [1-5]. In this work solution conductances of LiClO4 and (C2H5)4NBr in those solvents were measured at temperatures ranging from -30 ºC to +25 ºC. Limiting molar conductivities and association constants were evaluated through Viana and Calado [6] and Fuoss and Hsia’s equations [7]. Some thermodynamic functions resulting from (KA, T) variations are obtained and the results are discussed on this basis. The accuracy and precision of the results is high and they are where possible comparable to others previously published [8, 9].

Octadecanethiol self-assembled monolayer (OCSAM), which was first employed as one sensitizer for TiO2 nanoporous film electrode in the oxidation of methanol, was firstly investigated using electrochemical and photoelectrochemical methods. The photocurrent generated by using TiO2 nanoporous film electrode modified by OCSAM (18SH/ TiO2) is about 1.95 times larger than that without modification. In addition, the maximum absorption peak has shifted towards the infrared region for about 30nm when the TiO2 nanoporous film electrode modified with OCSAM. This paper has testified that the increased photocurrent could not be attributed to the photooxidation of octadecanethiol or the ethanol solvent simply. It was proposed that the change of surface structure of TiO2 nanoporous film electrode should be responsible for the phenomenon in some degree.

The surface analysis using interferometry studies clearly indicates that the oxidation cycling at high frequency at aluminium electrode in the 0.1M NaClO4 containing 10 mM sodium tetraphenylborate solution yields selective dissolution of faces other than (111) by forming pyramidal shape voids that is not due to the simultaneous dissolution and redeposition as in the case of Pt. It is also confirmed that surface smoothing and flattening on the aluminium surface is due to flattening process and the formation of voids are due to electrochemical dissolution.

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.