Portugaliae
Electrochimica Acta

The electroreduction behaviour of cysteine was investigated using cyclic, square wave and differencial pulse voltammetric techniques at a platinum working electrode. The reduction of cysteine occurs at a potential of -0.36 V independent of pH. It is a reversible process, controlled mainly by diffusion and in the mechanism of reduction 1 electron per molecule is involved. Using the voltammetric techniques: Cyclic Voltammetry, Square Wave Voltammetry and Differencial Pulse Voltammetry, different parameters (pH, frequency, step potential, pulse amplitude, scan rate) were optimized in order to develop an electrochemical procedure for determination of cysteine in pharmaceutical products. The repeatability, reproducibility, precision and accuracy of the methods were studied. No electroactive interferences from the excipient were found in the pharmaceutical compounds.

The objective of this investigation has been the study of corrosion resistance of Fe2.5Co64.5Cr3Si15B15, Fe3Co67Cr3Si15B12 and Fe5Co70Si15B10 amorphous metallic alloys obtained by the melt spinning technique, used as based materials to create a new type of giant magnetoimpedance (GMI) biosensor. The corrosion behaviour has been studied in phosphate buffered saline (PBS) solutions at pH 7.3 and 37.5 ºC. The electrochemical characterization of alloys has been made by means of DC techniques, obtaining the corrosion potential, pitting and protection potentials, as well as the perfect and imperfect passive regions of alloys. In this work, the experimental results obtained are discussed in order to study their corrosion behaviour in artificial biological solutions and thus determine their possible use as GMI-biosensor prototype materials.

A Langmuir-Schaefer (LS) film of a thiomacrocyclic (ThM) compound was deposited on the surface of a glassy carbon electrode (GCE) rod, from a subphase containing Cu(II) ions. The voltammetric response of this modified GCE when the ThM was bonded to Cu2+, showed that the LS film moved the oxidation peaks of copper to more positive values. On the other hand, a LS film of the ThM compound was deposited on the surface of a GCE rod from a subphase of pure water. When the voltammetric response of the GCE-ThM electrode was studied in a Cu2+-SO42- solution, it was found that the modified electrode increases its sensitivity respect to Cu2+ at low bulk Cu2+ concentrations in solution, and a surface-complexation reaction is proposed to explain the effect of the LS film on the GCE surface.

A nano-porous anodized aluminium oxide layer was synthesized using two step anodization process, in which the dents of aluminium formed in the first anodization step (one day) worked as the initial sites of the pore growth in the second anodization step (one hour). The influence of the voltage on the structure of aluminium oxide has been studied. For different anodization temperatures, it has been found that higher temperatures decrease the pore size, although lower temperatures yielded higher symmetry. The electrolyte used has high influence on the ordered pore domain size. Sulfuric acid generated very small pore sizes, phosphoric acid produced a very big pore size and the largest domain size was observed for samples anodized in oxalic acid.

In this paper we present experimental measurements of the temperature dependence of the electrical conductivity, s, in four ionic liquid compounds (ILs) in both the liquid and solid states and at atmospheric pressure. The chemicals measured are composed by the 1-ethyl-3-methyl-imidazolium (EMIM+) cation, which has been combined with four different anions: Cl-, Br-, BF4- and ethyl sulfate (ES-). In the liquid state, the temperature dependence of s, for the four ILs follows the Vogel-Tamman-Fulcher (VTF) equation with high precision. Around the transition between the solid and liquid states, the electrical conductivity of the three ILs with lighter anions presents an hysteresis loop, that can be explained as the apparition of supercooled liquid. In contrast, the EMIM-ES presents a smooth transition, without any jump in the s, value or any hysteresis loop (probably because its melting point is reported to be below the minimum temperature measured by us). Finally, the jump in s, is not related with the glass transition because its temperature value is well below the minimum temperature measured, and at that glass transition temperature the s, value is below the resolution of our conductivity meter (2 nS/cm).

The voltammetric behaviour of glutamate at a gold electrode was studied by means of cyclic voltammetry. From 2.6 to 11.5 pH values, glutamate originated a single cathodic peak. Potential of the peak, Ep, was -0.8 V, and it was independent from pH. The reduction of glutamate was found irreversible and mainly controlled by diffusion. An analytical approach for analysis of glutamate by means of square wave voltammetry was developed. Plots of current versus concentration presented a linear behaviour from 2.5×10-4 to 2.7×10-3 M. The detection limit was 6.3×10-5 M. Interference from compounds co-existing with glutamate in food was negligible, and in favour of an application of the proposed method to the analysis of real samples. Determination of glutamate in pure solutions resulted in acceptable deviation from the stated concentration. Relative errors ranged -2.5 to +1.6 %.

A new alkaline electrolytic solution based on ammonia for copper electroplating has been investigated. Upon addition of alkaline metal hydroxide solution and ammonia followed by the immediate addition of a strong inorganic acid, a highly exothermic reaction takes place. At optimum solution condition ammonia remains in solution as NH2 and forms a complexing agent.

The corrosion behaviour of 1018, 410 and 800 steels exposed to synthetic wastewater has been studied using linear polarization resistance (LPR), cyclic potentiodynamic curves (CPC), electrochemical noise (EN), and electrochemical impedance spectroscopy (EIS) tests. The conditions were: biochemical oxygen demand (BOD) of 776 ppm, a chemical oxygen demand (COD) of 1293 ppm, pH = 8 and the cell temperature was 24 °C. From the CPC and EN results, no localized corrosion was found for the stainless steels. However, the reverse was true for the 1018 steel. The EIS results showed that different corrosion mechanism occurred for the carbon steel compared with the stainless steels. This shows that the corrosion mechanism strongly depends on the type of steel. Overall, the 1018 steel exhibited the highest corrosion rate, followed by the 410 alloy. The highest corrosion resistance was achieved by the 800 alloy. In addition, SEM analyses were carried out to explain the experimental findings.

Electroless nickel (EN) process is the controlled auto catalytic reduction of nickel ions using suitable reducing agent such as sodium hypophosphite on certain catalytic surfaces, which results in the production of sound, coherent coatings for a number of applications. Frequently the rate of deposition of EN with hypophosphite as reducing agent is below 20 microns per hour. Hence the deposition of the electroless nickel with hypophosphite solution in the presence of small concentration of thiourea and its derivatives as accelerators is investigated. The mechanism of accelerated deposition is studied by impedance measurements. The performance of various additives during the plating process is being evaluated by increased Constant Phase Element (CPE) and decreased charge transfer resistance values. A model is proposed for the adsorptive behavior of additives.

The effect of different concentrations of 1,3-dihydroxypropane (DHP) on the electrodeposition of copper powder from acidified copper sulphate solution has been studied at different temperatures and different speeds of rotation. Copper powder was electrodeposited onto rotating cylinder electrode (RCE) that made of pure copper. The inhibition percentage, P, in the electrodeposited copper powder was 0.00 – 92.91%, depending on the experimental variables. P was affected by temperature and mole fraction of DHP, while rotation did not show any influence whatsoever. Values of the activation energy of electrodeposition process, Ea, were found to be less than 28 k J mol-1 indicating diffusion controlled process. The overall mass transfer correlations under the present conditions have been obtained using the dimensional analysis method. The data were valid for 80 < Sh (Sh = Sherwood number) < 3970, 290 < Sc (Sc = Schmidt number) < 59284 and 271 < Re (Re = Reynolds number) < 52705 and the results agreed with the previous studies of mass transfer to rotating cylinders in turbulent flow regimes. Experimental determination of the solution critical velocity was obtained for blank and 20% (v/v) DHP solutions at 298 K. The effect of time, DHP content, temperature and the speed of rotation on the morphological changes of the electrodeposited copper powder as well as deposits composition and particle size have been studied. Various particle sizes ranged 60.5 – 203.4 nm were obtained, characterized by EDS and XRD and found to be pure copper with small amount of oxygen. Different topographs proved that the rate of copper electrodeposition increased by increasing time, temperature and the speed of rotation. In addition, they proved that the deposition rate decreased by adding DHP to the solution. Therefore, the results obtained by SEM supported those results obtained by electrochemical measurements. The morphological structure of deposited copper powder from 20% (v/v) DHP at 1000 rpm and 298 K was unique, rounded-crystalline aggregates with voids.

This work presents the results of corrosion inhibition of muntz alloy (63% Cu,  37% Zn) in 1.0 M HCl by water extracts of some naturally occurring plants. These are: outer brown skin of onion (A), onion bulb (B), the cloves of garlic bulb (C), orange peels (D), and henna leaves (E). The techniques of measurements for the determination of the amount of each zinc and copper dissolved from the alloy in the aggressive solution were: weight-loss, galvanostatic polarization, linear polarization and atomic absorption spectroscopy. From these measurements the values of surface coverage, , and inhibition efficiency were calculated. It was found that the investigated extracts have high inhibition efficiency on the corrosion of muntz alloy in 1.0 M HCl. Their inhibition efficiency decreases according to the order: C > D > E > B > A. These extracts behave as mixed inhibitors, i.e., they affect both the cathodic and anodic processes. The activation energy of corrosion was calculated in absence and in presence of extracts. It was found that the presence of extracts in 1.0 M HCl solutions increases the values of activation energy of corrosion in that order of their inhibition efficiency. The inhibiting effect of these extracts results from their adsorption on the electrode surface via the adsorption centers of the compounds present in the extracts. The adsorption of these extracts onto the surface of muntz follows Frumkin,s isotherm. The atomic absorption spectroscopic measurements showed that the presence of these extracts greatly inhibits the preferential dissolution of zinc from the alloy and the occurrence of simultaneous dissolution of both zinc and copper.

The effect of natural occurring extract of artemisia on the corrosion of steel in 0.5 M H2SO4 in the temperature range 298 – 353 K is studied by weight loss method, electrochemical polarisation and linear polarisation Rp measurements. Results obtained reveal that extract reduces the corrosion rate. The inhibition efficiency increases with the increase of artemisia content at 10 g/L to reach 95% and 99% at 298 and 353 K, respectively. Results obtained by gravimetric and electrochemical polarisation are in good agreement. Polarisation studies clearly reveal that the presence of the natural artemisia does not change the mechanism of the hydrogen evolution reaction and acts as a mixed type inhibitor. The inhibition efficiency increases with temperature. The adsorption of artemisia on the steel follows Langmuir adsorption isotherm.

In this work the characteristics of corrosion and inhibition of 1008 stainless steel in 0.5 mol.L-1 H2SO4 were investigated by physical and electrochemical methods. Organic compounds containing the heteroatom of N as benzotriazole (BTAH), tolytriazole (TTAH) and mixtures of BTAH + TTAH were used as corrosion inhibitors. The techniques used were: open circuit potential, anodic potentiodynamic polarization measurements, chronoamperometry, electrochemical impedance spectroscopy, weight loss and optical microscopy. The anodic polarization showed that the increase of both concentrations BTAH or TTAH diminishes the current density in all the potential anodic range studied; these results together with the weight loss measurements suggest that the inhibitors act by blocking the surface and that the interaction among them is only of destructive nature when inhibiting efficiency is approximated 100%, revealing that at low concentration both inhibitors act without affecting the mechanisms of the cathodic processes, but the anodic polarization curves in presence of the TTAH showed a strong inhibitive effect in all the potential range studied. However, both gravimetric and electrochemical results suggest a synergic effect for the inhibitive efficiency of the mixture 1x10-3 mol.L-1 BTAH + 1x10-3 mol.L-1 TTAH.

This work is centered on the study of the corrosion inhibition of copper in 1 M HNO3 by N-phenyl oxalic dihydrazide (PODH) and oxalic N-phenylhydrazide N’-phenylthiosemicarbazide (OPHPT) synthesized in our laboratory. The inhibition efficiency obtained by chemical (weight loss) and electrochemical (potentiodynamic polarization) techniques shows that these compounds are very good inhibitors. Polarization curves indicate that OPHPT acts as a mixed type inhibitor while PODH acts on the cathodic reaction. The adsorption of these inhibitors is found to obey Langmuir adsorption isotherm. The thermodynamic functions of adsorption and dissolution processes were calculated.

The inhibitive action of the extract of Ficus nitida leaves toward general and pitting corrosion of C-steel, nickel and zinc in different aqueous media was investigated. Weight loss measurements, potentiostatic and potentiodynamic polarization techniques were used in this study. It was found that the presence of ficus extract in the corrosive media (acidic, neutral or alkaline) decreases the corrosion rates of the three tested metals. The inhibition efficiency increases as the extract concentration is increased. The inhibition efficiency depends on the type of corroded metal and on the corrosive solution. It was also found that the presence of ficus extract in the chloride containing solution shifts the pitting potentials of the tested metals toward the noble direction. The inhibitive action of ficus extract is discussed in view of adsorption of its components, the poly aromatic compounds, friedelin, epifriedelanol and nitidol, on the metal surface. It was found that such adsorption follows Langmuir adsorption isotherm. The calculated values of the free energy of adsorption indicated that the adsorption process is spontaneous.

Theoretical analyses show that in a sampled dc polarography on the static mercury drop electrode the response may consist of a pre-wave and a main wave that are separated by a minimum. This is a general characteristic of electrode reactions complicated by strong adsorption of the product. A steep pre-wave and the minimum are the indications of Frumkin isotherm. A flat plateau of the pre-wave is a special case of the steep plateau which appears at lower reactant concentrations. If the adsorption is weak, the pre-wave and the main wave are usually merged into a single wave.

A modified electrodialytic cell integrates electrodialysis and reduction of metal ions. The cell is able to recover metallic copper from wastewater containing 1000 ppm cupric ions and bring the concentration down to about 1 ppm. The kinetic data of decreasing copper ion concentration fit well in first order kinetics and allow calculation of the over all reaction rate constant. Effect of several parameters, namely, width of wastewater compartment, applied potential and concentration of anolyte and catholyte solution; on the over all reaction rate constant and specific energy consumption were studied. The best combination of parameters results in an overall rate constant of 7.84 × 10-4 sec-1 and specific energy consumption of 48.18 kW-h/kg copper.

By cyclic voltammetry the active/passive transition of Nb electrode has been investigated in concentration range from 0.1 M to 10 M aqueous solutions of H2SO4 and KOH. Results indicate the strong influence of the concentration and electrolyte nature to the active/passive transitions and stability of passive films. Depending on electrolyte concentrations, at potential of 1 V the calculated thickness of passive films varied from 2.2 nm to 3.2 nm. For the same concentrations of H2SO4 and KOH the formed passive films in KOH are thicker than in H2SO4. By multiple cycle sequences in which the final anodic potential is gradually enlarged, the barrier properties of passive films on Nb electrode were confirmed. In the first positive scan after the active/passive transition, no cathodic or reactivation peaks for both 1 M H2SO4 and KOH were observed. Only for higher concentrations of KOH (> 2 M) the small reactivation process was recorded. In concentration of 5 M and 10 M KOH, if after the passive film formation the Nb electrodes were maintained for 15 min at cathodic potential of -1.55 (SHE), the cyclic voltammograms indicated the complete dissolution of passive films. The voltammetric data have confirmed that the Nb electrode is more resistant in concentrated H2SO4 than in concentrated KOH solutions.