Portugaliae
Electrochimica Acta

Spontaneous deposition of tin (Sn) in polycrystalline platinum (Pt pc) from a sulfuric acid solution of tin salts, was evaluated by conventional electrochemical techniques, to determine its characteristics. Tin was deposited from a solution of SnCl2 or SnSO4 in sulfuric acid 1 M. The degree of surface coating by Sn was calculated by evaluating the region of hydrogen electrosorption through voltammograms performed using solutions containing only the supporting electrolyte, and the supporting electrolyte containing the Sn salt, respectively. The adsorption behavior of tin and oxidation have been studied by the results obtained with platinum electrodes and electrodes of platinum/tin, considering as comparative parameter the adsorption efficiency with respect to the area free of platinum.

The degradation of crystal violet (CV) and malachite green (MG) by potassium persulfate, was investigated by spectrophotometric methods. The behavior of degradation of crystal violet by persulfate was found to be similar to that of malachite green with only one important difference concerning the order with respect to the dye: the degradation is pseudo second order with respect to CV, but first order with respect to MG. The order with respect to persulfate is one in both cases. Degradation of CV by persulfate was effective at pH range of 2-8 and was found to increase with an increase in the initial concentrations of persulfate, temperature, and the presence of Ag+. The factors that were found to decrease the degradation rate of CV include: the initial concentration of CV, and the presence of halide salts and of the surfactant SDS. The rate of degradation remained the same after addition of Co(II), Ni(II) and Cu(II) salts. The activation parameters of the degradation reaction (Ea, ΔG#, ΔH# and ΔS#) were calculated. Finally, cytotoxic study revealed a decrease in the toxicity of the degradation products.

Silver nanoparticles (AgNP) were synthesized by electrochemical method using high purity metallic silver in the presence of the surfactant Pluronic® f127. The synthesized nanoparticles were characterized using UV-Vis spectroscopy, Dynamic Light Scattering (DLS), X-Ray Dispersion Spectroscopy (XDS) and Scanning Electron Microscopy (SEM). The antimicrobial activity of AgNP was evaluated against Escherichia Coli. In presence of surfactant, the prepared nanoparticles led to a monomodal distribution with an average size less than 100 nm. The minimal inhibitory concentration (MIC) of the resulting suspension was evaluated using micro dilution method and the MIC results are satisfactory.

The effect of various surfactants on the volume% codeposition of SiC in a nickel matrix was evaluated. Of the various surfactants tried, tetra methyl ammonium hydroxide (TMAH) was found to be the best for improving the quality of the deposits as well as the homogenous distribution of particles and with reasonable volume% of silicon carbide incorporation in the matrix. Composites were produced using 1 μm and 50 nanometer size powders. The effect of silicon carbide concentration and bath operating variables on the volume% of SiC incorporation in the deposit and the deposition rates were estimated. Substantial improvement in mechanical properties such as hardness and wear resistance was obtained with the nano SiC composite compared to the micro SiC composite.

Two poly(vinyl chloride) matrix membrane electrodes responsive to some drugs affecting the cardiovascular system, Amlodipine besilate (AM) and Valsartan (VL), were developed, described and characterized. A microsized graphite selective sensor was investigated with dibutylsebacate (DBS) as a plasticizer in a polymeric matrix of carboxylated polyvinyl chloride (PVC-COOH) in case of (sensor 1). This sensor 1 was prepared using 2- hydroxy propyl β-cyclodextrin (2HP β-CD) as an ionophore. While sensor 2 preparation was based on precipitation technique with bathophenanthroline iron (II) as electroactive materials in poly(vinyl chloride) (PVC) matrix. Fast and stable Nernstian responses near 1xE-5-1xE-3 M for the two drugs over the pH range 3-6 and 7-9 for the two sensors, respectively, were obtained. The method was successively applied for the determination of AM and VL in presence of each others, in their pharmaceutical formulations and in human plasma samples. The percentages recoveries for the determination of the drugs by the proposed selective electrodes were 99.78 ±0.382 %, 100.23 ± 0.440 %, for sensors 1 and 2, respectively. Statistical comparison between the results obtained by this method and those obtained by the official method and the reported method of the drugs was done and no significant difference was found.

Five solid membrane sensors responsive to memantine hydrochloride (MEM) and pramipexole dihydrochloride monohydrate (PXL) are described for simple and fast determination of these drugs in pharmaceutical preparation and human plasma. The first and the second sensors are based on the formation of an ion association complex between MEM as a cationic drug with Na tetra phenyl borate and ammonium reineckate (as anionic exchanger), respectively. The third sensor is based on the formation of an ion association complex between PXL with ammonium reineckate. The produced electroactive material is dispersed in PVC matrix. While the other fourth and fifth sensors are based on using functionalized lipophilic cyclodextrin derivative (2-hydroxypropyl-β-cyclodextrin) as sensor ionophore for the determination of MEM and PXL. The performance characteristics of these sensors-evaluated according to IUPAC recommendations-reveal fast, stable and near Nernstian response for 1x10-4-1x10-1 M and 1x10-6-1x10-2 M for (MEM) and (PXL), respectively. Many inorganic and organic substances such as drug excipients and diluents normally used in drug formulations do not interfere with drugs response. Statistical comparison between the results obtained by applying the proposed potentiometric method for the determination of the (MEM) and (PXL) in their pure powder forms and those obtained by applying the reported methods was done and no significant difference was found at p=0.05. Validation of the method according to ICH guidelines shows the suitability of the sensors for quality control analysis of the cited drugs in pharmaceutical formulations and human plasma. The proposed sensors can also be used as a detector for HPLC.

Conductivity measurements of solutions of terbium soaps in benzene-methanol show that these soaps behave as weak electrolytes in dilute solutions and Debye-Hückel-Onsager’s equation is not applicable to these soap solutions. The thermodynamic results indicate that the micellization process is favoured over the dissociation process.

Phenyltetrazole substituted compounds, namely 5-phenyl-1H-tetrazole (PT), 5-(4-chlorophenyl)-1H-tetrazole (Cl-PT), 5-(4-methoxyphenyl)-1H-tetrazole (MO-PT) and 5-p-tolyl-1H-tetrazole (M-PT) were synthesized to study their inhibition behavior on mild steel in 1 M HCl by weight loss measurement, potentiodynamic polarization studies and electrochemical impedance spectroscopy (EIS). It has been observed that corrosion rate decreases and inhibition efficiency increases with increasing in phenyltetrazole substituted concentration and dependence on molecular structure. Polarization data indicate that these compounds act as very good cathodic inhibitors of mild steel in 1 M HCl. The result showed that the inhibition efficiency decreased with temperature and increased with immersion time. EIS has the same trend of inhibitive effect as that of the polarization data, which indicates the formation of a protective layer on mild steel surface by molecules adsorption.

The corrosion behavior of mild steel (MS) in HCl solution containing various concentrations of electropolymerized P(p-phenylenediamine), P(pPD), has been investigated using weight loss and potentiodynamic polarization techniques. The data obtained from the two techniques are comparable and showed that the presence of P(pPD) in the acid solutions suppresses the corrosion rate of MS indicating that these polymers act as corrosion inhibitors (predominantly as anodic inhibitors). The inhibition efficiency (IE%) of the polymer enhances with increasing their concentrations and decrease with an increase in temperature. The inhibition occurs through adsorption and formation of barrier film on the metal surface which separates the metal from direct contact with the corrosive medium and hence protects the metal against the corrosion. The adsorption followed the Langmuir isotherm. The thermodynamic functions of the adsorption and dissolution processes were evaluated.

New imidazolium–based ionic liquids and some pyridinium-based ionic liquids, were tested as corrosion inhibitors of steel in 1.0 M HCl using potentiodynamic polarization, linear polarization and weight loss methods. The obtained results showed that all of the four ILs are good inhibitors for steel in 1.0 M HCl and the inhibition efficiency increased with increasing the inhibitor concentration. All of the ILs act as mixed-type inhibitors. Obtained results from Tafel polarization, linear polarization and weight loss are in good agreement with each other. The adsorption of the four ILs inhibitors, obey the Langmuir adsorption isotherm. From the adsorption isotherm, values of Gads for the adsorption process were calculated. The effect of temperature on the corrosion behavior of steel in presence of 1 × 10-3 M of compound II was studied in the temperature range 298-338 K.

The corrosion inhibition characteristics of nitrogen containing amino acid L-tryptophan on mild steel in 0.1 M HCl solution, in the temperature range of 30-50 oC, was studied by weight loss and potentiodynamic polarization measurements. L-tryptophan significantly reduces the corrosion rates of mild steel; the maximum inhibition efficiency being 83% at 50 oC in presence of inhibitor concentration of 500 ppm. The effect of the addition of very small concentrations of anionic surfactant, sodium dodecyl sulfate (SDS), and of cationic surfactant, cetyl trimethyl ammonium bromide (CTAB), on the corrosion inhibition behavior of L-tryptophan was also studied. The inhibition efficiency (IE) of L-tryptophan significantly improved in presence of both surfactants. The effect of SDS and CTAB on the corrosion inhibition behavior of L-tryptophan appears to be synergistic in nature, the values of synergism parameter being greater than unity. The adsorption of inhibitors on mild steel surface obeyed Langmuir’s adsorption isotherm. The calculated thermodynamic parameters for adsorption reveal a strong interaction between the inhibitors and the mild steel surface. The results obtained by electrochemical studies are consistent with the results of the weight loss measurements. L-tryptophan acts more anodic than cathodic inhibitor.

The effect of extract of Musa paradisiaca on corrosion inhibition of mild steel in aqueous 0.5 M sulphuric acid was investigated by weight loss method, potentiodynamics polarisation technique and electrochemical impedance spectroscopy (EIS). The inhibition efficiency is found to increase with increase in concentration of the extract. Polarization measurement indicates that Musa paradisiaca acts as a mixed-type inhibitor and the inhibition efficiency decreases with rise in temperature. The inhibition is assumed to occur via adsorption of inhibitor molecules on metal surface, which obeys Langmuir adsorption isotherm. The activation energy (Ea) and other thermodynamic parameters for inhibition process were calculated. These thermodynamic parameters indicate a strong interaction between the inhibitor and the mild steel surface. SEM and AFM studies confirmed the adsorption of inhibitor molecules on mild steel surface.

The inhibitive effect of ocimum gratissimum by seed extract on the corrosion of zinc – aluminium (ZA) alloy in 2 M hydrochloric acid (HCl) solution has been studied using gravimetric methods. Inhibition increases with concentration of extract but decreases with temperature. This observation implies that ocimum gratissimum seed extract is an effective and non toxic inhibitor of the corrosion of zinc – aluminium alloy. Adsorption of the extract on the surface obeyed the Freundlich adsorption isotherm. The calculated rate constant (k) shows a first order kinetics in the absence and presence of the inhibitor. The kinetic parameter B, measured for the reaction has a high negative value which implies that ocimum gratissimum becomes more effective as the temperature increases. Synergistic effect of halide additives shows an increase in the efficiency of the extract. However synergism parameter shows that synergism of halide additives decreases with increased concentration of inhibitor.

The inhibition efficiency (IE) of phyllanthus amarus extract (PAE)-Zn2+system, in controlling corrosion of carbon steel in an aqueous solution containing 60 ppm of Cl-, has been evaluated by weight loss method. Weight loss study reveals that the formulation consisting of 2 mL of PAE and 25 ppm of Zn2+ has 98% inhibition efficiency in controlling corrosion of carbon steel immersed in an aqueous solution containing 60 ppm of Cl-. Synergistic parameters suggest that a synergistic effect exists between PAE and Zn2+. Polarization study reveals that this system functions as mixed type of inhibitor controlling the cathodic reaction and anodic reaction to an equal extend. AC impedance spectra reveal that a protective film is formed on the metal surface. The FTIR spectra reveal that the protective film consists of Fe2+-phyllanthus complex.

The inhibition action of 3-ethyl-4-amino-5-mercapto-1,2,4-triazole (EAMT) on the corrosion of 6061-Al alloy in different concentrations of aqueous sodium hydroxide solution has been investigated at different temperatures, using potentiodynamic polarization and electrochemical impedance spectroscopic techniques. The surface morphology of the metal surface was investigated by scanning electron microscopy (SEM). The experimental results showed that the presence of EAMT in sodium hydroxide solution decreases the corrosion rates and the corrosion current densities (icorr), and increases the charge transfer resistance (Rp). It was found that the inhibitor efficiency depends on the concentration of the inhibitor, concentration of the corrosive media and temperature. The inhibition was assumed to occur through adsorption of the inhibitor molecule on the metal surface. The adsorption of the inhibitor on the metal surface is found to obey Langmuir adsorption isotherm. EAMT acts as a mixed inhibitor. Thermodynamic parameters for the adsorption processes were determined from the experimental data. The results obtained from both the techniques are in good agreement.

Special Issue Dedicated to the XII Iberian Meeting of Electrochemistry & XVI Meeting of the Portuguese Electrochemical Society

Metal carbonyl clusters are molecules or molecular ions perfectly defined in size, composition and structural details, which belong by size to the field of nanomaterials. Their molecular structures result from subtle balances between the metal-metal and metal-carbonyl interactions and usually adopt close-packed structures in which a chunk of cubic or hexagonal metal lattice is surrounded by a shell of CO ligands. Very often, such derivatives display extended redox activity affording reversible electron cascades. In many cases such activity increases if interstitial or semi-interstitial atoms of the main group elements (C, N, P, etc.) are inserted in their frames. This in fact triggers establishment of further metal-to-interstitial atom(s) interactions which not only contribute to the number of cluster valence electrons, but also modifies the bonding character of the frontier molecular orbitals.

The electrochemical properties of the one-dimensional coordination polymers [{CuX}2(YNC10H14O)]n (X=Cl: Y=NMe2 1a, Y=NHMe 1b, Y=NH2 1c; X=Br: Y=NH2 2c) and dimers [{Cu(YNC10H14O)}2(μ-X)2] (X=Cl: Y=NMe2 3a; Y=NHMe 3b; X=Br: Y=NMe2 4a; Y=NHMe 4b) were studied by cyclic voltammetry (CV) and controlled potential electrolysis (CPE). All complexes display anodic and cathodic processes. The anodic processes involve the oxidation of the metal site (Cu(I)Cu(II)), while the cathodic processes are based on the ligand. The substituent (Y) at the camphor hydrazones (YNC10H14O) plays a relevant role in the electrochemical properties and reactivity of Cu(I) coordination polymers and Cu(I) dimmers, as corroborated by CPE.

In this work LaNiO3 oxide was prepared by a self-combustion method using citric acid. The electrodes were obtained by coating a nickel foam support with the oxide suspension. Optical microscopy and cyclic voltammetry were used on the electrodes characterization. The evaluation of the electrodes electrocatalytic activity, towards the oxygen evolution reaction in alkaline medium, was performed by means of steady state measurements. The reaction follows a first order kinetics, with respect to OH- concentration, with Tafel slopes close to 40 mV, for low overpotentials. Based on the apparent and real current densities it was possible to conclude that the increase on the electrode activity, when compared with the published data, is mostly related to geometric factors. This fact has been associated with the high electrode/electrolyte contact area provided by the foam nickel substrate. Synergetic effects between the Ni foam and the perovskite oxide cannot be discarded.

The influence of the hydrodynamics of the system on the extent of the electrochemical degradation of phenol, using a boron doped diamond (BDD) anode was investigated. Two different electrochemical cells were used: a batch stirred cell (A), with a volume of 200 mL and a BDD anode of 17.5 cm2, and a batch with recirculation cell (B), with a BDD anode of 70 cm2 and connected to a tank of 30 L. Assays were performed at different stirring speeds and applied current density of 300 A m-2, for cell A, and various flow rates, with an imposed current density of 100 A m-2, for cell B. Chemical oxygen demand (COD) and total organic carbon (TOC) tests were performed to the samples collected throughout the assays, as well as UV-Vis spectrophotometric measurements. For cell A, after 2 h assay, COD removals between 84 and 94% and TOC removals ranging from 54 and 86 % were attained. For the assays run with cell B, during 10 h, COD and TOC removals varied from 27 to 51% and from 23 to 46%, respectively. The influence of the turbulence near the electrode’s surface in the combustion efficiency was also analysed.