Portugaliae
Electrochimica Acta

This work aimed to study the corrosion inhibition of carbon steel in 2 M HCl solutions using some aminopyrimidine derivatives. In this investigation the techniques of measurements were: weight loss, linear polarization, impedance and corrosion penetration. The inhibitory effect of the investigated compounds resulted from their adsorption onto the carbon steel. Their adsorption on the metallic surface obeyed Frumkin’s model of adsorption. The inhibition efficiency of the compound greatly depends on the electron density on the molecules, which in turn depends on its structure. The values of activation energy were determined in 2 M HCl in the absence and in the presence of inhibitors. The presence of inhibitors increases the values of the activation energy of corrosion. The adsorption of these compounds onto the metallic surface occurs via the N-atoms of amino-groups, O-atoms of hydroxy-groups, and S-atoms of mercapto group present in the compound. For this reason the inhibitory effect of the inhibitors varied according to the number and nature of adsorption centers in the molecules.

Corrosion inhibition of mild steel in 0.67 M H3PO4 by phenacyl dimethylsulfonium bromide and six of its p-substituted derivatives was studied using different chemical, electrochemical and scanning electron microscopy techniques. The order of increasing inhibition efficiency was correlated with its p-substituted through Hammett relation. Potentiodynamic polarization curves indicated that the compounds acted primarily as mixed-type inhibitors. Electrochemical impedance spectroscopy showed that the steel dissolution is controlled by charge-transfer mechanism. The kinetic-thermodynamic model of adsorption isotherm described the experimental findings. Number of active sites, binding constant and change of free energy were computed for all studied compounds. Depending on the inhibitor, it was found that each organic molecule replaced one or two adsorbed water molecules from the steel surface. The adsorption center was suggested to be the π electrons of the phenyl ring, and a flat configuration adsorption of the molecule may occur.

Differential Pulse Voltammetry (DPV) and Fourier Transformed Infrared Spectroscopy (FTIRS) techniques have been applied and compared, for the azithromycin determination in two different pharmaceutical formulations (capsules and suspension). Both methods were found to be highly precise with high recovery levels (more than 98 %) The analytical results obtained with the electrochemical method showed excellent correlation with the analytical results obtained by FTIRS. The proposed electrochemical methodology presents the advantage over FTIRS of being able to be used directly on azithromycin samples in capsules presentation as well as in suspension, since no interferences were observed from the excipients of the formulation, nevertheless the direct analysis of the suspension was not possible by FTIRS due to excipients interference.

This paper presents and discusses data collected on the electroreduction of O2 (ORR) in 1 M NaOH on a polycrystalline Pt surface that initially was pretreated by H2 evolution at -250 mV in deaerated 0.5 M H2SO4 and then evolved with time in a spontaneous and rather slow way towards a full Nekrasov (N-Pt) type behavior, both in the cathodic- and in the anodic potential scan. This fact represents a clear demonstration that a polycrystalline metal surface is a dynamic system and allowed us to carry out the thorough study of the mechanism and kinetics of the ORR on a full N-Pt surface that was not possible in the previous work reported in the Part II of this series. Essentially, we concluded that the change does not alter the ORR mechanism, which is always a series mechanism coupled to disproportionation of the HO2- produced; what varies in a given potential range is the value of the rate constants of the different mechanistic steps. It was possible to get absolute values of these constants only for some potential ranges, although the analysis of the data collected provided relative information for the remaining ranges.

Electrooxidation of carbon monoxide and formic acid was performed on Rh microparticles deposited on both, Glassy Carbon (Rh/GC) and polyaniline films (Rh/PANI), to evaluate the electrocatalytic activity of those electrodes. The deposit of Rh microparticles on those substrates provides a higher surface and exhibits a better electrocatalytic activity compared with that shown by smooth Rh for the electrochemical oxidation of formic acid in sulphuric acid. The oxidation of CO and HCOOH on Rh/GC and Rh/PANI was followed using in situ Multi-Step FTIR Spectroscopy (MS-FTIRS). Either linear or bridge bonded adsorbed CO (COl, COb), has been observed by FTIRS as the main species. In comparison to the adsorption of CO on smooth Rh surface, the IR features of CO adsorbed on Rh/GC and Rh/PANI electrodes showed an anomalous behaviour.

Liquid ammoniates, which are highly conductive electrolyte (more than 100 mS.cm-1 at 20 °C), are proposed as solvents for a redox battery working around room temperature, and up 80 °C. In the negative compartment, the anolyte is NaI • 3.3NH3, and the Na+/Na couple is proposed. It is reversible, as determined by cyclic voltammetry and galvanostatic cycling. In the positive compartment, the catholyte NaI • 3.3NH3 enriched in silver cations is proposed too. The cathodic material is merely this compound and the cathode is the silver metal. The couple Ag+/Ag is reversible too, as determined by cyclic voltammetry and galvanostatic cycling. Metallic sodium is extremely stable in this electrolyte; therefore NaI • 3.3NH3 could be used in a redox battery of high energy density and high power density. The maximum working temperature, which is proposed, 70 °C, is lower than the melting point of sodium (98 °C), and avoids high pressures of ammonia.

A conductivity technique has been developed, being highly useful for studying the association behaviour of cyclodextrins (CDs) with surfactants. In this paper, the complexation of β-cyclodextrin with alkyl sulfonates and alkyl sulfates with different carbon chains (from 10 to 16) is analysed. The effect of the ionic head group in the most hydrophobic surfactant is also analysed. The association constants for 1:1 and 1:2 complexes are calculated on the basis of a second and a third degree equation; in the latter a real solution of a Cardin-Tartagliae formula is applied to solve the third degree equation.

The corrosion and passivation behaviours of iron in ternary molten Li2CO3-Na2CO3-K2CO3 mixture at different temperatures (475- 550 oC) were studied. The techniques of measurements were: open- circuit potential, galvanostatic anodic polarization, and cyclic voltammetry. The amounts of iron dissolved in the melt were determined by atomic absorption spectroscopy after each experiment. Iron undergoes spontaneous passivation upon its immersion in the melt and the thickness of the oxide scales increases with the increase of temperature. During the passivation potential range different oxides and spinals are formed. These include a cubic solid solution of FeO and a-LiFeO2, a-LiFe5O8 and a-Fe2O3. At high anodic potentials the decomposition of carbonate takes place leading to passivity break down and CO2 and O2 gases evolution. Also, the spoliation of oxide scales in the carbonate melt occurs at the passivity breakdown. The values of corrosion parameters (Rp, io, icorr, ip) were calculated. The different values of activation energy of corrosion process were calculated. The results of cyclic voltammetric investigation indicate the formation of different oxides and spinals, and the cathodic and anodic decomposition of carbonate. The corrosion tests in 0.05 M HCl solution indicate that the oxide scales formed on iron are multilayered. The outer layer is less protective than the inner one.

The interfacial impedance (or shortly, the capacitance) of solid electrodes in the absence of faradaic reactions usually deviates from purely capacitive behaviour. On polycrystalline solid electrodes "frequency dispersion capacitance" is often observed. We present qualitative impedance experiments on pre-treated Ag-15%Cu alloy electrodes immersed in 0.1 M HClO4. The results suggest that models taking into account microscopic geometrical aspects only (i.e. roughness) are not suitable for interpreting the de facto link between roughness and capacitance dispersion. Instead, considering additional points, such as surface heterogeneities on an atomic scale (i.e. surface disorder) together with adsorption of certain ionic species, particularly the Cl-, on the surface of the metal can and do cause capacitance dispersion in the frequency range relevant in electrochemistry.

The inhibiting effect of N-cetyl N,N,N-trimethyl ammonium bromide (CTMAB) on the corrosion of mild steel in 1N H2SO4 solution has been studied by using techniques such as weight loss and electrochemical polarization, infrared (IR) and scanning electron microscopic (SEM) techniques. It is found that CTMAB is good inhibitor for corrosion of mild steel in 1N H2SO4. The maximum efficiency is about 96% for CTMAB at 10-1M concentration. The polarization curves indicate that the compound is mixed type inhibitor. The adsorption of inhibitor on mild steel surface is found to obey Langmuir adsorption isotherm. The activation energy Ea in the absence and presence of inhibitor is obtained by measuring the temperature dependence of corrosion method. The mechanisms of adsorption of CTMAB on mild steel have been investigated by IR spectra and SEM.

Copper nitroprusside (CuNP) was generated on the graphite paste electrode using a new methodology of the preparation. The electrochemical studies were carried out by cyclic voltammetry technique. The cyclic voltammogram of the modified electrode, CuNP showed two redox couples (E0’)1=0.22 and (E0’)2=0.88 V vs. SCE attributed to Cu(I)/Cu(II) and Fe(II)(CN)5NO/Fe(III)(CN)5NO, respectively. The nature of the cation affect the (E0’)1 and (E0’)2, as current intensity, shifting the E0’ for more positive potentials, for two redox processes. The voltammograms obtained with different KCl concentrations (0.1-3.0 mol L-1) exhibit a shift in the (E0’)1 to more positive potentials; this change was linear with the supporting electrolyte concentrations change. It was verified that the (E0’)1 remained practically constant at pH between 6 and 3. However, a new process with (E0’)3 (0.48 V) appears at pH<3 and it was ascribed to formation of intermediary species. The redox couple at (E0’)2=0.88 V presents an electrocatalytic response for N-acethylcysteine. The modified graphite paste electrode gives a linear response between 5.0´10-4 to 1.0´10-2 mol L-1 of N-acethylcysteine with a detection limit of 4.5´10-4 (±5%) mol L-1 (n=3) and an amperometric sensitivity of 4.9 mA/mmol L-1. The electrocatalytic oxidation of N-acethylcysteine compounds by the mediator has been used for the determination of N-acethylcysteine in a commercially pharmaceutical available product.

The electrodeposition of tin in presence of tartrate ions has been analysed by electrochemical techniques, mainly chronoamperometry, and by scanning electron microscopy (SEM). The obtained values of nucleus density with both techniques have been compared and discussed. The electrodeposition process follows an instantaneous nucleation with 3D growth under diffusion control at the initial times of the process, but a second nucleation process occurs at higher times. The influence of tartrate and of agitation conditions is also inferred from the crystal morphology.

The electro-oxidation of toluene in 0.1 M But4NPF6 + CH3CN solutions on glassy carbon electrodes was studied using electrochemical and spectroelectrochemical techniques. Toluene electro-oxidation yields an electrochemical inactive film on the electrode surface. In situ Fourier Transformed Infrared (FTIR) studies suggest the formation of a polymeric film, as the main product, on the surface. Depending on the experimental time scale toluene transformation is a complex reaction that could involve adsorption processes, pure charge transfer reaction and couple chemical reactions. Additions of small quantities of water to the electrolyte cause a cathodic displacement of the oxidation peak potential; in other words, the electro-oxidation reaction is favoured.

Azetidinone ester is the key intermediate during the synthesis of injectable cephalosporin compounds. This intermediate is undergoing electrochemical reaction during the process of conversion from penicillin to cephalosporin. The CV and RRDE studies clearly indicate that electro-reduction of this compound is taking place in three steps under the condition. The first two peaks correspond to the two, one electron transfers and the third one is due to the proton addition from the non-aqueous solvent. And during the process, the cyclisation is also taking place in order to neutralise the charge. Further CV and RRDE studies with the addition of small amount of water to the above system clearly confirm the mechanism. This was very well confirmed by the FTIR analysis of the product obtained from the bulk electrolysis.

The purpose of optimization consists in establishing suitable values for the operational parameters of a process, in order to achieve the results of that process to the greatest possible advantages. These ideal operating conditions are known as the optimum. The process parameters are current density, temperature, concentration, electrode materials, flow rate, cell voltage, mixing and diaphragm and engineering quantities are like current efficiency, space-time yield, energy consumption, yield and conversion. There are several possible targets in optimization of process and the main aim is to get maximum yield, high current efficiency, high purity and minimum energy consumption for mediated electrochemical oxidation. Mediated electro oxidation or indirect electrosynthesis is a cyclic process involving electrochemical generation of cerium(IV) and uses that agent to effect a chemical reaction. Cerium(IV) has attracted attention in electroorganic synthesis for synthesis of aldehydes and quinones.

The characteristics of corrosion inhibitors of the tolitriazole (TTA) for the carbon steel 1008, covered by zinc or tricationic phosphate (Mn, Zn, Ni), in the means of H2SO4 0.1 mol L-1 and NaCl 0.5 mol L-1, were investigated by physical and electrochemical methods. The results obtained in the absence of the inibidor TTA showed that the corrosion protection properties of the tricationic phosphate (Ptri) are better than those observed for the zinc phosphate (PZn), probably due to the presence of vacancies in the layer of PZn. The curves of anodic polarization presented current densities (j) smaller for the steel phosphatized with Ptri, when compared with the metal substrate without phosphate and with PZn. The results of impedance electrochemistry (IE) showed a polarization resistance (Rp) larger for the electrodes phosphatized with Ptri. The electrochemistry measurements of the samples of steel phosphatized, with PZn and/or Ptri, in the presence of [TTA] = 1x10-3 mol L-1, present smaller j and larger Rp when compared with the absence of TTA. In the process of phosphatization of the carbon steel with PZn + TTA, it was observed similar j and Rp in the samples of steel phosphatized with Ptri without TTA. These results suggest that the bathing of zinc phosphate with [TTA] = 1x10-3 mol L-1 can substitute the well known industrial bathing of tricationic phosphate. Corrosion testing by Salt Spray of steel phosphatized with PZn and PZn + TTA, painted with white polyester ink, confirmed the eletrochemical results.

It was developed a very sensitive analytical method to determine molybdenum in water and food grains, using the linear sweep voltammetry technique, with adsorptive preconcentration of a Mo(VI) 8-hydroxyquinoline complex, in an acetate buffer media of pH = 3.5 and KClO3 0.5 mol/L as a support electrolyte. The linear range of the methodology goes from 1.0 hg/L to 10 mg/L, with a limit of detection of 0.3 hg/L and quantification of 1.0 hg/L. The methodology is suitable in the range of mg/L and hg/L. The method was applied to grinded samples of rice, beans, lentil seeds and chickpeas with the following result: 6.2 mg/kg in lentil seeds, 6.0 mg/kg in chickpeas, 3.2 mg/kg in red beans, and 1.7 mg/kg in rice.

Some of the aluminium complexes are electroactive at positive potentials, being necessary a carbon vitreous electrode to their quantitative determination. Nevertheless the aluminium alizarine complex could be measured at negative potential with a mercury hanging electrode, but this complex is not electroactive at negative potential; instead of that, the residual alizarin non complex could be measured. The kinetic complex is very slow, so it is necessary to heat at 80 0C, for five minutes. The study of experimental conditions of complex formation was established at pH 9.25 and is determinant in the complex formation; besides, the buffers employed should be of high regulatory capacity to obtain derivative formation. The optimization of the measurement experimental conditions was done by square wave voltammetry (SWV). Detection limit got using standard deviation of seven blank solutions was 3 mg /L, the variation coefficient was 8%, and the mean recovery for 16 mg /L was 97%. The calibration curves were done in a linear interval of 8 to 64 mg /L. The method application to water samples is possible after previous evaporation.