Portugaliae
Electrochimica Acta

The corrosion behaviour of Zn – Al – Cu alloy was studied in pure HCl solutions and in the presence of different concentrations of inhibitors. The techniques of measurements were: weight loss, linear polarization, polarization impedance and corrosion penetration. It was found that the corrosion rate of this alloy is greatly higher than that of Zn or Al. The inhibitors used were: heterocyclic pyridine (p), 2-methyl pyridine (2 mp), 3-methyl pyridine (3 mp) and 4-methyl pyridine (4 mp). These inhibitors behave as mixed type and their inhibition efficiency increases according to the order: 2 mp < 3 mp < p <4 mp. The values of activation energy of corrosion were determined in pure acid and in the presence of inhibitors. It was found that the presence of these inhibitors increases the value of activation within the same order of their inhibitory effect. The adsorption of the investigated inhibitors on the surface of the alloy follows Frumkin’s isotherm.

The electrochemical behavior of citrazinic acid azo dye derivatives (I-V) was recorded in aqueous buffer solutions containing 10% (v/v) DMF using DC-polarography and cyclic voltammetry. The DC-polarograms displayed one or two reduction waves depending on the nature of substituent and the pH of the electrolyte solution. The reduction process takes place irreversibly and diffusion controlled at the electrode surface through the uptake of four electron/molecules for compound I and eight electron/molecules for each of compounds II-V. The recorded cyclic voltammograms displayed two irreversible cathodic peaks at different pH values. The kinetic parameters of the electrode process such as, αn, heterogeneous rate constant (ks) and activation energy (ΔG#) were evaluated and discussed from cyclic voltammetry. Also, the electrode reaction mechanism is proposed and discussed. The effect of introducing substituents on the half-wave potential of the reduction process is illustrated. The different thermodynamic parameters such as activation energy (ΔG#), enthalpy change (ΔH#) and entropy change (ΔS#) revealed the endergonic nature of the electrode reaction.

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.