Portugaliae
Electrochimica Acta

Novel miniaturized polyvinyl chloride (PVC) membrane sensors in all-solid state graphite and platinum wire supports were developed, electrochemically evaluated and used for the assay of orphenadrine citrate (ORP). The ORP sensors were based on the formation of an ion-association complex between the drug cation and tetrakis(4-chlorophenyl)borate (TpClPB) anionic exchanger as electroactive material dispersed in a PVC matrix. Linear responses of 1E-2 - 1E-5 M and 1E-2-1E-4 M with cationic slopes of 56.4 mV and 53.6 mV over the pH range 4-7 were obtained by using the ORP-coated graphite (sensor 1) and platinum wire (sensor 2) membrane sensors, respectively. The proposed methods displayed useful analytical characteristics for the determination of ORP in Norflex® tablets with average recoveries of 100.01±0.83, 100.09±0.90, and in plasma with average recoveries of 99.4±0.97 and 98.55±0.82, for sensor 1 and 2, respectively. The methods were also used to determine the intact drug in the presence of its degradate and thus could be used as stability indicating methods. The results obtained by the proposed procedures were statistically analyzed and compared with those obtained by using an official method. No significant difference for both accuracy and precision was observed.

The electrochemical reduction behavior of cefepime and cefpirome has been studied using cyclic voltammetry in Britton Robinson (BR) buffer ranging from pH 2.0 to 12.0. Cefepime and cefpirome are found to give two well defined peaks and these peaks are attributed to the reduction of the azomethine group (>C=N-) by two electron process in two steps. Kinetic parameters such as diffusion coefficient (D) and forward rate constant (Kof,h) values are evaluated and a reduction mechanism is proposed. The result indicates that the process of both the compounds is irreversible and diffusion controlled. A simple and rapid differential pulse polarographic method has been developed for the determination of cefepime and cefpirome in pharmaceutical formulations and spiked urine samples using the standard addition method. The lower detection limits are found to be 4.6E-8 M and 8.52E-8 M for cefepime and cefpirome, respectively.

The present paper contains the results relative to the reaction of NiO with Na2SO4 in presence of SO2 (g) at 1100 and 1200 K. Nickel oxide (NiO) is produced by the oxidation of metal alloys in many industrial operations. The oxidation of metal and metal oxides in presence of molten salts and gas environment at high temperature is often called as hot corrosion. In this investigation the studies have been carried out taking into consideration the influence of SO2 partial pressure. The thermo-gravimetric studies have been carried out as a function of Na2SO4 concentration in the mixture. The reaction products identified by XRD analysis and surface morphologies of the products were discussed on the basis of metallographic and scanning electron microscopic (SEM) studies. The pH of the aqueous solutions of the reaction products were measured to observe the changes in the system with varying the Na2SO4 concentration. A quantitative estimation of soluble metal ions has also been carried out with the help of an atomic absorption spectrophotometer. The possibility of chemical reaction is established by thermodynamic calculations of free energies of the reaction products.

This work presents the optimization process of mercury determination by Anodic Stripping Voltammetry (ASV), using a glassy carbon modified with a gold film as working electrode, in samples extracted by aqua regia. The samples may contain Cu, Fe or Pb in high concentrations. In these conditions the usual Hg determination by ASV is almost impossible. The effects of the supporting electrolyte on the sensitivity of the method and chloride and nitrate ions concentration were studied. Supporting electrolyte was selected between: HCl, HNO3, HClO4 and CH3COOH. Our results show that the best sensibility is obtained with HClO4 0.10 M. High chloride concentration decreases the useful potential window and the peak current. However, small chloride additions are required to obtain high peak currents because chloride prevents Au film poisoning. Another interference studied was that of Cu and Fe. This kind of interference can be avoided if the sample in the electrochemical cell is exchanged for fresh electrolytic solution (HCl 0.01 M -HClO4 0.10 M) before the stripping step.

The inhibition of corrosion of 304 SS in 2 N HCl solutions by N – furfuryl – N' phenyl thiourea (FPTU) has been investigated using potentiodynamic polarization techniques. The results obtained reveal that FPTU performs excellently as anodic inhibitor (IE> 93%) for 304 SS in HCl solution. The inhibitor functions through adsorption following Temkin’s adsorption isotherm, and the inhibition was governed by physisorption mechanism. The thermodynamic parameters deduced for the adsorption process reveal spontaneous adsorption of the compound on the alloy steel surface.

Two microsized graphite-design sensors based on ionophore technique, polyvinyl chloride carboxylated (PVC-COOH) and β-cyclodextrin (β-CD), are used for fabrication of two membrane sensors for the two studied drugs, metronidazole (MZ), sensor 1, and spiramycin (SP), sensor 2. Fast and stable Nernstian responses near 1E-5 - 1E-3 M for MZ and 1E-5-1E-2 M for SP over pH range 5.5-7.5 for the two electrodes reveal the performance characteristics of these electrodes which have been evaluated according to IUPAC recommendations. The aim of this work is to develop a new, simple, accurate and precise method for the determination of MZ and SP in their binary mixtures, which can be applied in routine quality control. The method is successively applied for the determination of the two drugs in their pharmaceutical formulations. Validation of the method according to the quality assurance standards shows suitability of the proposed electrodes for the use in the quality control assessment of these drugs. The recovery percentages for the determination of the two drugs by the two proposed selective electrodes are 99.86 ± 0.249 % and 99.69 ± 0.856% for sensors 1 and 2, respectively. Statistical comparison between the results obtained by this method and the reported one is done and no significant difference is found.

Bright nickel deposits were electrolytically applied on steel in the nickel Watts bath. The effect of some operational parameters on metal deposition in bright nickel plating was investigated. The investigation indicated that the weight of bright nickel deposited on metal during the process of electroplating was affected by plating temperature, voltage, current density, plating bath pH and plating time. The study established that the deposition of best bright nickel was obtained at a plating temperature of 56 ºC, current density of 6 A/dm2 and plating time of 18 minutes. Brightener is used in applications requiring outstanding appearance with minimum thickness of applied nickel plating. It can also be used for heavy deposit applications because it exhibits unparalleled ductility and low stress. Brightener was used in this study to determine the best nickel plating in the process. Boric acid was added for fixing the bath pH. The compositions of the brightener and nickel solution used are included in the text.

The performance of triethylenetetramine-tribenzylidene (TTTB) and triethylenetetramine-trisalicylidene (TTTS) as corrosion inhibitors for zinc in hydrochloric acid is investigated. At lower concentrations, both inhibitors accelerate the attack but inhibit corrosion at higher concentrations, e.g., 96–100% with 1.0% concentration in 0.5 M and 1.0 M HCl. The efficiency of TTTB decreases while that of TTTS remains almost constant (≥ 99.7%) up to 120 minutes and in the temperature range 35 – 65 ºC. The activation energies are higher in inhibited than in plain acid with both inhibitors. The free energy of adsorption (ΔGads) and heat of adsorption (Qads) are negative, which suggests that there is spontaneous adsorption on metal surface, and from the values of (ΔGads) and (Qads), the values of entropy of adsorption (ΔSads) were calculated. Galvanostatic polarization shows that corrosion is under mixed control with predominance of the cathodic part. In uninhibited 1.0 M HCl, complete cathodic protection is achieved at a current density of 4.2224 Adm-2, but in presence of these inhibitors, much lower current densities are required. Plot of log (θ/1-θ) versus log Cinh gives a straight line, suggesting that inhibitors cover both the anodic and cathodic regions through general adsorption following Langmuir isotherm. The mechanism of inhibition has been proposed.

Two sensitive voltammetric techniques (adsorptive and catalytic cathodic stripping) are described for the determination of timonacic, in the presence of copper or nickel ions, respectively. The measured peaks which correspond to the reduction or copper (I) or nickel – timonacic complexes are measured at -773 and -782 mV, respectively. The different experimental parameters have been carefully studied. The methods have been fully validated. The detection limits were 1.5 and 13.6 ng mL-1, respectively. The methods have been applied for the determination of timonacic in pharmaceutical tablets. The obtained results have been compared statistically with those obtained from a published method.

Essential oil from fennel (Foeniculum vulgare) (FM) was tested as corrosion inhibitor of carbon steel in 1 M HCl using electrochemical impedance spectroscopy (EIS), Tafel polarisation methods and weight loss measurements. The results show that the increase of the charge-transfer resistance (Rct) with the oil concentration supports the molecules of oil adsorption on the metallic surface. The polarization plots reveal that the addition of natural oil shifts the cathodic and anodic branches towards lower currents. Such shifts indicate that FM oil acts as a mixed-type inhibitor. The global rate of corrosion estimated by weight loss measurements confirms the above results. The inhibition efficiency attains a maximum of 76 % at 3 mL/L, but decreases with the rise of temperature. The analysis of FM oil, obtained by hydro-distillation, using Gas Chromatography (GC) and Gas Chromatography/Mass Spectrometry (GC/MS) showed that the major components were limonene (20.8 %) and β-pinene (17.8%). The adsorption of FM on the steel surface has been discussed according to the chemical composition of the oil, giving an explanation to the obtained results.

Aromatic nitrones have been synthesized and investigated for their function as inhibitor for the corrosion of mild steel in the organic acid media – 20% formic acid and 20% acetic acid at various inhibitor concentrations in the range of 50 - 150 ppm. Weight loss measurements and potentio-dynamic polarization studies reveal that the nitrones behave as mixed type inhibitors. Among the nitrones tested, para-methyl substituted derivatives perform better as inhibitors than the un-substituted nitrones.

The inhibitive effect and adsorption properties of petiole extract obtained from destructive distillation of cocos nucifera for the corrosion of mild steel in 0.5 M H2SO4 and 1 M HCl were investigated using mass loss, polarization and electrochemical impedance techniques of monitoring corrosion. The results obtained indicate that petiole extract of cocos nucifera behaves as good inhibitor for the corrosion of mild steel in 0.5 M H2SO4 and 1 M HCl. Activation energy of thermodynamic parameter was evaluated from temperature studies result. The adsorption of the inhibitor on mild steel surface was found to be spontaneous, endothermic and consistent with the assumptions of Langmuir adsorption isotherm. The electrochemical measurements reveal that the petiole extract behaved like mixed type inhibitor. Efforts are made to analyze the effectiveness of petiole extract of coconut palm in industrial processes.

Properties of electrodeposited Ni-composite coatings containing ceramic particles are very much dependant on the bath used, current density, duration of deposition, particle content in the bath, etc. In the present study, the influence of process parameters like the concentration of particles, current density and time of deposition on the area fraction of yttria stabilized zirconia (YSZ), the microhardness and the thickness of the electrodeposited nickel (Ni)-YSZ composite coating was analyzed by Taguchi Design method and analysis of variance (ANOVA). According to the experimental results and ANOVA, the interaction of current and time are the most significant factors influencing the thickness of the coating; interaction of concentration of particles in the electrolyte bath and current are the most significant factors influencing the microhardness; and concentration of particles in the electrolyte bath is the most significant factor affecting the area fraction of particles in the Ni matrix. Models were developed for predicting the microhardness and thickness of the composite coating and area fraction of particles incorporated in the nickel matrix. They were found to be in good agreement with the experimental results. The models were tested for experimental conditions and were found to be close to predicted values. The thickness of the deposit was mainly dependent on the current density and duration of plating. On the other hand, the microhardness of the coating and area fraction of particles present in the nickel matrix were mainly dependent on the amount of particles present in the bath.

The inhibition efficiency of natural phosphate and kaolin systems in controlling corrosion of stainless steel in HClO4 (0.1 M) solution has been evaluated by electrochemical polarization methods, and electrochemical impedance spectroscopy. The electrochemical data show that the corrosion resistance is greatly enhanced after surface modification. The best protection is obtained with natural phosphate. The uniform coatings of phosphate and kaolin on iron substrates were obtained by electro deposition.

The effect of N-[N`-(4-methoxyphenyl)benzenesulphonamido]-3-carboxy-4-methyl-4-(4-methylphenyl)-3-butenamide 1, N-(N`-phenylbenzenesulphonamido)-3-carboxy-4-methyl-4-(4-methylphenyl)-3-butenamide 2, and N-[N`-(4-chlorophenyl)benzenesul-phonamido]-3-carboxy-4-methyl-4-(4-methylphenyl)-3-butenamide 3, on the corrosion of low carbon steel in 1 M hydrochloric acid solution has been studied by weight loss measurements and potentiodynamic technique over the temperature range 30-60 °C. Results obtained showed that protection efficiency increased with the increase in concentration of the different organic compounds used and decreased with the increase in temperature. Langmuir adsorption isotherm at all temperatures was studied. The phenomenon of physical adsorption is proposed from the values of Ea and ∆G˚ads. Results obtained showed that compound 1 is the best inhibitor, and the protection efficiency follows the order: 1 > 2 > 3.

The influence of 2-[3-(2-oxo-2-phenylethylidene)-1,4-dihydroquinoxaline-2(1H)-ylidene]-1-phenylethanone (Qx1) and amino-2-aniline (Diam1) on the corrosion of steel in 1 M HCl solution has been investigated by weight loss measurements, potentiodynamic polarisation and impedance spectroscopy (EIS) methods. The inhibiting action increases with the concentration of Qx1 and Diam1. The highest efficiency (85%) is obtained at the 10-4 M Qx1. There is good agreement between gravimetric and electrochemical methods (potentiodynamic polarisation and EIS). Polarisation measurements also show that Qx1 and Diam1 act as mixed inhibitors. The cathodic curves indicate that the reduction of proton at the steel surface happens with an activating mechanism. Qx1 adsorbs on the steel surface according to Langmuir adsorption model. Effect of temperature is also studied between 308 and 353 K.

The corrosion behavior of mild steel in dimethyl sulfoxide containing various concentrations, i.e., 0.001, 0.005, 0.010, 0.015 and 0.020 M of hydrochloric acid has been studied in presence of 0.1 M tetra ethyl ammonium chloride as supporting electrolyte by applying electrochemical techniques. The results of these experiments have been supplemented by surface analysis of the samples exposed to corrosive solution under different experimental conditions. The electrochemical investigation was done potentiostatically by varying the concentration of HCl in DMSO and the experimental temperature as well.

The corrosion inhibition effect of acid extract of the pericarp of the fruit Garcinia mangostana on mild steel in hydrochloric acid medium is studied. Using weight-loss data, corrosion rate (CR) and thermodynamic parameters such as energy of activation for corrosion of mild steel (E), heat of adsorption of the inhibitors on the metal surface (Q) and change in free energy (∆G) are calculated. Adsorption isotherm is also recorded between log C and θ. Electrochemical parameters are evaluated from AC impedance and Tafel polarization studies in the presence and absence of different concentrations of inhibitor. Infra-red spectra are recorded for the extract and the adsorption product to find out the changes in the functional group frequencies of the organic components of the extract. The surface morphological changes are analysed using the SEM images. The entire study shows that the pericarp extract of the fruit G. mangostana is a non-toxic, cost-effective corrosion inhibitor for mild steel in acid medium.

The corrosion inhibition of stainless steel in 0.9% NaCl solution in presence of poly(vinyl alcohol) (PVA) is discussed according to electrochemical measurements, such as potentiodynamic polarization and electrochemical impedance spectroscopy (EIS).The morphology of the surface was analyzed using Scanning Electron Microscopy (SEM). The composition of the layer formed on stainless steel surface was estimated using X-ray Photoelectron Spectroscopy (XPS) technique. Electrochemical measurements indicated that the presence of PVA in NaCl solution decreases the corrosion current and increases the polarization resistance. The values of inhibition efficiency obtained from polarization curves and EIS measurements are in good agreement. In absence and in presence of PVA, SEM images showed that the stainless steel surface was covered with a non-uniform layer and a uniform adsorbed film, respectively. XPS analysis indicated that the surface layer consists of PVA containing a small amount of other elements, such as Na and Cl.

The electrochemical oxidation of toluene in aqueous-acid medium was studied on monolithic and reticulated glassy carbon electrodes using cyclic voltammetry. The oxidation potential was 1.7 V for both electrodes. However, the normalized oxidation currents were higher on reticulated glassy carbon, showing more catalytic activity than on the monolithic glassy carbon. To normalize the obtained anodic current values, a method for the determination of the electrodes effective area was used, which consists in the underpotential deposition of a copper monolayer.