Portugaliae
Electrochimica Acta

The rate of copper dissolution in presence of phosphoric acid tert-butanol electropolishing mixtures was studied by measuring the limiting current which represents the rate of electropolishing. The rate of dissolution decreases by increasing phosphoric acid concentration, electrode height and the mole fraction of alcohol. The data show that addition of tert-butanol to phosphoric acid decreases the rate of dissolution of copper by an amount ranging from 68 % to 90 % depending on the dielectric constant of mixtures. Thermodynamic parameters are given.

A solid of Fe2O3 based carbon-epoxy composite electrode was investigated for use as a potentiometric pH sensor. The electrode was constructed with a mixture of sulfated iron(III) oxide, carbon power and epoxy resin that was deposited directly onto a glass tube. The effect of composition (Fe2O3, carbon and epoxy resin) on the electrode response and its calibration curve (mV/pH) were investigated. The analytical behaviour of the electrode in acid-base titrations was compared with that of a glass electrode. A linear response from pH 1.7 to 12.2 with a slope of -39.7 ± 0.6 mV/pH (at 25 ºC) was observed.

The formation and stability of alkali metal complexes with crown ethers containing an anthraquinone unit has been investigated in methanol and acetonitrile solutions by potentiometric methods. Complexes of 1:1 stoichiometry were observed for all the studied systems; 2:1 complexes were only detected for the larger alkali cations (Rb+ and Cs+) and with the ligand with the larger macrocyclic cavity (AQ21C7). The 1:1 complexes with the highest stability are formed by K+ ion with the AQ18C6 ligand and by Cs+ ion with the AQ21C7 ligand. In the alkali cation group and in both solvents, the stability of the 1:1 complexes was found to vary in the following order, Li+ < Na+ < K+ > Rb+ > Cs+ with AQ18C6 ligand and Li+ < Na+ < K+ < Rb+ < Cs+ with AQ21C7 ligand. The stability of the complexes of AQ21C7 is smaller than those of their AQ18C6 analogues. The stability of the silver complexes of AQ18C6 and AQ21C7 was found to be smaller than that of the alkali metal complexes of comparable size. The incorporation of an anthraquinone unit in crown ethers induces some loss of complexation stability for all cations, but the cation selectivity is not changed. The results obtained are analysed and discussed regarding the effects of the relative sizes of the cations and macrocyclic cavity of the ligands, solvating ability of the solvents towards the cation and influence of the anthraquinone unit.

The electrochemical behavior of Pt(IV) ions in the presence of dimethylglyoxime has been investigated using adsorptive stripping voltammetric method. The results indicated that the platinum is reduced from its adsorbed state as bis-dimethylglyoxi-mate platinum(IV) complex. The peak current is linearly proportional to the Pt(IV) concentration in the solution, thus representing the basis for a quantitative analysis. Various experimental parameters affecting the stripping response, indicating pH, the ligand concentration or the accumulation time and potential, and the scan rate, were studied to obtain a highly linear response. From the results the scheme for the reduction process of the platinum dimethylglyoximate complex in acidic media is presented.

In liquid ammonia, the anodic behavior of n- and p-type III-V semiconductors are strongly different from those observed in aqueous media. For both types, the anodic process in liquid ammonia involves the formation of an adsorbed film, onto the semiconductor surface just before its dissolution. This film results from the chemisorption of N atoms, issued from the oxidation of the solvent.

The cathodic electrochemical behavior of InSb was studied in liquid ammonia. In neutral unbuffered media, in spite of strong negative polarization no electrochemical modification of the interface was observed. However, in acidic media, a cathodic decomposition was detected after scanning InSb in the potential range of hydrogen evolution. Hydrogen embrittlement results from a strong interaction of hydrogen radical (H.) and indium surface. A coated film of indium results from this cathodic decomposition.

The effect of some thiourea derivatives on the corrosion inhibition of copper has been investigated in 2 M HNO3 solution. Weight-loss and polarization measurements were applied to analyse the metal corrosion behaviour in the absence and presence of the inhibitor. The results show that all the studied thiourea derivatives inhibit the corrosion of copper in the solution and that the inhibiting efficiency increases with a decrease in temperature or an increase in concentration of the thiourea derivatives. The obtained results show that the adsorption of the additives used follow the Temkin adsorption isotherm. Moreover, the thermodynamic activation parameters of the copper corrosion reaction were calculated and discussed.

Studies on the initial stages of the atmospheric corrosion of copper exposed to the urban atmosphere of the city of Lisboa, at a site called Campo Grande, from November 2000 till February 2001 (winter period), are presented in this paper. Average corrosion rates of 0.139 and 0.105 g/(m2 day) were obtained of copper samples after one and three months of exposure, respectively. From SEM/EDS data it was concluded that during the first month of exposure corrosion products were mainly composed by copper oxides and minor amounts of copper chlorides, while exposures of two and three months lead to the formation of copper sulphates apart from copper oxides and copper chlorides. A uniform and adherent film was visible on the surface after three months of exposure. During the period under evaluation concentrations of NO2 were between 32 and 36 ppb and deposition rates of SO2 between 17 and 20 mg/(m2 day), without significant changes during the three month period. The degree of humidity and the average precipitation were both quite high (winter season). The SO2 present in the atmosphere with a high degree of humidity explains the formation of copper sulphates.

An ideal low temperature synthetic route to prepare vanadates such as LiMnVO4 and LiNiVO4 with high purity, better morphology and good electrochemical behaviour was attempted via, Self Propagating High Temperature (SPHT) method using an internal fuel. Conventional solid-state fusion method and acetate precursor method were also carried out at high temperatures such as 1073 K and 873 K respectively and the results were compared with that of SPHT method. Samples obtained from these three methods were examined for their FTIR spectra, XRD pattern, surface area, particle size analysis and SEM. Both the compounds viz. LiMnVO4 with an orthorhombic spinel-related structure and LiNiVO4 with an inverse spinel structure were found to be synthesized stoichiometrically at a lower temperature (573 K) through SPHT method. Charge-discharge studies carried out at a constant current drain (0.1 mA) showed that these vanadates were electrochemically active. To understand the role of fuels in SPHT method, three fuels namely Urea (U) (FCV=6) and two other fuels with a high fuel calorific value viz. Glycine (FCV=9) and Pthalyl DiHydrazide (PDH) (FCV=37) were employed in the synthesis. Among the three fuels better products were obtained for Glycine and Pthalyl DiHydrazide (PDH) rather than Urea in terms of surface homogeneity and better morphology. Advantages of the SPHT method of synthesis and the role of fuels were discussed in detail.

The acidic dissociation constants of the protonated form of 2-amino-(2-hydroxymethyl)-propane-1,3-diol (tris) and 2-amino-2-methylpropane-1,3-diol (bis) in a series of water/1,2 dimethoxy ethane mixtures at 5 ºC temperature intervals from 25 to 45 ºC are determined using a cell without liquid junction potential. The corresponding thermodynamic parameters (ΔGº, ΔHº, and ΔSº) of the protonated form of these buffer indicators in the various solvent mixtures are evaluated. The variation of the standard free energy change of transfer with solvent compositions for both indicators indicates a characteristic minimum which is attributed to selective solvation of the individual species participating in the equilibrium process. Solubility measurements of these indicators at different temperatures are determined in order to evaluate the thermodynamic functions of the transfer of the free bases which may provide a better insight on the nature of the ion-solvent interactions and the relative basicity of the solvent.

This investigation deals with the stress corrosion cracking of AISI 304 stainless steel in concentrated boiling H3BO3 solution with NaCl additions. The characterization of the passive film was carried out using samples submitted to the corrosive environment, in both the annealed and cold worked states. The study shows that plastic deformation of the metallic material reduces polarization resistance and film stability (in terms of cathodic reduction). Furthermore, from the capacitance measurements, using the Mott-Schottky approach, it can be concluded that the influence of plastic deformation is higher when the passive film reveals p-type conductivity. Based on the electronic band structure of n-type and p-type semi-conductors, a representation for a possible correlation of charges of opposite sign between space charges and near surface dislocation is given.

Extensive quantities, like electrode reaction rates and double layer parameters, have to be referred to the unit real surface of the electrode, in order to compare the activity of different electrode materials and / or different preparation methods for the same material. The purpose of this paper is to review three methods usually used for the estimation of the real surface of CO3O4 and NiCO2O4 thin film electrodes. Two of them, capacitance ratio and voltammetric charge, are based on the voltammetric technique whereas the third one, Zn2+ ion adsorption (ZIA), is based on the ionic interaction at the solid-liquid interface. The applicability, advantages and limitations of the three methods are compared.

The influence of some chalcones on the corrosion of nickel in 1 N nitric acid solution has been studied using weight loss and galvanostatic polarization techniques. In general, at constant acid concentration, inhibitor efficiency increases with increase of concentration of inhibitor and decreases with rise in temperature. Polarization studies reveal that the compounds behave as cathodic inhibitors. The effect of temperature on corrosion inhibition has been studied and activation energy has been evaluated. The adsorption of the inhibitor on the nickel surface is found to obey the Frumkin adsorption isotherm. Some thermodynamic parameters are calculated and discussed.

Propylene carbonate (PC) and -butyrolactone (GBL) are important dipolar aprotic solvents good to prepare electrolyte solutions of lithium perchlorate applicable to rechargeable high energy batteries, as we have shown before [1-5]. In this work solution conductances of LiClO4 and (C2H5)4NBr in those solvents were measured at temperatures ranging from -30 ºC to +25 ºC. Limiting molar conductivities and association constants were evaluated through Viana and Calado [6] and Fuoss and Hsia’s equations [7]. Some thermodynamic functions resulting from (KA, T) variations are obtained and the results are discussed on this basis. The accuracy and precision of the results is high and they are where possible comparable to others previously published [8, 9].

Octadecanethiol self-assembled monolayer (OCSAM), which was first employed as one sensitizer for TiO2 nanoporous film electrode in the oxidation of methanol, was firstly investigated using electrochemical and photoelectrochemical methods. The photocurrent generated by using TiO2 nanoporous film electrode modified by OCSAM (18SH/ TiO2) is about 1.95 times larger than that without modification. In addition, the maximum absorption peak has shifted towards the infrared region for about 30nm when the TiO2 nanoporous film electrode modified with OCSAM. This paper has testified that the increased photocurrent could not be attributed to the photooxidation of octadecanethiol or the ethanol solvent simply. It was proposed that the change of surface structure of TiO2 nanoporous film electrode should be responsible for the phenomenon in some degree.

The surface analysis using interferometry studies clearly indicates that the oxidation cycling at high frequency at aluminium electrode in the 0.1M NaClO4 containing 10 mM sodium tetraphenylborate solution yields selective dissolution of faces other than (111) by forming pyramidal shape voids that is not due to the simultaneous dissolution and redeposition as in the case of Pt. It is also confirmed that surface smoothing and flattening on the aluminium surface is due to flattening process and the formation of voids are due to electrochemical dissolution.

Platinized carbon micro electrodes (~ 10 um diameter), positioned close (~ 5 um) to the cell membrane of a human fibroblast, the ensemble constituting a semi-artificial synapse, are used to monitor events at the cellular level. A few tens of femlomoles of reactive oxygen species produced and emitted by the cell upon mechanical pricking with a glass micropipette (~ 1 um diameter) are released into the liquid film of some hundred femtoliires comprised between the cytoplasmic membrane and the electrode surface, leading to a sudden and drastic rise in their concentrations (in the order of several micromoles). This oxidative stress-type response aims at disarming the aggressor and is thought to be shared by many (if not all) eukaryotic cells. This method allows to detect in real time and quantify the species constituting the oxidative burst cocktail: hydrogen peroxide, H2O2, peroxynitrite, ONO2-, nitrogen monoxide, N0°, and nitrite, NO2-. They are likely to derive ultimately from superoxide anion, O2°-, and nitrogen monoxide, NO*, synthesised by NADPH oxidase and NO synthase enzyme systems, respectively. By placing the microelectrode at different positions about the injured area of the cell membrane, it was concluded that the signals correspond to a spherical diffusion of the emitted electroactive species from a point-source.

There are many examples in the literature, and a few in commerce, of ihe employment of the electrochemistry of redox enzymes for analytical purposes. Though attention to the direct electrochemistry of redox enzymes is growing, so far, there have been no commercial applications of redox enzymes. That might be about to change.

Electrochemistry offers many methods for decreasing the contamination produced by different industrial processes. Among these, Electrochemical Synthesis and Electrochemical Degradation of Organic Compounds in wastewater are two of the more promising ones. There is no doubt that the best way to avoid pollution is to eliminate the process that produces it and in this way very polluting chemical syntheses can be substituted by more cleaner electrochemical processes. As an example of this, the electrochemical synthesis of L-cysteine derivatives from L-cystine will be described. As far as the second method is concerned, the electrochemical degradation of wastewater containing organics appears to be very effective when the wastewater can not be biologically treated and when the oxidation is only carried out until the wastewater can be treated by biological methods. Details of the procedure followed for the development of a pre-industrial process will be shown for the treatment of industrial wastewater containing phenol.