Portugaliae
Electrochimica Acta

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Scatchard, Buffle and van den Berg/Ruzic graphical methods for the analysis of complexation reactions of metal ions by soil and water organic matter (SWOM) samples using a simple 1:1 model, were assessed. Simulated and experimental (SWOM sample: Laurentian fulvic acid, FA) data sets were analysed. Simulated data shows that the presence of ligand heterogeneity results in deviations from linearity of the Scatchard and Buffle plots, while van den Berg/Ruzic plots remains linear. Average conditional equilibrium constants (K) and ligand concentration (LT) can be estimated from van den Berg/Ruzic plots. The graphical analysis of experimental data for the system FA plus Cu(TJ), obtained with a Cu(II) ion selective electrode, shows that the sample is characterised by ligand heterogeneity. The equilibrium parameters obtained from van den Berg/Ruzic plots are the following (standard deviation in parentheses): Log K = 5.56(1); LT = 44(2) M [{L}= 1.76(8) mmol/gFA].

Proton ligand dissociation and metal ligand stability constants of acrylonitrile (AN) with some transition metal ions in 0.1 M KC1 and 50% (v/v) methanol-water mixture were determined potentiometrically. In the presence of 2,2'-azobisisobutyronirrile as initiator of the proton-polymeric ligand dissociation of the metal polymeric ligand stability constants were also evaluated. The influences of temperature on the dissociation of AN and the stability of its metal complexes in the monomelic and polymeric forms were critically studied. Based on the thermodynamic functions, the dissociation process of AN was found non-spontaneous, endothennic and entropically unfavourable. The metal complexes formed exhibit spontaneous, endothermic and entropically favourable behavior. Key words: acrylonitrile, dissociation constants, stability constants, thermodynamics . Recently considerable interest has been focused on aqueous organometallic chemistry due to environmental and toxicological importance [1], In solution, polymer-metal complexes form microheterogeneous regions occupied by polymer backbone [2,3]. Polymeric ligands are used to select a specific metal ion from a mixture, thus to isolate important metals from wastewater [4]. Therefore, the aim of the present work was focused on studying the dissociation constants of AN and the stability constants for its metal complexes with Pb2+ , Cd2+ , Hg2+ , U022'1' and Th4+ in monomelic and polymeric forms employing potentiometric and thermodynamic measurements.

The effect of l-benzoyl-4-phenyl-3-thiosemicarbazide derivatives as inhibitors for the corrosion of copper in 3M HNO3 acid has been studied using weight loss, galvanostatic polarization and C.V. techniques. The inhibition efficiency calculated from the three measurements are in good agreement and depends on the type and concentration of inhibitor. Application of Hammett equation supported the obtained results. The effect of temperature was studied over a temperature range 30-60°C using weight loss technique. The inhibitors appear to function through general adsorption following the Langmuir adsorption isotherm. Quantum calculations were performed to give a more general picture of the nature of corrosion inhibition.

Electrochemical deposition and dissolution of zinc in acidic zinc plating bath solutions containing formaldehyde (FA), benzaldehyde (BA) or furfuraldehyde (FUA) has been studied by cyclicvoltammetric technique. Shapes of voltammograms, peak potentials and Tafel slopes are not affected by the presence of aldehydes in the plating bath solution. Aldehydes decreased the peak currents and the extent of decrease is in the order: FUA>BA>FA. This is accounted by Langmuir type of adsorption of aldehydes on zinc-solution interface and the electron density of oxygen atoms in aldehydes. SEM photographs of zinc deposits indicated the gradual decrease of grain size from FA to BA and FUA. Negative values of S°a and G°a suggested the considerable interaction of aldehydes on zinc surface with rigid adsorbed species.

Nickel secondary systems have high energy density, power density, rugged physical structure and good low temperature performance. Nickel secondary systems are mainly used in electric vehicles as well as in military and commercial aircraft. Electrochemical impregnation of active material onto porous nickel foam electrode containing different additives has been studied to give best electrochemical reversibility. The development of additives for nickel electrode is mainly aimed at increasing the discharge capacity and minimizing self-discharge. Porous nickel electrodes have been evaluated using cyclic voltammetry, potentiodynamic polarization and AC impedance measurements. All the addition agents favour the reversibility of the charge-storage reaction of nickel electrode. Addition of copper and thiourea increases the capacity values.

The redox reaction of V(V) in ammonia buffer solution of pH = 8.60 was studied by means of square-wave (SWV) and cyclic (CV) voltammetry. The redox reaction studied exhibits properties of a surface redox process in which both reactant and product of the redox reaction are immobilized to the electrode surface. A mathematical model for the electrode mechanism proposed was developed under conditions of square-wave voltammetry. In agreement with the theoretical findings, the phenomena of "split SW peaks" and "quasi-reversible maximum" are demonstrated experimentally. These unique features of the SWV response of a surface redox reaction are utilized for characterization of the redox process of V(V) in ammonia buffer. The following values of the kinetic parameters of the investigated redox reaction were estimated: standard rate constant Ks = 120 ± 10 s-1 and electron transfer coefficient = 0.4 ± 0.05. The formal potential of the redox couple V(V)/V(IV) in ammonia buffer with pH = 8.6 is E° = -0.56 V vs. Ag/AgCl (sat. KC1).

Electrochemical noise measurements and impedance analyses were performed on A516-70 carbon steel in chromate/chloride solutions. The noise data were used to calculate the noise resistance in the time domain, and the spectral noise resistance and spectral noise impedance in the frequency domain. It is shown that the spectral noise impedance is equivalent to the modules of electrode impedance over the experimental frequency range. This relationship is independent of the corrosion types (passivity, general corrosion and metastable pitting). The noise resistance coincides with the charge-transfer resistance and the spectral noise resistance only under certain conditions, such as passive state or general corrosion, where the noise resistance provides an indication of the corrosion resistance of the material under study. During pitting corrosion, the noise resistance is much lower than the charge-transfer resistance and only qualitatively, rather than quantitatively, tracks the change of corrosion resistance. The spectral noise resistance is consistent with the charge-transfer resistance and can be used to indicate the corrosivity of carbon steel under studied conditions.

A Nafion-coated wall-jet mercury film electrode was tested to determine cadmium and lead at trace levels in flowing systems by square wave anodic stripping voltammetry. Optimization of the experimental conditions and square wave parameters is discussed. For a 60 s deposition time, the detection limits restricted by the amount of cadmium and lead in the blank solution were lxlO"9 M and lxlO"10 M respectively. In comparison with the conventional mercury film electrode, the resistance to Triton X-100 interference was found to be superior with the Nafion film coating. The modified electrode was applied to trace metal detenninations in real environmental samples.

Sodium benzoate is a salt widely used in industry as preserver. Its activity coefficients in aqueous solutions can be estimated by Pitzer theory, once the necessary parameters are known. Electromotive force (emf) measurements on galvanic cells without liquid junction have been used for the determination of mean activity coefficients of sodium chloride in the ternary system sodium chloride + sodium benzoate (NaB) + water. Pitzer theory has been applied to this system and those data enabled the evaluation of the specific parameters for sodium benzoate at 298.15 K by linear regression. The values obtained for beta(NaB)(0) and beta(NaB)(1) parameters, respectively 0.177 kg mol-1 and 0.34 kg mol-1 are close to those published already for other carboxylic acid salts.

The procedure for the voltammetric determination of palladium(II) involves an adsorptive accumulation of palladium(II)-DMG complex on a stationary mercury electrode, followed by a stripping voltammetric measurement of reduction current of adsorbed palladium(II) chelate complex. The optimum conditions for the analysis of palladium(II) included pH (3.5 - 4) and an accumulation potential between 0 and - 0.3 V (vs. Ag/AgCl). The peak current is proportional to the Pd-DMG concentration and increases linearly with a preconcentration time.

Electrochemical oxidation of 3,5 dimethylpyrazole-pentacyanoferrate (II) complexes at glassy carbon electrode has been determined from analyses of voltamperometric curves at different pH values. The complex species formed are dependent on the pH media. Voltammograms display only a well-defined wave but the peak potentials shift to less positive values as the pH medium increases. The possible mechanisms of the reaction have been discussed.

Mutual differential diffusion coefficients of lithium chloride in moderately concentrated (0.1 to 1.0 mol dm-3) aqueous solutions were measured at 25 °C using an improved open-ended conductimetric capillary cell method. A critical examination of the response of the technique to such improvements is done on the basis of the analysis of different experimental parameters as well as from the reproducibility of the diffusion coefficients. The imprecision of the diffusion coefficients, generally < 1 %, is not affected by the improvements. The reported LiCl mutual diffusion coefficients agree with those obtained previously by Gouy interferometry, and Rayleigh interferometry within lower deviations than 2-3 %. Agar's semi-empirical equation for the concentration dependence of electrolyte diffusion coefficients is in good agreement with the experimental data.

Ni(II) and Zn(II) complexes of a macrocyclic diimine crown ether, 2,3,11,12-dibenzo-5,9-diaza-4,9(l0)-cisdiimino-l,13-dioxacyclopentadecane (O-en-N-tn), were prepared and their electrochemical behaviours were reported. Reduction of the complexes was studied in ethyl alcohol-water mixture (1:5 v/v) using sampled current polarographic, differential pulse polarographic, cyclic voltammetric, chronoamperometric and chronocoulometric techniques. A mechanism for the electrode reaction was proposed. Heterogeneous rate constant and the transfer coefficient of the electrode reaction were determined. Adsorptional characteristics of the complexes on mercury electrode were studied and adsorbed amounts of the complexes were found to be 10-11 mol/cm2 and 10-12 mol/cm2 order of magnitude in acidic and basic media respectively.

When experimental data for activity coefficients are analyzed,one typical feature of the variation of logarithm of mean ionic activity coefficient with ionic strength is the presence of a minimum in the curve. The presence of this minimum gives rise to a new point at which lnγ± takes a value of zero in spite of the ionic strength being different from zero. In this paper the position of both points has been determined for 151 electrolytes using Pitzer equations to describe the dependence of lnγ± on ionic strength.

Annual Index

The first and second dissociation constants of adipic acid have been determined by a precise emf method in water and in aqueous binary mixtures of ethanol, over a wide range of solvent composition (0-70%(w/w) ethanol). Measurements were conducted at different temperatures ranging from 30 °C to 60 °C at intervals of 10 °C, by emp measurements. The thermodynamic parameters (∆H°, ∆G° and ∆S°) for the first and second dissociation reactions have been computed, analyzed and discussed in terms of solute-solvent interactions. The pK values have been correlated with the mole fraction of the organic solvent and the effect of dielectric constant on the dissociation processes was then discussed.

Electrochemical behaviour of zinc-glycine complex has been studied in the presence of thiourea on platinum by cyclic voltammetric technique. The obtained results revealed that zinc-glycine complex undergoes successive reduction to zinc with the formation of monovalent zinc complex in a slow step. Thiourea in the plating bath solution hindered both the deposition and dissolution processes. Hydrogen evolution reaction was hindered by the adsorption of thiourea, which strengthened the M-H(ads) bond and raised the hydrogen over voltage.

The corrosion inhibition of nitrogen, oxygen and sulphur containing compounds in acidic chloride solutions was studied by chemical and electrochemical techniques. The percentage inhibition efficiency was calculated from both methods is in good agreement. The corrosion rate was found to be a function of the nature, concentration and temperature of the medium. The increase in temperature was found to increase the corrosion rate. The inhibitors are adsorbed on the C-steel surface according to the Langmuir isotherm. Some thermodynamic functions were computed and discussed. Quantum data were calculated and the results of HOMO and LUMO showed that the increase in energy of HOMO corresponds to an increase in the inhibition efficiency.

The study of the electronic structure of the passive films formed on high purity Fe-Cr, Fe-Cr-Ni and Fe-Cr-Ni-Mo stainless steels was carried out by photoelectrochemical measurements. Bandgap energy and quantum efficiency are associated with the changes promoted by the alloying elements in the bilayer structure of the passive films. The analysis of the photocurrent as a function of the applied potential reveals a Poole-Frenkel effect. The donor densities extracted from photoelectrochemical measurements are compared to those obtained in previous works from capacitance measurements.