Portugaliae
Electrochimica Acta

The present work describes the development of a new experimental set-up to allow determination of water in solid materials that can not be analysed by the traditional Karl Fischer's coulometric titration, as they are not soluble in Karl Fischer's solution and they only release water at high temperatures. For this purpose, a new tubular oven was designed and tested, where the temperature can be raised up to 1000 °C, which can be coupled to a Karl Fischer Coulometer. Different geological samples were tested, and the results were crossed with other traditionally used methods for this type of determinations.

Metallothioneins are a group of low molecular weight proteins, existing virtually in all living organisms. These proteins are rich in cysteine clusters, which are known to have the ability to complex heavy metals through their thiol groups. Metallothionein production and accumulation is induced by the presence of heavy metals; in the cell, metallothioneins act as protecting molecules by complexing the cations (avoiding them to be free). This (particular) characteristic allows these proteins to be used as biomarkers for heavy metal exposure of the living organisms in their biota. In order to overcome some of the actual limitations of the existing methods for metallothionein determination, a square wave adsorptive stripping voltammetric method is proposed in the present work using a hanging mercury drop electrode. The method development involved optimisation studies of instrumental and analytical conditions, and influence of interferences. The method was applied to samples extracted from fish liver, which was previously submitted to a controlled heavy metal exposure. The method presents a detection limit of 10-9 M of metallothionein and has a good linear range. The results obtained show that the method can be easily applied to metallothionein determination in trout liver samples submitted only to simple separation procedures.

An analytical equation corresponding to a reversible EE mechanism in differential pulse voltammetry is deduced. This technique is more suitable than d.c. voltammetry in order to distinguish an EE process from a single E mechanism when the formal potential values of both steps are close.

Theoretical expressions are presented which correspond to the response of adsorbed molecules which exhibit a reversible or totally irreversible charge transfer in Reciprocal Derivative Chronopotentiometry with constant current (RDC) and Reciprocal Derivative Chronopotentiometry with exponential current (RDCE). In spite of the fact that RDC enjoys an important role in the elucidation of electrode processes, when this technique is applied to adsorbed molecules exhibiting an irreversible charge transfer, peaks are not observed. Under these conditions RDCE turns out to be more suitable than RDC . Furthermore, the use of programmed currents makes the selection of an appropriate range of transition times easier than does the use of constant currents. Equations for the peak currents and peak potentials of the (dt/dE)/E and (de(wt)/dE)/E curves for reversible and totally irreversible process are given in order to show the advantages of the use of the (de(wt)/dE)/E curve when an exponential current time function is used.

The influence of the tetrabutyl ammonium cation (TBuA+) on the redox behaviour of the Cd(Hg)/Cd(II) couple in 1M chloride media was examined using differential pulse and cyclic voltammetry techniques. It was found that at low scan rates (tens to hundreds of mV/s) and with an increase in tetra butylammonium cation concentration, some results from both techniques are consistent with observations made in classic polarographic studies (e.g. the increase in tetrabutylammonium ion concentration in a solution containing the cadmium(II) ion, in chloride media, promotes a similar decrease in the cathodic current intensity of the metallic cation). At higher scan rates, the results of cyclic voltammetry experiments are suprisingly different. Among various effects observed, the presence of the tetrabutylammonium cation, below a certain concentration, seems to have no effect on the electroreduction of cadmium(ll) ions.

The cyclic voltametric behaviour of the biscyanamide-dihydride Mo complex [MoH2(NCNH2)2(dppe)2] [BF4]2 (dppe = Ph2PCH2CH2PPh2) is reported and interpreted by considering the occurrence of a cathodically-induced monodehydrogenation process of the cyanamide ligand.

The cyclic voltammetric behaviour of the allenylidene complexes trans- [FeBr(L)(depe)2][BF4] (L = CCC(Me)Ph 1a, CCCPh2 1b, CCCEt2 lc) is reported and the electrochemical PL and EL ligand parameters for the allenylidene ligands are estimated showing that they behave as slightly weaker net electron-donors than organonitriles.

By cyclic voltammetry in a methanolic solution, the amavadin model [Bu4N]2[V(HIDA)2] (HIDA= tribasic form of the N-hydroxy-,’ iminodiacetic acid) acts as an electron-transfer mediator in the electrocatalytic oxidation of pyrogallol (1,2,3-trihydroxybenzene). The mechanism of this anodic process is being investigated by digital simulation of cyclic voltammograms and a preliminary result is presented.

The cyclic voltarnmetric study of the following isocyanide complexes derived from trans-[FeH(CN)(dppe)2] 1 (dppe = Ph2PCH2CH2PPh2) enabled the estimate of the values of the electrochemical Pickett PL and Lever EL ligand parameters for the corresponding cyano-derived ligands: trans-[FeH(CNR)(dppe)2]A 2 [R = H(2a) or Et(2c), A- = BF4-; R = Me(2b), A- = I-; R = C(O)Ph(2d), A- = Cl-] or trans[FeH(CNX)(dppe)2] 3 [X = BPh3 (3a), VCl3(thf)2 (3b) or BF3 (3c)].

The reactions of 5 mM benzaldehyde at platinum in 0.1 M HClO4 were studied by means of differential electrochemical mass spectrometry (DEMS) combined with cyclic voltammetry. It was observed that this compound oxidises completely to CO2 at E > 0.60 V vs. RHE (reference hydrogen electrode), but also produces toluene, benzene and cyclohexane in the potential region for hydrogen adsorption and H2 evolution. No partial or total hydrogenation of toluene was observed. These results are compared and discussed with those previously obtained for benzyl alcohol and benzoic acid.

Annual Index

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.