Portugaliae
Electrochimica Acta

The electrochemical study of 4,iVii2,l,2,4,Triazole-pentacyanoferrate (II), 1,^/^2,1,2,4, Triazole-pentacyanoferrate (II) and Benzotriazole-pentacyanoferrate (II) complexes was carried out in a platinum rotating electrode at different pH values. For these complexes, values of the electrode rate constant, ko, and the transfer coefficient, ana have been estimated. The results obtained for these complexes were found to be PH-independent in a wide pH range.

Activity coefficients of NaCl and Na2Mal (Sodium Malonate) in their aqueous mixtures, NaCl+Na2Mal+H20 at 298.15 K, were determined from emf measurements of galvanic cells. The measurements were carried out at stoichiometric ionic strengths of 0.025, 0.05, 0.075, 0.1, 0.25, 0.5, 0.75, 1,2, ajid 3 mol kg1 and at series of ionic-strength fractions yB=3mB/(mA+3mB) within each of these ionic strengths. The results obtained were analysed by the Harned rule, Pitzer (s#clMal=0.0380 kg mol"1; i/'NaC1Mai =-0.0257 kg2mof2) and Lim (HOLL) equations.

A novel zinc-N.N'-dichlorodimethylhydantoin (Zn/DDH) organic primary cell system has been studied under various current drains. The attractive feature of this system Is its high open circuit (2.12 V) and closed circuit voltages (2.00 V) and non-toxicity in comparison with conventional aromatic nitro compounds (eg. Zn/m-dltrobenzene OCV: 1.10; CCV: 0.90 V). Several battery parameters viz. Internal resistance, capacity, energy density and coulomblc efficiency have been calculated.

The impedance spectra obtained at short exposure times for Al 7075-T6 immersed in 0.5 N NaCI do not agree with the pitting model proposed by Mansfeld and co-workers. Instead of a transmission line impedance, an inductive loop is observed at low frequencies. This result is due to the non-linearity of the system at short exposure times, when Ecorr is close to Ep^. The application of the anodic portion of the ac signal causes this non-linear behavior. The observed impedance is therefore a system response and not a true impedance. For longer exposure times agreement with the pitting model is observed.

The passivation and localized corrosion of (a-/3) brass is studied in borate-boric acid buffer + 0.5 M NaCl aqueous solution (pH 9) in the 0 -60 °C range using electrochemical techniques combined with surface microscopy. Passivation of a-fi brass in borate-boric acid buffer is due to the electroformation of a complex ZnO.xH 0 and Cu 0/CuO/Cu(HO) anodic 2 2 2 layer. In borate-boric acid buffer + 0.5 M NaCl passivity breakdown leading to pitting corrosion is observed when the applied potential exceeds E, the b breakdown potential. The E Vs T plot gives a straight line with slope b -5x10 V/ C. At a constant T the localized corrosion resistance of a-0 brass results considerably lower than that of polycrystalline Cu.

Voltammetric investigation of a chemical species after accumulation on a hanging mercury drop electrode can be very interesting chemically. Histidine-containing peptides, for example, are frequently found to have accumulated as copper(I) complexes. The mechanism of adsorption can be altered by modifying the HMDE, for example with polyaminoacids. Chemical transformations can be observed to be occurring on the electrode surface, particularly during repeated cycles in cyclic voltammetry. But is adsorptive stripping voltammetry (AdSV) useful analytically? It is an accepted method for the determination of at least eight trace metals in seawater and other water samples. Its acceptance is partly owing to the ease of destroying organic interferences by means of UV irradiation. But what about the determination of organics? Here, usually, some form of separation is required, and the use of solid phase extraction cartridges seems to be particularly appropriate.

The determination of sulfanilic acid in the colouring matter tartrazine was undertaken. Sulfanilic acid is determined by differential pulse polarography after derivatisation to an azo compound (diazotisation /coupling procedure). Being an azo compound too, tartrazine will interfere in the polarographic determination of derivatized sulfanilic acid. To solve this problem two approaches were tried successfully: a chromatographic separation and a masking procedure for tartrazine. Separation was achieved in a column packed with Amberlite XAD-2. Addition of gelatine caused the elimination of polarographic peak of tartrazine, allowing the determination of derivatized sulfanilic acid without need of a separation procedure.

A method for the determination of ascorbic acid is described, based on the derivatization of dehidroascorbic acid, (oxidazed form of ascorbic acid) with o-phenylenediamine to form aquinoxaline type compound, that is determined using differential-pulse polarography. A linear range between 10ug/L to lOxlO^ug/L was obtained. The method was applied to the determination of ascorbic acid in commercial fruit juices and compared with an established spectrophotometric method.

Square wave voltammetry (SWV) is applied to the anodic reaction of uric acid at a carbon electrode in aqueous solution. The electrochemical oxidation and adsorption characteristics of uric acid under these conditions are presented. In order to evaluate the analytical potentialities of this technique on the oxidative mode on a carbon electrode, reproducibility and sensitivity tests were carried out.

Results of an investigation of electrochemically catalysed nucleophilic substitution of 4-bromobenzophenone in the presence of phenoxide ions, as nucleophile, are reported. The data obtained are analysed by comparing the limiting currents of the reduction waves in the presence and in the absence of the nucleophile, and a value of 1.9 x 105 M"1 s-1 was obtained for the rate constant of the substitution reaction.

The electro oxidation of dimethylmineborane (DMAB) on specpure and electrolytic Ni and Co in succinate containing solutions of slightly acid pH has been studied by cyclic voltammetry and potential time measurements. The results have shown differences in electroactivity which can be related with surface morphologies. The study has been extended to electroless Ni-B and Co-B substrates and the electrocatalytic properties of the alloys compared to those of pure metals.

The electrochemical behaviour and complexation properties of synthetic macrocyclic crown ethers with six and seven oxygen donor atoms in the macroring. and attached to an anthraquinone unit towards the alkaline-earth cations, Mg2+ and Ba2+, are examined by cyclic voltammetry. The results indicate different cation binding enhancements with these electrochemically switched systems. The larger cation size and larger number of macroring donor atoms favour a stronger interaction between the cations and the reduced ligands.

The electro oxidation of methanol and iso-propanol has been examined on Au(100) and compared with the electro oxidation on non-modified and modified glassy carbon electrodes. The studies with Au(100) have pointed out the role of the OH" layer formation for the beginning of the electro oxidation of alcohols in alkaline solution (1,2) on noble metals. The small differences in the structure of alcohol (CH3OH and iso-propanol) cause the big differences in an electrochemical behaviour on noble electrochemical surfaces. The usually prepared GC electrode (3) is not active for the electro oxidation of alcohols at all and modified, Ag/GC electrode shows the different way of the adsorption-desorption processes of both alcohols connected with the electrochemical activity only deeply in the oxide region (5). The comparison with the electro oxidation of CH2O on noble (4) and non-noble electrodes (5) indicates that the most simple molecule with the aldehyde group exhibits the unexpected electrocatalytic behaviour at Ag-GC surfaces in the region of the potential before the oxide formation. That means the general conclusion is not possible to make; each molecule requires a special explanation of the electrocatalytic phenomena.