Portugaliae
Electrochimica Acta

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.

The electrochemical investigation, by cyclic voltammetry and controlled potential electrolysis at Pt electrodes, is reported for complexes trans- or eis-[PtHRL„][R = cyano- or trifluoro-alkyl, phenyl or cyano-phenyl; L = tertiary phosphine or. 1/2 diphosphine] in acetonitrile, and they are shown to undergo Pt-H bond cleavage (with proton extrusion) upon anodic oxidation at ca. 0.3 ~ 1.7V vs. SCE.

The redox properties of various Schiff bases (HL, derived from condensation of tryptamine with salicylaldehyde derivatives) and of some of their complexes [ML^] (M = Co, Ni or Cu) have been studied by cyclic voltammetry and cathodic controlled potential electrolysis, in aprotic media. They present both anodic and cathodic processes which, in the case of the complexes, can be either ligand or metal centred, the redox potentials of the latter, for both the M(II/III) and M(I/II) redoxpairs, following the order: Co < Ni < Cu.

Reaction of ^a»s-[ReCl(N2)(dppe)2] (dppe=Ph2PCH2CH2PPh2) with NCR (R=aryl) in toluene and under sunlight gives the corresponding cis-[ReCl(NCR)(dppe)2] compounds. The electrochemistry of these complexes in 0.2M [Bu4N][BF4]/THF is studied by cyclic voltammetry and controlled potential electrolysis, at Pt electrodes, which indicate the occurrence of an electro induced cis-to-trans isomerization.

The electrochemical behaviour of some camphor(+) derivatives (L) is reported and their electronic properties discussed. The redox properties of some new palladium(II) complexes, [PdC^], [PdCl2(NCMe)L] and [PdCl2L], with such ligands, are also discussed. This study was done by cyclic voltmmetry at a Pt electrode in an aprotic medium, commonly 0.2M [Bu4N][BF4]/CH2Cl2- In a few cases, controlled potential electrolysis was performed at a Hg pool or a Pt gauze electrode, for cathodic or anodic processes, respectively.

The cyclic voltammetric behaviour of fraA7S-[ReX(CN)(dppe)2][NBu4] (X=CI or H) and of products derived from alkylation, such as trans-[ReH(CNMe)(dppe)2], in aprotic medium and at a Pt electrode, is reported.

By cyclic voltammetry, at a Pt electrode, complexes trans-rReXLL'41 (X = N3, NCO or NCS; L = N2 or CO; L'= 1/2 Ph2PCH2CH2PPh2 or PMe2Ph), in aprotic medium, undergo a first gi/as/'-reversible single electron anodic process at Eox1/2 in the range 0.12-0.36 (L = N2) or 0.55-0.73 V vs. SCE (L = CO). Within each of these series, the variation of the oxidation potential is determined by the effect of the pseudo-halide ligand (X) (Eox1/2 increases in the order N3 < NCO < NCS), following their net electron donor/acceptor ability, as measured by the PL ligand parameter (N3" > NCO" > NCS" » N2 > CO), and the observed values of Eox1/2 for the dinitrogen complexes are shown to agree with those predicted on the basis of the additive EL parameter. The values of the electron-richness parameter, Es, for the metal sites trans-{ReX(dppe)2} (X = N3, NCO or NCS) have also been obtained, and linear correlations between Eox1/2 and IR v(N2) or v(CO) frequencies have been recognised.

Electroreduction of methylisocyanide to dimethylamine has been achieved by controlled potential electrolysis at the cathodic wave of trans-[FeH(CNMe)(PhoPCH2CH2PPh2)0] [BF4], in 0.2 M [Bu4N] [BF4J/ tetrahydrofuran and in the presence of phenol.

Mean activity coefficients of NaCl have been calculated, by means of the Pitzer theory, in mixed solutions of NaCl and NaH2P04 and of NaCl and Na2HP04, with ionic strength lower than 2 mol kg-1, at 25 °C. Electromotive force measurements, using a silver, silver chloride and a sodium ion selective electrodes, show nernstian response, in those mixed electrolyte solutions.

Liquid junction potentials between 0,099 mol kg-1 HC1 and 0,100 mol kgaKCl were measured without and with bridge solutions. Ag.AgCl electrodes of the thermal electolytic type and a novel cell design with liquid junction formed in a capilary tube with cylindrical geometry were used. The results were compared with calculated and experimental values obtained with a similar system but more complicated than the present one.

The electrochemical oxidation mechanism of a group of dimeric Vinca alkaloid type antineoplastic agents showed that homogeneous chemical reactions are coupled with a mu-ltistep electron transfer process that is also dependent on pH. The irreversibility of the process leads to final products that form an unreactive film that strongly adsorbs on the electrode surface. The differences encountered in the anodic oxidation mechanisms of the compounds studied can be explained in relation to their toxic effects.

Voltammetric studies of the octahedral Co(III) complexes with a pentadentate ligand (PICDIEN) and a ligand X (X= Cl~, Br", H20) in aqueous solution of 0.1M in KN03, pH 7 at Pt electrode were performed and the corresponding results are presented in this paper. Experimental values of E° for the complexes under study were estimated. Peaks A'/ A at potentials between -0.4 e +0.4 V vs e s c were attributed to the reduction and oxidation of Co(III), while peaks B/B' at E°= +0.90 V vs e s c are most probably due to the oxidation of Br -.

A kinetic study of the electroless autocatalitic deposition of copper-cobalt alloys from hypophosphit solutions is presented. As the cobalt is present in solution, it deposits. Its function is to promote the reducing agent oxidation which is not catalysed by copper surfaces. Degreased aluminium wires were used as substract. Several parameters were varied around standard conditions. Deposition rates were determined by gravimetric measurements and the elementar composition of the metallic deposits by X rays analysis (EDS/WDS). Also the ionic composition of each solution was calculated presupposing thermodinamic equilibrium. Distinct mechanisms were found. Activ centers only control deposition rates when cobalt molar percentages lower than 5. Hypophosphit concentration is a determinant factor if bellow 600 mol/m3. The deposition rate is sensivel to small pH variations only between 8.5 and 11. Citrate and boric acid are important to increase the reaction but should not exceed 50 mol/m3 and 400 mol/m3 respectively. It is suggested that the species resulting from the boric acid ionization are protons accepters along the metalization process.