This work describes the determination of pentachlorophenol (PCP) in pure and polluted waters by square wave voltammetry (SWV) and the influence of the electrode potential on PCP oxidation on boron doped diamond (BDD) electrodes. Measurements carried out in solutions of the Britton-Robinson buffer with pH = 5.5 revealed a single oxidation peak at 0.80 V vs. Ag/AgCl for PCP in a process that is controlled by the adsorption of the species. The detection limits obtained were 5.5 mg L-1 in pure water and 15.5 mg L-1 for water taken from a local creek, respectively. Controlled potential electrolyses were carried at 0.9, 2.0 and 3.0 V vs. Ag/AgCl and the solutions analysed by SWV, HPLC, chloride ion selective electrode and UV-vis spectroscopy. At low positive potential (0.9 V), the formation of an adherent film on the electrode surface involving the transference of 1 electron per PCP molecule was observed. At potentials close to the onset of O2 evolution (2.0 V), the formation of the corresponding quinone was detected. Electrolyses carried out well into the region of oxygen evolution (3.0 V) lead to the total combustion of PCP to CO2 and H2O as well as to the release into solution of 5 Cl- ions per PCP molecule destroyed.

A oxidação eletrocatalítica de ácido fórmico e formaldeído foi estudada por voltametria cíclica sobre eletrodos de Pd e da liga amorfa de Pd(Si), com os resultados obtidos sendo comparados com aqueles similares disponíveis na literatura. A oxidação do HCOOH ocorre sobre Pd por meio de desidrogenação catalítica direta via (:C(OH)2)ads, enquanto que sobre Pd(Si) não existem evidências da formação deste intermediário. Este fenômeno é uma conseqüência da presença de sítios inertes de Si na superfície, que diminui a probabilidade da ocorrência de sítios ativos adjacentes disponíveis para esta adsorção. Em altas concentrações de HCOOH, este intermediário sofre desidratação sobre a superfície de Pd e um pico de oxidação associado ao COads pode ser observado. Para o HCHO, o mecanismo de oxidação sobre os dois eletrodos é similar àqueles já discutidos na literatura para a Pt. Entretanto, os óxidos formados sobre a liga amorfa Pd(Si) são mais reativos que aqueles sobre Pd, afetando, assim, a cinética total do processo para ambas as moléculas orgânicas, o que é evidenciado pelo aumento nas correntes anódicas observado nos voltamogramas.

The electrocatalytic oxidation of formic acid and formaldehyde on Pd and on amorphous Pd(Si) was studied by cyclic voltammetry and the results compared with the literature for similar systems. The oxidation of HCOOH on Pd occurs through direct catalytic dehydrogenation via (:C(OH)2)ads while on Pd(Si) this intermediate does not appear to be formed. This is a consequence of the presence of inert Si on the surface that diminishes the probability of adjacent free sites. At high HCOOH concentrations, that intermediate undergoes dehydration on the Pd surface and COads oxidation peak is observed. For HCHO, the oxidation mechanism on both electrode materials appears similar to that previously proposed for Pt. However, the oxides formed on the amorphous Pd(Si) alloy are more reactive than those on Pd thus affecting the overall kinetics of the process for both organic molecules, a fact revealed by the increase in anodic currents observed in the voltammograms.

A eletrocristalização de níquel foi estudada sobre ultramicroeletrodo (UME) de platina. O comportamento dos transientes de corrente indicou um processo de nucleação instantânea sob controle cinético. A aplicação do modelo de Abyaneh mostrou que os núcleos crescem com uma geometria do tipo calota esférica, possuindo valores diferentes para as constantes de velocidade no sentido paralelo e perpendicular à superfície. As constantes de velocidade para o crescimento de núcleos no sentido paralelo variaram de 5,97 x 10-7 a 1,92 x 10-7 mol cm-2 s-1, com o aumento do potencial de -0,57 a -0,63 V, enquanto que as correspondentes constantes no sentido perpendicular apresentaram valores em torno de 1,44 x 10-7 mol cm-2 s-1. As relações entre estas forneceram os ângulos de contato, cujos valores aumentaram de 27,5 a 87,6 graus. Finalmente, o número de núcleos formados aumentou com o potencial, de 0,43 x 10(7) a 379,09 x 10(7) cm-2.

The electrocrystallization of nickel was studied on a platinum ultramicroelectrode (UME). The behaviour of the current transients pointed to an instantaneous nucleation process under kinetic control. The application of Abyaneh's model showed that nuclei grow in a spherical cap geometry with different values for both rate constants in the parallel and perpendicular directions. The parallel rate constants for nuclei growth varied from 5.97 x 10-7 to 1.92 x 10-7 mol cm-2 s-1 by increasing the step potential from -0.57 to -0.63 V, while the perpendicular rate constant remained constant around 1.44 x 10-7 mol cm-2 s-1. The relationships between these two constants give the contact angles of the crystallites, whose values also increased with potential from 27.5 to 87.6 degrees. Finally, the number of nuclei formed on the surface increased with potential from 0.43 x 10(7) to 379.09 x 10(7) cm-2.