Portugaliae
Electrochimica Acta

A comparative study of the electrodeposition of tin (II) sulphide on tin and tin amalgam, has been done in bicarbonate solutions using a potentiodynamic technique. The influence of sweep rate and sulphide concentration, on the peak -current has been studied. The results show, that in the early stages, the deposition takes place by a different mechanism in each substrate. However the thickning of the film seems to occur by diffusion through the solid phase, in both substrates, for high sulphide concentrations, while for low concentrations the rate-determining step is the diffusion in solution.

The formation of complexes [M0L2(dppe)^], [MoL^Cdppe)^] and [MoLL'(dppe^] (L=CNBut, L'=C0) (by imposing a cathodic clamp beyond the reduction peak of [MoC^idppe^] in the presence of L and L') was monitored by ramp-clamp reverse ramp voltammetry and, from the analysis of the product distribution, it was concluded that the unsaturated centres of the types {Moidppe^}, {MoLCdppe)^} and {MoL' (dppe^} discriminate (although equally) between the two substrates, the addition of CO at each site occurring 2.3 times as fast as the addition of CNBu*" to the corresponding site.

Electrochemical P ligand and E (electron-richness) and S (8) 3 (polarisability) metal site parameters, defined by other authors for octahedral 18-electron complexes with 16-electron square pyramidmetal centres, are extended to octahedral 18-electron compounds of the types [*1 L ] with 14-electron (n = 2) or facial 12-electron (n = 3) metal sites, by defining new overall P^^ ligand parameters as, e.g.,PnL-E1/2[Cr(CO)6-nLJ " E1/2[Cr, {cis-M(C0)4> (M = Cr, Mo, W) and {fac-M(CO)^ } (M = Cr, Mo) binding metal centres, and the corresponding ligand P^L and metal site (Es,B) parameters are estimated. For the c_is_- 14-electron metal sites (n = 2), bidentate chelating ligands (LL) are also considered.

A new on line technique, the packed bed wall-jet electrode, for the determination of total iron in hydroponic solutions has been developed. To avoid interference from 0- and to convert Fe(III) to Fe(II) a packed bed electrode is inserted upstream of a wall-jet electrode. The total iron is then determined by the oxidation of Fe(II) on the downstream electrode. The bed is shown to remove 99% of the 02 present in air saturated solutions, thereby removing the necessity for deoxygenating the test solution by bubbling N2. Linear calibration plots have been obtained for iron concentrations from 0.050 to 1.5 mmol dm and the technique has been tested in samples of nutrient film solution.

A model -is presented of the space charge layer formed •in semiconductors deposited as a thin layer on an insula tor substrate. The variations in charge transfer coefficients is stu died for tin oxide electrodes. The anodic current density arising from a tunnel effect is calculated by a simplified expression, which considers the variation in barrier width with applied potential.

Apresentam-se e discutem-se os resultados obtidos no estudo do comportamento de eléctrodos selectivos de suporte de resina condutora com sensor de sulfureto de prata (I) em resposta a baixas concentrações de catião prata(I). As calibrações foram realizadas com soluções padrões de nitrato de prata(I) (>10 M), com soluções tampões de complexaçao obtidos com o sistema catiao prata(I)-EDTA (10 a ~ 10 M em catião prata(I) livre), com soluções tampões de precipitação obtidas com halogenetos de prata(I) (10 a 10 M em catião livre) e com soluções de sulfureto com concentração de anião sulfureto fixada por variação de pC (10 a 10 M em catião prata(I) livre). Observou-se resposta linear em toda a gama de concentrações de 10 a -22 10 M em catiao prata(I).

The electron donor/acceptor character of a variety of organic groups (X),-CH(R) (NR') ,-CH(R) (OR') ,-CH(R) (OCOR' ) ,-CH(R)N and ^C=Y, was Z X investigated by studying the redox properties (by cyclic voltammetry), the electronic absorption and the ''"H n.m.r. spectra of substituted ferrocene complexes (FcX) with those groups as substituents. An ordering of these groups(X) according to their electron acceptor character was proposed on the basis of the electrochemical data, and the results were discussed in terms of the energy dependence of the involved molecular orbitals of FcX on the electronic properties of the substituent group(X).

The polarographic reduction of vitamin B1 was studied as a function of pH by using three different buffer solutions, namelly, acetate, phophate, and ammoniacal solutions. The results show the possibility of following electrochemical1y the transformations of thiamine in its yellow and thiole forms by measuring the height of the corresponding reduction waves for thiamine and its yellow form, and the height of the oxidation wave of the thiole form. A way of calculating the pK value of the transformation TH - YF is also suggested.

Existem muitos casos onde as respostas de frequência (curvas de impedância) nao podem ser predictas pela consideração de um circuito equivalente de Randles. As reacções nao são tão simples como as consideradas (oxidação ou redução num uníco passo) e um tratamento teórico mais elaborado é necessário para interpretar os resultados. Entre estes processos estão aqueles que envolvem intermediários adsorvidos. Nesta comunicação apresentam-se exemplos de reacções de corrosão onde esses intermediários se encontram presentes, as suas respostas no plano complexo de impedâncias e como a simulação de um circuito equivalente correspondente a esses processos mostra a presença desses mesmos intermediários. Estes reflectem-se no dito circuito equivalente pela presença de uma combinação em paralelo de uma resistência e de um condensador o qual, por sua vez, se encontra em série com a resistência de transferencia de carga e a resistência de transferência de massa (impedância de Warburg). Por sua vez, estes elementos estão em paralelo com a capacidade da dupla camada.