Portugaliae
Electrochimica Acta

The polarographic behaviour of Co(II) in Britton-Robinson media is studied by Kalousek's polarographic methods. The experimental contribution at the study of the reduction of Co(II) and re-oxidation of Co(0), mechanism which takes place on the d.m.e.is discussed. The na^ and na^ values correspond to processes where monoelectronic transfer steps are coupled with chemical-physical steps.

Differential pulse polarography and anodic stripping voltammetry can be used directly in suspensions without pre-filtration; heavy metals like copper(II) and lead(II) adsorbed on hydrous oxides such as silica and ct-goethite are not being released during the time scale of the techniques and are thus not sensed by these techniques. In samples with colloids and organics such as EDTA and NTA in the presence of heavy metals, when the complexes are non-labile but electro-chemically reducible at a more negative potential, one can determine the ligand concentration and the binding capacity of the colloid from the voltammetric measurements of individual batches with different amounts of cations like copper(II) and lead(II). From the differential pulse measurements of solutions with heavy metals and the organic ligands EDTA and NTA, it is possible to know directly the amount of metal bound to each one of the ligands and the free cation. This can also be determined by anodic stripping voltammetry at different deposition potentials.

The synthesis and the differential pulse polarographic analysis of ten 6-acyl-2(3)-benzoxazolone derivatives are presented. pH 3.00 Britton-Robinson buffer was found to be.the best supporting electrolyte. The electrochemical determination is based on the reduction of carbonyl group present in all molecules. Linear response was observed from 0.2-2 to 25-65 mg.L depending on the molecules.

The Electrochemical behaviour of mild steel in equimolar carbonate/bicarbonate solutions was investigated by means of cyclic voltammetry. In the pre-passive region the presence of three anodic peaks (peak I, II and III) indicated the participation of Fe(0), Fe(II) and Fe(III) in the electrochemical processes. The participation of carbonate/bicarbonate species in this region was detected by X-Rays diffraction. At potentials > + 0.800 V (SCE) the transpassive region begins with a sharp increase in current giving place to a novel peak preceding oxygen evolution. This peak is associated to Fe(VI) species, also being observed by the authors in the absence of carbonate/bicarbonate for solutions of NaOH with pH > 11.06. The reduction peaks detected in the reverse scan (peaks VI, VIII and IX) correspond to the anodic peaks V, III and II respectively. This characterization was made by sweep reversal studies. A subsequent chemical reaction is thought to be coupled to the electrochemical processes taking place in the transpassive region. The present research work aims to contribute determination of the mechanism in the transpassive Further work related to the characterization transpassive peak is in progress.

Studies of passive film on AISI 304 stainless steel in alkaline medium (pH = 9.2) with and without Cl~ addition were carried out by using photoelectrochemical and impedance techniques. For the band gap energy of the oxide film values of 2.65 - 2.75 eV were estimated and for the flat potentials values between -430 and -530mV (SCE) were calculated. The variation of the capacitance of the film with the potential of its formation was also studied.