Application of thin-shielded mercury microelectrodes in anodic stripping voltammetry.

The performance in anodic stripping voltammetry (ASV) of hemispherical mercury microelectrodes, fabricated by electrodeposition of liquid mercury on the surface of Pt microdisks which were surrounded by a rather thick or thin insulating shield, was compared. The Pt microdisks were produced by sealing a wire of 25 microm diameter into a glass capillary, and by coating the cylindrical length of the Pt wire with a cathodic electrophoretic paint. The ratio of the overall tip radius b, to the basal radius of the electrode a, so-called RG=b/a, was equal to 110+/-10 and 1.52+/-0.01 for the thick- and thin-shielded microdisk, respectively. The mercury microelectrodes were characterized by cyclic voltammetry at 1 mVs(-1), in 1mM Ru(NH(3))(6)(3+) aqueous solution. The steady-state voltammogram recorded with the thin-shielded mercury microelectrode displayed less hysteresis, while the steady-state current was about 30% higher than that of the thicker one. This was a consequence of the additional flux due to diffusion from behind the plane of the electrode. The flux enhancement, which was operative at the thin-shielded mercury microelectrode during the deposition step in the ASV experiments, allowed recording stripping peaks for Cd and Pb, which resulted about 32% larger than those recorded at the thicker shielded mercury microelectrode, under same experimental conditions. The usefulness of the thin-shielded mercury microelectrode for ASV measurements in real samples was verified by determining the content of heavy metal ions released in the pore water (pH 4.5) of a soil slurry.

Analysis of pellets found in the north area of the Asiago plateau (Vicenza, Italy), a locality of the 1914-1918 World War.

Pellets of unknown material contained in an aluminium cylinder were found in the north area of the Asiago plateau (Vicenza), a locality of the First World War (1914-1918). Elemental analysis, infrared, chromatography and NMR experiments indicate that the main product is pentaerythritol tetranitrate (PETN). This substance was probably an igniting primer used by Austro-Hungarian (A.U.) military engineering. Hypothesis of medical use of the pellets can be disregarded while it seems improbable the use of this explosive by German Army during the Second War World.

Steady-state voltammetry of hydroxide ion oxidation in aqueous solutions containing ammonia.

An oxidation process observed in dilute aqueous solutions of ammonia was investigated under steady-state conditions with gold microelectrodes with radii in the range 2.5-30 microm. Over the ammonia concentration range 0.1-10 mM, a well-defined voltammetric wave was observed at approximately 1.4 V versus Ag/AgCl. It was attributed to the oxidation of hydroxide ions that arise from the dissociation of the weak base. The steady-state limiting current was found to depend on the concentration of supporting electrolyte, and in solution with low electrolyte, it was enhanced by migration contribution, as expected for a negatively charged species that oxidizes on a positively charged electrode. In addition, the steady-state limiting current was proportional to both the ammonia concentration and the electrode radius. The overall electrode process was analyzed in terms of a CE mechanism (homogeneous chemical reaction preceding the heterogeneous electron transfer) with a fast chemical reaction when measurements were carried out in solutions containing NH3 at < or = 5 mM and with electrodes having a radius of > or = 5 microm. This was ascertained by comparing experimental and theoretical data obtained by simulation. The formation of the soluble complex species Au(NH3)2+ was also considered as a possible alternative to explain the presence of the oxidation wave. This process however was ruled out, as the experimental data did not fit theoretical predictions in any of the conditions employed in the investigation. Instead, the direct oxidation of NH3, probably to N2O, was invoked to explain the anomalous currents found when the CE process was strongly kinetically hindered. Throughout this study, a parallel was made between the CE mechanism investigated here and that known to occur during the hydrogen evolution reaction from weak acids.

Oxygen as redox mediator in scanning electrochemical microscopy. Application to the study of localised acid attack of marble.

Oxygen from air-saturated aqueous solutions was employed as redox mediator in SECM experiments. Accurate approach curves under negative-feedback conditions were obtained using platinum and gold microelectrodes. Imaging experiments were also carried out, using a 2.5 microns gold microelectrode and oxygen that acted as distance mediator. The topographic images of a glass surface and that of a marble surface prior and after localised acid attack were recorded. High concentrations of hydrogen ions were produced locally, at the microelectrode tip held 3 microns above the marble surface, by applying a large enough positive potential within the oxygen evolution region. Under these conditions, the dissolution of CaCO3 occurred. Pits were produced, and the crater volumes thus obtained were linearly dependent on the electrolysis time.

Effects of the mercury ion concentration on the preparation of mercury deposits on platinum micro-disk electrodes.

In this paper, the effect of mercury ion concentration on the preparation of mercury microelectrodes fabricated on a platinum microdisk of 10 microns radius was studied. The preparation of the mercury microelectrodes was followed by chronoamperometry, and the measurements were performed in Hg2(NO3)2 solutions with concentrations over the range 0.1-12.5 mM. The mercury microelectrode size was referred to as h/a, where h is the height of the mercury deposit and a is the radius of the inlaid microdisk. The mercury microelectrodes investigated had h/a values over the range 0-2. The results obtained indicated that from concentrated mercury ion solutions (> 0.5 mM) the mercury growth was somehow erratic due to the coalescence of small mercury droplets. On the other hand, from dilute solutions (< 0.5 mM), the mercury deposits grew smoothly. Under the latter conditions the geometric coefficient k, which characterises the steady state diffusion limiting current expression at a mercury microelectrode, was determined with good accuracy (within 5% error) from the chronoamperometric curves recorded during the mercury microelectrode preparation. In general, the coefficient k was calculated from theoretical expressions derived for a sphere cap and spheroidal geometry, which may model the shape of the mercury deposits. The comparison between theoretical and experimental k values suggested that both geometries equally modelled the experimental mercury deposit obtained. Finally, for the sphere cap geometry an algebraic expression relating k and h/a was derived for an easier k evaluation from experimental h/a values.