REPRODUCIBLE, RELIABLE AND RUGGED HG-PLATED IR-BASED MICROELECTRODE FOR IN-SITU MEASUREMENTS IN NATURAL-WATERS

Construction of a reproducible, reliable and rugged iridium-based microelectrode is described. Perfect electrical contact, Ir-glass sealing and Ir disk morphology are the key points for obtaining reproducible voltammetric sensors. These points are discussed in detail and optimal fabrication conditions are given. Electron bombardment under vacuum yielded good soldering between Ir and Cu electrical cable. Reproducible polishing of the microelectrode to a mirror like Ir disk surface is obtained with silicon carbide pads and diamond paste using an automatic home made polisher. Cyclic voltammetry and optical microscopy have been used to characterize this microelectrode. Hg-plated Ir-based microelectrodes are prepared by electrodeposition of mercury on the iridium disk substrate. Reproducibility and reliability close to 100% have been obtained for Ir-based microelectrode preparation, mercury layer deposition and trace metal measurements by differential pulse anodic stripping voltammetry (DPASV) and square wave anodic stripping voltammetry (SWASV) in synthetic solutions even at low ionic strength (10(-3) M). Continuous measurements over long periods of time indicate that the Hg-plated Ir-based microelectrodes can be used for several days without renewal of the mercury layer. These microelectrodes were applied to lead and cadmium speciation studies directly in river waters by SWASV without any separation. The results show that free Pb-II and Cd-II concentrations as low as 0.5 nM and 0.1 nM respectively can be determined by direct measurements, without perturbing the medium. The lifetime of these microelectrodes in the present state of the art is more than 2 years.