Scanning electrochemical microscopy. 48. Hg/Pt hemispherical ultramicroelectrodes: fabrication and characterization

Hg/Pt hemispherical ultramicroelectrodes (UMEs) (25-[micro]m diameter) were prepared either by electrodeposition from a mercuric ion solution or by simple contact of the Pt disk to a hanging mercury drop electrode. The two methods produced equivalent tips. Optical inspection and electrochemical characterization of these Hg tips with methyl viologen, cobalt sepulchrate trichloride, and hexamineruthenium(III) chloride confirm the hemispherical nature of the UME. The scanning electrochemical microscopy approach curves for all three redox couples over a conductive substrate fit theoretical plots for hemispherical electrodes. The numerical solution of the diffusion equations for substrate generation--tip collection (SG-TC) transients for a finite Pt disk and Hg/Pt hemispherical UME are reported and compared to experimental results. A diffusion layer approximation is presented, and diffusion coefficients are extracted from the simulation. The SG-TC results reveal the enhanced sensitivity of hemispherical UME to radial diffusion.