DIFFUSION-COEFFICIENTS DETERMINED WITH MICROELECTRODES

Steady-state amperometry at microdisk electrodes results in a current which is proportional to the electrode radius, the concentration of electroactive species, and the diffusion coefficient of the species electrolyzed. Measurements were made in this manner with platinum disks (r almost-equal-to 5-mu-m) in solutions of Fe(CN)6(3-) (0.5 M KCl, pH 3.0), ferrocene (0.2 M tetrabutylammonium perchlorate in acetonitrile), and Ru(NH3)6(3+) (phosphate buffer, pH 7.0). Subsequently, the electrodes' radii were determined by scanning electron microscopy and the diffusion coefficients were determined from the measured currents and other relevant parameters. In addition, chronoamperometric measurements were made with electrodes of 50-mu-m radius under conditions where time dependent currents are observed. The diffusion coefficients determined in this way were in good agreement with those determined under steady-state conditions. The values obtained at 25-degrees were 7.17 x 10(-6) cm2 s-1 for Fe(CN)6(3-), 2.17 x 10(-5) cm2 s-1 for ferrocene, and 5.48 x 10(-6) cm2 s-1 for Ru(NH3)6(3+).