VOLTAMMETRIC MEASUREMENT OF ULTRASLOW DIFFUSION RATES IN POLYMERIC MEDIA WITH MICRODISK ELECTRODES

Cyclic voltammetry and chronoamperometry were used in conjunction with microdisk electrodes in polymeric solvents to measure diffusion coefficients of ferrocene redox solutes down to 10‒12 cm2/s. Experimental manipulation of the parameter τ (τ = 4Dt/r2) results In diffusion profiles dominated by radial diffusion, linear diffusion, or a combination of the two. In the former two cases, adherence to the steady-state current equation for a microdisk and the Cottrell equation, respectively, was observed. Under conditions of mixed contribution, evaluation of diffusion coefficients was based on comparison of experimental and simulated current-time curves. Generally, diffusion coefficients >10‒7 cm2/s were evaluated from steady-state potential sweeps. Diffusion coefficients <10‒11 cm2/s could be evaluated by a standard Cottrell analysis. Values in the “transition region” were evaluated from the slope of nonzero Intercept Cottrell plots. A numerical guideline, based on τ, is presented. © 1990, American Chemical Society. All rights reserved.