Enhancement of Diffusion at a Microelectrode

The response of a spherical electrode to a potential-step function in the limiting current region is given by 

At long times, spherical diffusion is predominant, the current becomes independent of time, and Eq. (14.46) takes the form 

in a spherical field of diffusion (which is achieved for a microelectrode after a time determined by its radius), one obtains an equation similar to that given for semiinfinite linear diffusion, except that the radius of the electrode plays the role of the Nernst diffusion-layer thickness and the limiting current density is independent of time. The validity of Eq. (14.47) is one of the incentives for fabricating 

When an ultra-microelectrode is used as an electroanalytical tool, the diffusion-limited current density is not affected by the rate of flow, which typically creates a diffusion layer thickness of the order of50-100 pm. This can be a great asset for online monitoring in industrial applications, where the flow rate may fluctuate and would otherwise have to be measured and corrected for. 

---