For the macroelectrode, one can use the Warburg impedance to describe the diffusion coefficient. This Demonstration shows the analytical solution for the diffusional impedance of a micro-disk electrode. It is a function of frequency, kinetics, concentration, and the radius of the microelectrode.
The diffusional impedance can be simulated by the finite-element method (FEM) or by the finite-difference method (FDM).  formulates the analytical expression for the real and the imaginary parts of the diffusional impedance:
= radius of microelectrode,
= diffusion coefficient,
= angular frequency.
The numeric values of the functions and are given in this Demonstration; you can vary the parameters , , and to investigate the diffusional impedance .
 M. Fleischmann and S. Pons, "The Behavior of Microdisk and Microring Electrodes. Mass Transport to the Disk in the Unsteady State: The AC Response," Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 250(2), 1988 pp. 277–283.
 M. Keddam, N. Portail, D. Trinh, and V. Vivier, "Progress in Scanning Electrochemical Microscopy by Coupling with Electrochemical Impedance and Quartz Crystal Microbalance," ChemPhysChem, 10(18), 2009 pp. 3175–3182.