Dear Raj,

I would say that the main reason why the ferri/ferrocyanide redox couple is typically used in those EIS experiments is because it follows an outer-sphere one electron transfer. This mechanism means that the rate of the electron transfer is going to depend on the electrode surface.

Furthermore it is a quite reversible process (but not totally) in most types of electrodes, so it is easy to work in conditions near to equilibrium. Typically EIS experiments are performed with small excitation potentials from the equilibrium potential of the redox couple (it is normally necessary to use a mixture of the redox couple ferri/ferro at the same concentration).

Hope it helps.

But sir i have problem to calculate Ret value from impedance Curve  by applying Nyquist plot. Can you help me for this Ret calculation.