Is it necessary to do EIS measurements for the analyte? Also in ferro/ferri slution?
It depends on the amplitude-frequency potential applied, because the energy required for the REDOX reaction comes from your external electromotive force.
No, it is not necessary to perform electrochemical impedance spectroscopy (EIS) measurements on the analyte solution alone when doing EIS characterizations. The key EIS data to collect is on the bare electrode and with the target redox species that will be analyzed.
Typically, EIS is used to characterize the interface, surface reactions, double layer effects, and charge transfer kinetics. So measurements are primarily done on:
The ferro/ferricyanide redox couple is a common benchmark probe for characterizing electrodes and interfaces. Any conductive electrode material will show a redox response.
But running EIS on just the Fe(CN)6 itself in electrolyte without an electrode does not tend to provide meaningful data. The key information comes from the interfacial impedance changes induced by the redox couple.
Thank you so much, Ahmed Emad Fathy Abbas and Zhangmin Zhang for your informative response.