Dear Sanjukta,

From my experience, this can have multiple reasons:

1) Changes in the electrical conductance of the electrolyte. You can counter check your EIS results with the help of a conductometer to get a feeling of the standard deviation of the electrolyte's conductance and Rs for multiple experiments.

2) Assuming the solution resistance is the only ohmic resistance at high frequency is a simplification in case the contacts and cables leading to the potentiostat are having basically no ohmic resistance. For instance, a slightly non-optimal back contact of the working electrode is a possible issue encountered in practice that easily adds couple of Ohms. In this case, re-adjustment of the back contact can easily be performed.

3) The reference electrode has an ohmic resistance that distorts the ideal high-frequency impedance modulus-plateau scenario if you are not using a platinum wire in parallel with the RE. Therefore, conventional RE's set practical limits for the high frequency part. It is typically in the range of 50 kHz. Depending until which frequency you are analysing your spectra this effect can alter the resulting R(s+eq)-value.

Please also consider the experimental chapters of these two wonderful EIS textbooks:

Impedance Spectroscopy: Theory, Experiment, and Applications

DOI:10.1002/0471716243

and

Electrochemical Impedance Spectroscopy

DOI:10.1002/9780470381588

Hope that helps and best regards

Dear Zamindar,

Based on my experience and adding to what Dr Otto explained, you also have to take into consideration whether the measurement you will take will be in OCP or in specific potentials, electrolyte and area of your sample, all of this will influence your analysis, and if your medium reaction is in an aqueous system, the pH of the solution is a crucial factor, in the literature you can find works on this, which may help to better understand

https://doi.org/10.1016/j.ssi.2019.115198, https://doi.org/10.1016/j.mseb.2022.115833, https://doi.org/10.1007/s10008-020-04532-2

Best Regards

Thank you so much, respected Martin Otto and João Carlos Martins da Costa for your valuable responses and suggestions.

The conductivity and permittivity of the solutions varied between the experiments. You could try this innovative method to check your samples:

Measurement Science an Technology DOI:10.1088/0957-0233/16/5/025

Jp Wu,John P. W. Stark

Thank you so much for your suggestion Jp Wu ..