Usually, we evaluate the anti-corrosion performance of our specimens via EIS in a 100 ml beaker filled with 3.5% salt solution. In our research facility, we employ CH instruments electrochemical workstation for that purpose. We adopt a three-electrode assembly in which our specimen acts as working electrode, Pt wire as counter, and Ag/AgCl as a reference electrode. For each run, I always try to maintain the same distance between electrodes. However, for a similar sample (but a different specimen), I barely get similar results. There is always a difference of 5 to 20% in impedance values. I know external factors like immersion time and sample size also matter and I do consider those things.
Now I am thinking to reduce the distance between the electrodes. Will it work?
Generally we take the same distance between working electrode and reference electrode for each sample.
The distance should be large enough to prevent the current from directly bypassing the cells between the electrodes
Dear Nisar Ahmed ,
can you, please, both[1] Nyquist plot(s), in single graph[2] ?
1. Nisar Ahmed , quot.: "However, for a similar sample (but a different specimen), I barely get similar results. There is always a difference of 5 to 20% in impedance values."
2. A single Ohmic contact in each one specimen (but different in specimens) is a common issue in 'different' Nyquist plot(s), an Z_real shifting (control).
Did you check the stability of your reference electrode ? Also, how do you know that your samples are identical as you tell that these are from different specimens.
If you suspect non constant cell parameters are a source of error, I think reducing electrode distance may increase the error. Inasmuch smaller distances are more difficult to control than large distances so relative error in Rs will increase. If you repeat your experiment with the same specimen (not only a duplicate measurement, but also cleaning, remounting, fresh electrolyte, slightly different temp if working at RT etc...), do you observe a similar error?
Dear Nisar,
reading through your description, I feel that ~5% accuracy of your impedance measurements is to be expected in view of the fact that the usual, standard 3 electrode configuration in Electrochemical Impedance Spectroscopy is quite badly defined (geometry of the cell and electrodes) .
My suggestion is the same as that of Artur Braun : "Why don't you try out first."
There is no easy way around your problem and you will have to perform a whole number of calibration tests, testing the dependencies you talk about.
Although I am not a specialist on 3-electrodes corrosion EIS investigations, the few experiments of this kind we performed led us to the following conclusions in regards the experimental set-up:
1. All electrodes should be of the same shape (let us say disc like), the ~same area and they should be placed in parallel.
2. If the reference electrode, for example, is point-like (single wire sticking out of Ag/AgCl assembly), the impedance between the working electrode and the reference electrode is ill-defined and not really sufficiently reproducible
3. The optimal distance between the working electrode and the reference electrode has to be calibrated with sufficient precision. The important parameter here is the concentration of ions in the solution within the cell. Higher the concentration, the narrower is the interface region (that is , the sharp interface ~"Helmholtz planes" and the "Diffusion layer")
4. The more quasi-one dimensional is your cell, the better.
With best regards
Petr
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