I am trying to understand the ohmic drop occur in the measurement of polarization resistance of steel embedded in concrete. In Linear polarization technique, ohmic drop concept is clear. In eletrochemical impedance spectroscopy, ohmic drop is not clear. We are applying a constant voltage and measure the frequency responses. How is that related to ohmic drop measurement?
dear Sripriya Rengaraju,
show us some impedance (data or graphs) for steel embedded in concrete. Then we will say how to find the ohmic drop.
Dear Sripriya,
Please see the links below. It may help you.
http://www.ecochemie.nl/download/Applicationnotes/Autolab_Application_Note_EC03.pdf
http://www.ecochemie.nl/download/Applicationnotes/Autolab_Application_Note_EC04.pdf
Dear Ioannis Samaras
I have attached a sample file which i got for one of my specimen. I want to understand how the ohmic drop occurs in the system when EIS is performed? Since we are measuring the frequency responses alone, how it is related to ohmic drop?
Dear Ram Pyar Singh,
Thank you for your notes.I need a clarification.
Are you saying the time constant in the EIS circuit is the ohmic drop?Can you please elaborate.
Dear Sripriya,
Time constant in the EIS circuit is NOT ohmic drop.
Ohmic drop depends on Ohmic resistance. And Ohmic resistance depends on the sample geometry and electrical conductivity. There are few formulas given in above links. You can use formula according to your sample geometry.
However, Using AC impedance spectroscopy, you can directly measure the ohmic resistance by fitting the impedance spectra. and then calculate the ohmic drop by using formula given below.
Also, I would like to say that I think that you are applying constant voltage and measuring the corresponding current at different frequency; i.e. frequency is sweeping.
By doing this, you will get a impedance spectra which is called as Nyquist plot. By fitting this spectra using software like Z View 2 can give ohmic resistance of your material.
dear Sripriya Rengaraju,
thank you for your impedance data. In time-domain exp. the ohmic drop will be equivalent to about 92 Ohms. You may find it from the : Zreal(1kHz to 10kHz) ~ (92+-5) Ohms.
Note : Zreal(~1kHz) is about the Rs in a Randles circuit [https://en.wikipedia.org/wiki/Randles_circuit]
Respected Ioannis Samaras sir,
I think, the value 92 ohm is not ohmic drop. Rather it is Ohmic Resistance of sample. Ohmic drop is a voltage drop which is a difference between applied voltage and actual voltage inside the sample.
I have also fitted the Impedance data, which gives R = 87.12 Ohms.
Dear Ram Pyar Singh,
good work. I said "the ohmic drop will be equivalent to... ". Your R = 87.12 Ohms is in the range Zreal~ (92 -5) Ohms. She may find Zreal with a spot (f~1kHz) measurement without a fitting.
Dear Ram Pyar Singh,
Ohmic drop= Current* R = I*87.12.
How will i get Current in this case?
Can you please elaborate?
For the high frequency measurement part (AC signal) the ohmic drop idea may be ignored.
In EIS measurement the potentiostat function is still in use: the WORK electrode is maintained at a constant potential (not constant voltage). If the measurement is made at OCP, the current (DC) is zero (and also the ohmic drop is zero).
You may also ignore the charging/discharging of the double layer (the WORK potential is maintained constant).
If the potential maintained at WORK during EIS is far from OCP, use the current value at this potential, measured in a previous recorded CV, to estimate iR drop.
An ohmic drop is relevant if > 1mV.
For your actual case: R= 90 ohm. The current must be > 10 μA to have a significant iR> 1 mV.
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