I have just started my master and I have never encountered with electrochemistry in depth before. I am currently doing a potential-step chronoamperometry to determine the diffusion coefficient of 1mM FcMeOH and 1 mM K3IrCl6 both in 0.5 M KCl. It was done by using a 25 micrometer diameter platinum electrode. I stepped the potential from 0 to 0.3 V for FcMeOH and from 0.5 to 0.85 V for K3IrCl6. The acquisition of the data was 2.5 ms. The thing was that I got a really high current for the first data and it was around 25 nA. The data was also taken with the interval time of 10 ms and interestingly, the current for the first data was a bit lower than before and it was around 15 nA. This is really important because I want to fit the experimental data with theoretical current and it would not fit well if the current for the first data is too high. But why do I get this really high current for the first data and does it really matter or can we just ignore the first few data? Please kindly help and advice me
Hi Saiful,
In practice, measures may have experimental limitations as Cottrell equation predicts, high current at short times. For example, limitations on the voltage/current output of the potentiostat make that the registry system saturated. There is also the limitation imposed by the charge double layer at t = 0. I thinks that you have to ignore those first points.
I hope this may help you
Hi Alvaro and Manzar, thank you so much for the answers. I found a book written by Zoski in 2007 and she said that this kind of error can be avoided by not taking any data for 5 to 10 time constant after the potential step. But how am I suppose to tell the NOVA 1.9 Autolab PGSTAT101 to do that?
It may help to export the data into another program that allows you to fit the data. You can do the fit with and also without the fist points and compare the results. That would be interesting to see.
I am currently using the Origin to interpret the data but have never used any other software other than Origin to treat the data. Apparently, K3IrCl6 gace a much lower current for the 1st data rather than the FcMeOH. So I would probably stick woth K3IrCl6 for now. Thank you for the help and support.
This may be helpful: http://www.asdlib.org/onlineArticles/elabware/kuwanaEC_lab/PDF-21-Experiment3.pdf
Hi Saiful, is the microelectrode you are using a disk? If so, I think that you may be complicating your life by doing chronoamperometry. I actually use 1 mM FcMeOH daily to characterize my electrodes (we make them ourselves so we need to determine the true size). If you want to determine the diffusion coefficient, I would suggest using cyclic voltammetry. Try going from -100mV up to 400mV. You should see an s-shape like the one I've attached here. Notice that there is a plateau at around 300 mV. This is called the steady-state current. Having this value makes your life easy since there is a nice equation you can then use to determine the diffusion coefficient:
I = 4 n F D C* a
I = steady-state current
n = number of electrons exchanged on the reaction = 1 for the FcMeOH oxidation
C* = Concentration of your species = 1 mmol for FcMeOH
a = radius of your electrode = 12.5 um for you
D = diffusion coefficient of your species = what you want!
Hopefully this helps!
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