I have to determine how many electrons are involved in each reduction peak in a cyclic voltagramme of a poly quinoidal system. I do not have access to polarography. Can I use computational (DFT) chemistry to determine the likelihood that 1 or 2 electrons are moving in each peak or is there an experimental way to get the data from my CV experiment. The equipment I am using is a Princeton Applied Research Potentiostat/Galvanostat 263A.
Ok... This is not an easy issue because coulometry can't really tell you how many electrons are in a peak of CV due to the different time scale (unless you have a very simple system) (imagine that your product gets electrolysed, then slowly reacts to form another product that is electrolysed at the same potential!)
I would recommend to read papers by Dennis H. Evans (he studied multiple electron transfers) or by Christian Amatore (he published a paper about how to measure the absolute electron counts for CV).
Chandan, attached the CV data, I'm sorry but I can't reveal the structure. I can tell you that its a poly-para-quinonimine.
Based on your CV and your equipment capability you can do the controlled potential coulometry to answer your question.
Many thanks Yurii V Geletii, any chance of a reference to look up on this. I'm not an electrochemist and will need to read up on the expt procedure.
Any textbooks on electrochemistry + a manual to your instrument. Personally, I like a book
Electrochemistry for Chemists, Donald T. Sawyer, Andrzej Sobkowiak
Ok... This is not an easy issue because coulometry can't really tell you how many electrons are in a peak of CV due to the different time scale (unless you have a very simple system) (imagine that your product gets electrolysed, then slowly reacts to form another product that is electrolysed at the same potential!)
I would recommend to read papers by Dennis H. Evans (he studied multiple electron transfers) or by Christian Amatore (he published a paper about how to measure the absolute electron counts for CV).
Yes, I recommend Yurii V. Geletii advice. Controlled potential coulometry is the best way, how to determine the number of electrons. Also, some information can be obtained from the height of the CV peak comparing with a standard. But it must be measured under limiting conditions, it means, not to be influenced by a chemical reaction = dissociation/protonation.
In my opinion, in a simple cyclic voltammogram each sharp peak is a consequence to a single electron process. On the other hand if you have a broad peak it may be a consequence to a processes that involve more than one electron. So you can try with recording cyclic voltammograms at very slow scan rates and maybe each broad peak will divide into single sharp peaks that involve only one electron.
Dear Sasho,
to my best knowledge this statement is not correct; "In my opinion, in a simple cyclic voltammogram each sharp peak is a consequence to a single electron process. On the other hand if you have a broad peak it may be a consequence to a processes that involve more than one electron"
Dear Kevin,
I agree with you that a bulk electrolysis is often not applicable. Based on practically reversible CV, it may work in this case. A digital simulation of CV might be helpful, but requires deep understanding of electrochemistry. Please, provide references on "papers by Dennis H. Evans (he studied multiple electron transfers) or by Christian Amatore (he published a paper about how to measure the absolute electron counts for CV) "
The simple answer is you can't.
However like Kevin i would suggest looking at for instance this paper by Christian Amatore (http://www.sciencedirect.com/science/article/pii/002207289080024Z)
But you need a second method either a potential step, microelectrode, rotating disc electrode experiment. And then preferably you should compare your results with a standard (for instance ferrocene). In that way you can often reduce the equation governing you electrochemical data to only contain two variable (electron count and diffusion constant). But if your peak is poorly seperated it can still be complicated and digital simulation might be needed.
Magnus, many thanks for your input, the paper you recommended is very interesting and quite relevant to our chemistry. Where can I learn about digital simulations?
In my opinion all peaks are an evidence for a one electron process, except the cathodic peak at -0.8 V that is for a two electron process (reduction from 0 to -2 if the numbers you have mentioned on the plot are the oxidation states).
The main idea is that you have the same quantity of your compound (measured in moles) that is reduced or oxidised at each step.
Then:
- use the potentiostat software to change the abscissa from potential to time (in seconds); now you see the same image, but it is a "chronoamperogram" (not a standard one, because the potential is swept): 4 anodic peaks, 3 cathodic peaks.
- try to deconvolute this new plot (the baseline is not horizontal) and to integrate the area below each separated peak (it is proportional to the quantity of charge).
- you will observe that all peaks have the same area/charge, except the cathodic one at -0.8 V that has an area /charge two times higher.
If still in doubt, send me a Excel file with your CV data, to try a deconvolution.
Dear Cornel, Super advice and we will get right on it. Many thanks! We will definitely get in touch if we run into difficulties. I appreciate your offer for help.
About digital simulation:
https://www.researchgate.net/publication/294885612_Digital_Simulation_in_Electrochemistry
Book Digital Simulation in Electrochemistry
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