Your question does not have a simple answer. What is happening  on counter electrode depends on numerous factors. You may use 4 -electrode two compartment electrochemical cell to get some ideas about the processes on the counter electrode

Dear Mr Amon, in the counter electrode doesn't occur redox reaction, because it doesn't undergo polarization by the concentration. Counter electrode just serves to close the electrical circuit between working electrode and it. Previously, voltametry has been proposed to use two electrodes, where the reference electrode was used to applied and to measure the potential. In this last case, the reference electrode was not polarized. However, a distorced voltamogram was obtained with high resistance solutions. To avoid the polarization of the counter electrode, its area is many times higher than working electrode. For this, the counter electrode is a platinum coil. Besides, the counter electrode is connected to out of the opamp that the potential is applied. Therefore, the device doesn't measure electrical current of the redox reaction on the counter electrode. Of course, this is the theory. If a reaction is occuring in the counter electrode, this must be wrong. Best regards.

Thank your for your anwers!

Dear Mr. Fernandes,

if there is no redox reaction at the counter electrode, where does the current flow to?

Here is more detail about the practical problem that I am facing:

I have an analyte which can be oxidised and reduced. So when I analyse the solution after the experiment I find products from the oxidation of my analyte and also products which I suspect to result from the reduction of the analyte.

So my immediate question is: can it be that my analyte is oxidised at the working electrode and simultaneously reduced at the counter electrode?

Dear Mr Amon, I think that analyte is oxidised and reduced only at working electrode. Although, the electrical current flows between counter electrode and working electrode, the first is not polarized, because its area ís very large. The potentiostat applies a positive and negative potential. In positive potential, the substance must be reduced in working electrode, because the device supplies electrons to working electrode. When the potential is negative, occurs the opposite in working electrode. Pay attention, this is the convention for potential. Electrons have negative charge, therefore should be more logical that potential was negative. Best regards.

Thank you for your excellent answer Christian.

Suppose that my CE has similar dimensions as the WE ( not a good setup, but still..). Is it then possible that faradaic currents are the main contribution at the CE?  It appears to me, that in this case the cell is basically similar to an electrolysis cell? Am i wrong? Especially if the analyte concentration is very high -> high currents at the WE.

it is theoretically to  have WE: CE = 1: infinity ;

we  do not want CE to  interfere in the WE electro chemistry -

better use minimum 10 times area- use  divided  cells

Yes, but what happens if the areas are similar? I am interested in the consequences.

What processes would occur? How large are the errors of the measurement?

In your case, I think it is very likely that you are oxidizing the analyte at WE and simultaneously reducing it at CE. If, say, the analyte can not be reduced, what we might see at CE is the reduction of water, which is why we sometimes see bubbles on CE.

i shall explain -- theoretically the area of WE:CE=1:infinity

it means that the current density at the CE is negligible so that it remains as an ideally polarizable electrode

now in reality we can have 10 times the area of WE

the material you use as a CE should have a wide electrochemical window

the CE should not undergo dissolution/ passivation and so on

you can use a divided cell  so that the products formed if any at CE should interfere the WE

We do not bother about CE electro chemistry in CV

Thanks a lot Christian! That helped me alot. This Part of experiments is never covered in textbooks. Thank you for your competent answer!

Hi Christian,

Excellent. I have the same concern as Alfred. In addition, is the any reference that describes these relation (distance from CE to WE for ex.)?

Thanks

google primary current distribution in electrochemical cells