Cyclic voltammetry peaks

[jayakumar]

In cyclic voltammogram of a reversible single electron transfer
system, why do the reduction and oxidation peaks result at different
potentials (voltages)? Should not they come at the same voltages ?

When this Q was asked during an interview, my answer was if the
reduced/oxidized specie is adsorbed to the electrode, then the peaks
may come at the same potential. Since they have to diffuse through the
solution to reach the electrode surface, that results in the shift ie
the lag in the peaks. But the interviewer was not convinced. I could
not find answer is standard text books.

Can anyone help me to get the answer?

[Dieter Britz]

Post by jayakumar
In cyclic voltammogram of a reversible single electron transfer
system, why do the reduction and oxidation peaks result at different
potentials (voltages)? Should not they come at the same voltages ?
When this Q was asked during an interview, my answer was if the
reduced/oxidized specie is adsorbed to the electrode, then the peaks
may come at the same potential. Since they have to diffuse through the
solution to reach the electrode surface, that results in the shift ie
the lag in the peaks. But the interviewer was not convinced. I could
not find answer is standard text books.
Can anyone help me to get the answer?

Not a bad answer. There is a clear description in Bard & Faulkner 2nd Ed,
page 227. The lag has to do with the lowering of the surface concentration
of the electroactive substance, increasing the concentration gradient, and
this shifts the peak potential past the equilibrium potential. Further on,m
the solution gets depleted so the current drops again.

[jayakumar]

Post by Dieter Britz

Post by jayakumar
In cyclic voltammogram of a reversible single electron transfer
system, why do the reduction and oxidation peaks result at different
potentials (voltages)? Should not they come at the same voltages ?
When this Q was asked during an interview, my answer was if the
reduced/oxidized specie is adsorbed to the electrode, then the peaks
may come at the same potential. Since they have to diffuse through the
solution to reach the electrode surface, that results in the shift ie
the lag in the peaks. But the interviewer was not convinced. I could
not find answer is standard text books.
Can anyone help me to get the answer?

Not a bad answer. There is a clear description in Bard & Faulkner 2nd Ed,
page 227. The lag has to do with the lowering of the surface concentration
of the electroactive substance, increasing the concentration gradient, and
this shifts the peak potential past the equilibrium potential. Further on,m
the solution gets depleted so the current drops again.
--
Dieter Britz (dieterhansbritz<at>gmail.com)

Thanks.When I saw the Bard and Faulkner p.227 it explain the general
mechanism under which the redox process occurs during a CV
measurement. Does not answer directly my Q. However, from your answer
may I relate the drop in concentration with the diffusion of Red
species.

Part of the Red specie, after electron transfer in the forward step,
diffuses away from the electrode surface. As a result, the E value
shifts as given by Nernst equation. I this right ? But this brings
another Q, if part of the Red species is diffused how come the ratio
of peak currents (Ic/Ia) can be still be unity. Doesnt this mean that
the part of Red specie which has escaped is also oxidised
subsequently?

[Dieter Britz]

Post by jayakumar
Part of the Red specie, after electron transfer in the forward step,
diffuses away from the electrode surface. As a result, the E value
shifts as given by Nernst equation. I this right ? But this brings
another Q, if part of the Red species is diffused how come the ratio
of peak currents (Ic/Ia) can be still be unity. Doesnt this mean that
the part of Red specie which has escaped is also oxidised
subsequently?

Basically and briefly, during the forward scan there are changes out
to a certain distance, meaning that there has been transport to and
from that far away from the electrode; and on the reverse scan the
same thing happens, so nothing escapes - it can all be reoxidised.