Why does current change direction in cyclic voltammetry?

[LogicX]

You start at a certain voltage. Then you decrease this voltage to be more negative which reduces the analyte. Then you switch at a set voltage and increase the potential so that it is becoming more positive. Why does this switch the direction of the current so that on the reverse sweep the species is oxidized?

At the same magnitude current on the forward and reverse sweeps the species is either getting reduced or oxidized, respectively. I would have initially thought that because the voltage is the same at this point on the reverse scan, when you reach that voltage again you would simply continue to reduce the product more. But instead the flow of electrons reverses; why?

The difference is not in the magnitude of voltage but the direction of the sweep. How can this matter? Isn't a certain negative voltage still the same negative voltage regardless of which direction you are sweeping"? Shouldn't the current flow in the direction based on the sign of the voltage, not the direction of the sweep?

I'm fundamentally confused about this process.

 

[Borek]

I would have initially thought that because the voltage is the same at this point on the reverse scan, when you reach that voltage again you would simply continue to reduce the product more.

That would be true if the solution composition around electrode was the same all the time. Is it?

 

[LogicX]

That would be true if the solution composition around electrode was the same all the time. Is it?

No, you build up an amount of reduced product around the electrode.

But my same question stands. Why does that reduced product get oxidized on the reverse scan, but does nothing on the forward scan at the same voltage once it has been formed?

Take a snapshot in time at the same voltage on the forward and reverse scans. What is different so that current flows in opposite directions during each? I know the point I'm missing is that it is an equilibrium that has disrupted, but I can't wrap my head around what this actually means. The problem is that my understanding of echem is such that I think of a certain applied potential as driving an electron transfer process in one direction. So I don't see how the same voltage could also produce an electron transfer in he opposite direction.

I hope I'm being clear about my thought process.

(thanks for responding to both my threads! Sometimes I make the same thread on a chemistry forum when I am worried it is a more chemistry related problem)

 

[Borek]

Why does that reduced product get oxidized on the reverse scan, but does nothing on the forward scan at the same voltage once it has been formed?

When you sweep potential ratio of concentrations on the electrode surface is given by the Nernst equation (assuming reaction is reversible and fast enough). As the potential changes linearly with time during a forward scan concentration of the reduced form is always lower than the one dictated by the potential. That means during a forward sweep there is only one possible direction of the reaction.