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.

 

[LudusRex]

Cyclic voltammetry is a technique used to study the redox behavior of a species in a solution. It involves sweeping the voltage applied to an electrode between two set values, typically in a sawtooth pattern. As the voltage is swept, the resulting current is measured and plotted against the voltage. This technique allows us to observe the oxidation and reduction reactions of the species as the voltage changes.

In cyclic voltammetry, the direction of the current is determined by the direction of the voltage sweep. When the voltage is swept from a more negative value to a more positive value, the current will flow in one direction. This is known as the forward sweep. However, when the voltage is swept in the opposite direction, from a more positive value to a more negative value, the current will flow in the opposite direction. This is known as the reverse sweep.

The reason for this change in current direction is due to the nature of the redox reactions occurring at the electrode surface. During the forward sweep, the applied voltage is more negative than the reduction potential of the species, causing it to undergo reduction and resulting in a flow of electrons from the electrode to the species. On the reverse sweep, the applied voltage is more positive than the oxidation potential of the species, causing it to undergo oxidation and resulting in a flow of electrons from the species to the electrode. This is why the current changes direction on the reverse sweep.

It is important to note that the magnitude of the voltage does not determine the direction of the current, but rather the direction of the voltage sweep. This is because the species in solution has a specific reduction potential and oxidation potential, and the direction of the current is determined by whether the applied voltage is more negative or more positive than these potentials.

In summary, the direction of the current in cyclic voltammetry is determined by the direction of the voltage sweep and the redox potentials of the species in solution. The change in current direction on the reverse sweep is due to the reversal of the redox reactions at the electrode surface.