Cyclic voltammetry

Links

- JavaScript version - runs directly in web browser (created by Leo Smith, 2023 Concordia chemistry graduate)
- (LabVIEW version is still in creation)

Description

This web-based simulator assumes a solution of either the oxidized (Ox) or reduced (Red) form of an Ox/Red redox couple:

Ox + ne− ⇌ Red

It shows how the voltammogram responds when the following parameters are changed:

Half-reaction

- standard reduction potential, Eo
- standard rate constant, ko
- number of electrons, n
- transfer coefficient, α
- diffusion coefficient, D

Triangle waveform

- starting potential, Einitial
- switching potential, Eswitch
- scan rate, ν

Experimental conditions

- bulk concentration of Ox, Cb
- electrode area, A
- temperature, T

Randles–Sevcik equation

The peak current (ip) in a cyclic voltammetry experiment at a stationary electrode is described by the Randles–Sevcik equation:

ip = 0.4463 nFACb ( nFνD / RT )1/2

which includes the constants not seen above:

Faraday constant, F (96485 C/mol)
Gas constant, R (8.314 J K−1 mol−1)

EC mechanism

An EC mechanism occurs when a chemical reaction (C) follows an electrochemical reaction (E). Changing the 1st order rate constant (k1) shows the effect of this parameter on the resulting voltammogram.

Electrode rotation

The shape of a voltammogram changes significantly when the solution is stirred. One of the most straightforward methods of generating a constant rate of stirring is by rotating the electrode, which brings the solution to the electrode surface at a constant and controlled rate. The dynamics of the rotating disk electrode (RDE, shown at right) have been thoroughly examined. The limiting current (il) at an RDE is described by the Levich equation:

il = 0.62 nFAD2/3ν-1/6ω1/2Cb

which includes the parameters not seen above:

rotation rate, ω
kinematic viscosity, ν (which in the Levich equation is not the scan rate)

Both ω and ν can be adjusted in the simulator to see how they affect the voltammogram.

Guiding questions

(Refresh the screen as you begin each question to return to the default parameters.)

If you double the concentration (Cb), what happens to the peak current (ip)? Is the change in ip consistent with what you would predict from the Randles–Sevcik equation? Explain.
What is the value of the peak current (ip) if you start with a scan rate (ν) of 25 mV/s? What about 100 mV/s? Is this change in ip consistent with what you would predict from the Randles–Sevcik equation? Explain.
How does the voltammogram change when n increases from 1 to 2 to 3? The difference between the potentials of the peak anodic current (Epa) and the peak cathodic current (Epc) is expected to follow the relationship: Epa - Epc = 0.059/n. Estimate the values of Epa and Epc at n = 1, 2, and 3 to determine if this is true.
Beginning with the 1st order rate constant (k1) equal to zero, start increasing the value of k1 by dragging the slider to the right. How does the voltammogram change? How can you explain this behavior?
How does increasing the electrode rotation rate (ω) change the voltammogram? What happens to the oxidation signal? How do you explain this?

Direct questions or comments to Leo Smith or Prof. Mark Jensen

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