Characterization
The utility of cyclic voltammetry is highly dependent on the analyte being studied. The analyte has to be redox active within the experimental potential window. It is also highly desirable for the analyte to display a reversible wave. A reversible wave is when an analyte is reduced or oxidized on a forward scan and is then reoxidized or rereduced in a predictable way on the return scan as shown in the first figure.
Even reversible couples contain polarization overpotential and thus display a hysteresis between absolute potential between the reduction (Epc) and oxidation peak (Epa). This overpotential emerges from a combination of analyte diffusion rates and the intrinsic activation barrier of transferring electrons from an electrode to analyte. A theoretical description of polarization overpotential is in part described by the Butler-Volmer equation and Cottrell equation. Conveniently in an ideal system the relationships reduces to, for an n electron process.
Reversible couples will display a ratio of the peak currents passed at reduction (ipc) and oxidation (ipa) that is near unity (1 = ipa/ipc). This ratio can be perturbed for reversible couples in the presence of a following chemical reaction, stripping wave, or nucleation event.
When such reversible peaks are observed thermodynamic information in the form of half cell potential E01/2 can be determined. When waves are semi-reversible such as when ipa/ipc is less than or greater than 1, it can be possible to determine even more information especially kinetic processes like following chemical reaction.
When waves are non-reversible it is impossible to determine what their thermodynamic E01/2 is with cyclic voltammetry. This E01/2 can be determined, however it often requires equal quantities of the analyte in both oxidation states. When a wave is non-reversible cyclic voltammetry can not determine if the wave is at its thermodynamic potential or shifted to a more extreme potential by some form of overpotential. The couple could be irreversible because of a following chemical process, a common example for transition metals is a shift in the geometry of the coordination sphere. If this is the case, then higher scan rates may show a reversible wave. It is also possible that the wave is irreversible due to a physical process most commonly some form of precipitation as discussed below. Some speculation can be made in regards to irreversible waves however they are generally outside the scope of cyclic voltammetry.
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