Cyclic Voltammetry

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Cyclic Voltammetry
Introduction
Electrochemical analyses can be thought of in terms of two broad classes of
measurement, one in which the potential that develops between two electrodes is measured
(potentiometry) and another in which the current that flows between two electrodes is measured
(amperometry).
In potentiometry, it often proves helpful to arrange things such that the current
is very low (e.g., by placing a high-resistance voltmeter in series between two electrodes).
The
electrochemical potential of one electrode (the reference electrode) is usually fixed, so the
measured cell potential can be interpreted in terms of an equilibrium half-cell reaction involving
an analyte species in contact with the other electrode (the working electrode).
In favorable cases,
one can use data from potentiometric measurements to calculate analyte concentrations directly
from the Nernst equation.
Potentiometry is a simple and straightforward analytical method, and
is routinely used to solve many problems in the analysis of electrochemically active and/or
charged analytes.
Figure 1. Schematic diagrams for two-electrode electrochemical experiments.
Left; Potentiometry. Right; Amperometry
An important assumption in potentiometry is that the measured potential accurately
reflects the equilibrium position of a well-defined electrochemical cell reaction.
Often this is not
the case, however, and potentiometric methods cannot be used.
In many situations, it is instead
more appropriate to control the potential of the working electrode (relative to a reference
electrode) and to measure the resulting current.
(Recall that current is simply the flow rate of
electrons in a circuit; an ampere of current corresponds to a coulomb of charge flowing per
second.)
The magnitude of the resulting current and its dependence on the applied potential then
provide the analytical information.
An experiment in which the potential applied to the working
electrode is swept at a constant sweep rate and the resulting current measured as a function of
potential is called a voltammetry experiment, and much of the recent interest in electroanalytical
chemistry stems from the use of voltammetry to obtain analytical (e.g., concentration),
thermodynamic (e.g., redox potentials and equilibrium constants), kinetic (e.g., rate constants for