Page 43 - Analytical Electrochemistry 2d Ed - Jospeh Wang

Analytical Electrochemistry, Second Edition. Joseph Wang
Copyright # 2000 Wiley-VCH
ISBNs: 0-471-28272-3 (Hardback); 0-471-22823-0 (Electronic)

CHAPTER 2

STUDY OF ELECTRODE REACTIONS

2-1 CYCLIC VOLTAMMETRY

Cyclic voltammetry is the most widely used technique for acquiring qualitative
information about electrochemical reactions. The power of cyclic voltammetry
results from its ability to rapidly provide considerable information on the thermo-
dynamics of redox processes, on the kinetics of heterogeneous electron-transfer
reactions, and on coupled chemical reactions or adsorption processes. Cyclic
voltammetry is often the ®rst experiment performed in an electroanalytical study.
In particular, it offers a rapid location of redox potentials of the electroactive species,
and convenient evaluation of the effect of media upon the redox process.
Cyclic voltammetry consists of scanning linearly the potential of a stationary
working electrode (in an unstirred solution) using a triangular potential waveform
(Figure 2-1). Depending on the information sought, single or multiple cycles can be
used. During the potential sweep, the potentiostat measures the current resulting
from the applied potential. The resulting plot of current versus potential is termed a
cyclic voltammogram. The cyclic voltammogram is a complicated, time-dependent
function of a large number of physical and chemical parameters.
Figure 2-2 illustrates the expected response of a reversible redox couple during a
single potential cycle. It is assumed that only the oxidized form O is present initially.
Thus, a negative-going potential scan is chosen for the ®rst half-cycle, starting from
a value where no reduction occurs. As the applied potential approaches the
characteristic E for the redox process, a cathodic current begins to increase, until

a peak is reached. After traversing the potential region in which the reduction
process takes place (at least 90=n mV beyond the peak), the direction of the potential
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