Page 50 - Analytical Electrochemistry 2d Ed - Jospeh Wang

2-1 CYCLIC VOLTAMMETRY 35

time scale is around 0.1±1000 ms. Ultramicroelectrodes (discussed in Section 4-5.4)
offer the use of much faster scan rates and hence the possibility of shifting the upper
limit of follow-up rate constants measurable by cyclic voltammetry (3). For example,
highly reactive species generated by the electron transfer, and living for 25 ns can be
6
1
detected using a scan rate of 10 Vs . A wide variety of fast reactions (including
isomerization and dimerization) can thus be probed. The extraction of such
information commonly requires background subtraction to correct for the large
charging current contribution associated with ultrafast scan rates.
A special case of the EC mechanism is the catalytic regeneration of O during the
chemical step:

O ‡ ne „ R 2-8†

R ‡ A „ O 2-9†

An example of such a catalytic EC process is the oxidation of dopamine in the
presence of ascorbic acid (4). The dopamine quinone formed in the redox step is
reduced back to dopamine by the ascorbate ion. The peak ratio for such a catalytic
reaction is always unity.
Other reaction mechanisms can be elucidated in a similar fashion. For example,
for a CE mechanism, where a slow chemical reaction precedes the electron transfer,
the ratio of i =i is generally larger than unity, and approaches unity as the scan
p;r p;f
rate decreases. The reverse peak is usually not affected by the coupled reaction,
while the forward peak is no longer proportional to the square root of the scan rate.
ECE processes, with a chemical step being interposed between electron transfer
steps,

O ‡ ne „ R ! O ‡ ne ! R 2 2-10†
1
1
2
are also easily explored by cyclic voltammetry, because the two redox couples can be
observed separately. The rate constant of the chemical step can thus be estimated
from the relative sizes of the two cyclic voltammetric peaks.
Many anodic oxidations involve an ECE pathway. For example, the neurotrans-
mitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to
leukoadrenochrome. The latter can rapidly undergo electron transfer to form
adrenochrome (5). The electrochemical oxidation of aniline is another classical
example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a
dimerization reaction to yield an easily oxidized p-aminodiphenylamine product.
Another example (of industrial relevance) is the reductive coupling of activated
ole®ns to yield a radical anion, which reacts with the parent ole®n to give a reducible
dimer (7). If the chemical step is very fast (in comparison to the electron-transfer
process), the system will behave as an EE mechanism (of two successive charge-
transfer steps). Table 2-1 summarizes common electrochemical mechanisms invol-
ving coupled chemical reactions. Powerful cyclic voltammetric computational
simulators, exploring the behavior of virtually any user-speci®c mechanism, have