Page 19 - Analytical Electrochemistry 2d Ed - Jospeh Wang

4 FUNDAMENTAL CONCEPTS

current because it obeys Faraday's law (i.e. the reaction of 1mole of substance
involves a change of n 96,487 coulombs). The faradaic current is a direct measure
of the rate of the redox reaction. The resulting current±potential plot, known as the
voltammogram, is a display of current signal (vertical axis) versus the excitation
potential (horizontal axis). The exact shape and magnitude of the voltammetric
response is governed by the processes involved in the electrode reaction. The
total current is the summation of the faradaic currents for the sample and blank
solutions, as well as the nonfaradaic charging background current (discussed in
Section 1-3).
The pathway of the electrode reaction can be quite complicated, and takes place
in a sequence that involves several steps. The rate of such reactions is determined by
the slowest step in the sequence. Simple reactions involve only mass transport of the
electroactive species to the electrode surface, the electron transfer across the
interface, and the transport of the product back to the bulk solution. More complex
reactions include additional chemical and surface processes that precede or follow
the actual electron transfer. The net rate of the reaction, and hence the measured
current, may be limited either by mass transport of the reactant or by the rate of
electron transfer. The more sluggish process will be the rate-determining step.
Whether a given reaction is controlled by the mass transport or electron transfer is
usually determined by the type of compound being measured and by various
experimental conditions (electrode material, media, operating potential, mode of
mass transport, time scale, etc.). For a given system, the rate-determining step may
thus depend on the potential range under investigation. When the overall reaction is
controlled solely by the rate at which the electroactive species reach the surface (i.e.,
a facile electron transfer), the current is said to be mass transport-limited. Such
reactions are called nernstian or reversible, because they obey thermodynamic
relationships. Several important techniques (discussed in Chapter 4) rely on such
mass transport-limited conditions.

1-2.1 Mass Transport-Controlled Reactions
Mass transport occurs by three different modes:

DiffusionÐthe spontaneous movement under the in¯uence of concentration
gradient (i.e., from regions of high concentration to regions of lower concen-
tration), aimed at minimizing concentration differences.
ConvectionÐtransport to the electrode by a gross physical movement; such
¯uid ¯ow occurs with stirring or ¯ow of the solution and with rotation or
vibration of the electrode (i.e., forced convection) or due to density gradients
(i.e., natural convection);
MigrationÐmovement of charged particles along an electrical ®eld (i.e., the
charge is carried through the solution by ions according to their transference
number).
These modes of mass transport are illustrated in Figure 1-1.