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Analytical Electrochemistry: The Basic Concepts
1. Potential
Potential can be described as the work required to move an electron or other point
reference charge from an infinite distance away to a point of interest – inside of a metallic
electrode, for example. The magnitude of the potential at the surface of an electrode depends on
the excess charge that exists there above that of the metal alone. An external power supply is
capable of forcing excess electrons into or out of the electrode, leading to a non-equilibrium
condition at the interface. If a large excess of electrons is present
in the electrode, the potential of the electrode is caused to be very
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negative, with the energy of the electrons in the electrode being
Power supply
very high. Likewise, excess positive charge caused by the removal
of electrons from the electrode leads to a positive potential at the
electrode, and low electron energy.
The measurement of electrode energy requires that a
V
reference point be established, and that individual electrode
energies be taken relative to a potential that is fixed. This is
normally done by introducing a second electrode, called the
reference electrode, whose chemical composition is fixed and its
potential energy unchanging. Thus, the term potential, in an
electrochemical sense means the voltage difference between two
electrodes, and is denoted “E”, with units of volts. The
arrangement of a metal electrode immersed in a solution of
charged electrolyte along with a reference electrode is referred to
as an electrochemical cell (Figure 1), and E the cell potential.
Charge movement can occur between the metal electrode
Figure 1
and a solution species when the cell potential is sufficient to either
promote an electron into the lowest unoccupied molecular orbital (LUMO) of the solution
species (called a reduction) or to allow movement from the highest occupied molecular orbital of
the solution species (HOMO) into the metal electrode (called an oxidation). In either case, the
electrons involved in charge movement end up in a place offering them the lowest available
energy (“electrons are lazy”). This process is illustrated in Figure 2.
Because work is being done by the metal electrode, it is commonly referred to as the
working electrode. In the simplest description, the potential at which an electron transfer takes
place can be related to the overall free energy change (∆G ) of the reaction by
∆G = - n F E
where n is the stoichiometric number of electrons involved in the reaction and F is the Faraday
constant which relates the total charge (in coulombs, C) of the reaction to the amount of product
formed (96,485 C/mole for n = 1).
Figure 2
Ox + e-  Red
e-
LUMO
LUMO
Increasing
electron
energy
(potential)
HOMO
HOMO
There exists a well-defined, critical energy required for any electrode process, called the
E0. Tables of E0 values are available for many redox couples. The potentials found there were
measured for each half-cell with all species present at unit activity relative to the standard
hydrogen electrode. The standard hydrogen electrode (SHE) is a half-cell composed of an inert
solid electrode like platinum on which hydrogen gas is adsorbed at unit activity, immersed in a
solution containing hydrogen ions at unit activity. The half-cell reaction is given by
2H+ (aq) + 2 e-  H 2 (g)
E0 = 0.000 V
and it has arbitrarily been assigned a half-cell potential of 0.000 V.
At this point, it is adequate to view the oxidation/reduction process of a solution species
as a function of potential applied to the working electrode. If the working electrode is more
positive than the E0 for the solution species, the oxidized form of the species is stable at the
electrode surface. Further, if the working electrode is more negative than the E0, the reduced
form of the species will be stable. Together, the oxidized and reduced forms of the species
represent a redox couple.
Electron transfers of the type just described in which electrons are transferred across the
metal-solution interface with a resulting oxidation or reduction of a solution species are called
faradaic processes. Study of these charge transfer reactions is the goal of most techniques in
analytical electrochemistry. Unfortunately, the complexity of the interfacial system is such that
even in the absence of faradaic electron transfer, other processes do occur that can affect
electrode behavior. These processes include adsorption, desorption, and charging of the
interface as a result of changing electrode potential. These are called nonfaradaic processes, and
lead us to a short discussion of what is termed the electrical double layer.