ELECTROCHEMICAL METHODS Fundamentals and Applications - Allen.J.Bard

12 Chapter 1. Introduction and Overview of Electrode Processes
Metal Solution Metal Solution
(a)
(b)
Figure 1.2.2 The metal-solution
interface as a capacitor with a
charge on the metal, q M , (a)
negative and (b) positive.
where q is the charge stored on the capacitor (in coulombs, С), Е is the potential across the
capacitor (in volts, V), and С is the capacitance (in farads, F). When a potential is applied
across a capacitor, charge will accumulate on its metal plates until q satisfies equation
1.2.4. During this charging process, a current (called the charging current) will flow. The
charge on the capacitor consists of an excess of electrons on one plate and a deficiency of
electrons on the other (Figure 1.2.1b). For example, if a 2-V battery is placed across a 10-
/л¥ capacitor, current will flow until 20 /лС has accumulated on the capacitor plates. The
magnitude of the current depends on the resistance in the circuit (see also Section 1.2.4).
The electrode-solution interface has been shown experimentally to behave like a capacitor,
and a model of the interfacial region somewhat resembling a capacitor can be
given. At a given potential, there will exist a charge on the metal electrode, q M , and a
charge in the solution, q s
(Figure 1.2.2). Whether the charge on the metal is negative or
positive with respect to the solution depends on the potential across the interface and the
composition of the solution. At all times, however, q M — -q s . (In an actual experimental
arrangement, two metal electrodes, and thus two interfaces, would have to be considered;
we concentrate our attention here on one of these and ignore what happens at the other.)
The charge on the metal, q M , represents an excess or deficiency of electrons and resides in
a very thin layer (