Page 70 - Corrosion Engineering Principles and Practice

50 C h a p t e r 4 C o r r o s i o n T h e r m o d y n a m i c s 51

The equilibrium constant (K ) for the same reaction can be
eq
obtained with Eq. (4.7)
RT ln K eq = −∆ G = nFE (4.7)
0
0

4.2 Standard Electrode Potentials
The potential difference across an electrochemical cell is the potential
difference measured between two electronic conductors connected to
the electrodes. In the external circuit, the electrons will flow from the
most negative point to the most positive point and, by convention, the
current will flow in the opposite direction. Since the electrode potential
can be either positive or negative, the electrons in the external circuit
can also be said to flow from the least positive electrode to the most
positive electrode. A voltmeter may be used to measure the potential
differences across electrochemical cells but cannot measure directly
the actual potential of any single electrode. Nevertheless, it is
convenient to assign part of the cell potential to one electrode and part
to the other.
There are several potential benchmarks in common use, but the
most ancient is the half-cell in which hydrogen gas is bubbled over a
platinum electrode immersed in a solution having a known
concentration of hydrogen ions. This historically important reference
electrode is called the standard hydrogen electrode (SHE) if a
standard solution of acid is used. By definition, the equilibrium
potential of this electrode is zero at any temperature. However, the
SHE can be somewhat inconvenient to use because of the need to
supply hydrogen gas. Therefore, other reference electrodes are much
preferred for practical considerations.
The potential difference across a reversible cell made up of any
electrode and a SHE is called the reversible potential of that electrode,
E. If this other electrode is also being operated under standard
conditions of pressure and concentration, then the reversible potential
difference across the cell is the standard electrode potential E of that
0
electrode.
Tables of standard electrode potentials such as Table 4.1 and
Table 4.2 can be obtained if any one electrode, operated under
standard conditions, is designated as the standard electrode or
standard reference electrode with which other electrodes can be
compared.
Since an electrochemical reaction can be written either as an
oxidation or a reduction causing confusion in relation to the sign of
the potential of that reaction, a convention was adopted in Stockholm
in 1953 to write the standard potential of a reaction in reference to its
reduction (E 0 ) as shown in Table 4.1 and Table 4.2.
red