Environmental Engineering Reference
In-Depth Information
reaction, the standard single-electrode potential will be
0.76 V (SHE). Thus
the cell potential as an algebraic sum of these two values will yield a value of
1.1 V. This has a positive sign and, therefore,
will yield a negative value
(Eq. 2.17) and the reactions as considered, i.e., the oxidation of zinc metal and
reduction of Cu
2
ions are thermodynamically favored. Thus in any electrochemi-
cal reaction, the most negative or active half-cell tends to be oxidized and the
most positive or noble half-cell tends to be reduced. The larger positive value
of redox potential is synonymous with high oxidizing ability of the reaction.
It is immediately apparent from the emf series that the metals below hydrogen
will corrode (at least, they will have the tendency to corrode) in oxygen-free acid
solutions with the evolution of hydrogen. If oxygen is available, the other reaction
of oxygen reduction forming water will also take place. The metals above hydro-
gen will have no tendency to react or corrode with the evolution of hydrogen in
oxygen-free acid solutions, e.g., Cu in H
2
SO
4
solution. However, if oxygen is
available, copper will readily corrode in acid solutions owing to the higher po-
sition of E
O
2
/H
2
O
with respect to E
Cu
/Cu
in the series. Copper will also have a ten-
dency to corrode in neutral solutions with dissolved oxygen because E
O
2
/OH
E
Cu
/Cu
. Gold and platinum are noble metals because they exhibit large positive
redox potentials and have no tendency to corrode except in the presence of ex-
tremely powerful oxidizing agents.
All of these considerations, however, are applicable to the systems at unit
activity. With the change of concentration of reactants and products, single-elec-
trode potentials change, and the tendency for reaction also changes accordingly.
The change in electrode potential with change in concentration and temperature
is calculated from the Nernst equation:
∆
G
°
RT
nF
log
a
oxid
a
red
E
E
0
2.3
(2.23)
where E is the single-electrode potential
E
0
is the standard single-electrode potential
R
is the gas constant
T
is absolute temperature
n
is the number of electrons transferred in the reaction
F
is the Faraday constant
a
oxid
and
a
red
are the activities of oxidized and reduced species, respectively.
Note that with an increase in the activity (concentration) of the oxidized spe-
cies, the potential becomes more positive. Thus, potential can be considered to
be a measure of the oxidizing power of the solution.
2.2.5 Potential-pH Diagrams
Potential-pH diagrams, also known as Pourbaix diagrams, are the graphical repre-
sentation of the stability of a metal and its corrosion products as a function of
