Environmental Engineering Reference
In-Depth Information
Figure 2.10
Hydrogen-hydrogen ion exchange current densities on different metals.
graphically in Fig. 2.10. On the mercury surface, the equilibrium is attained with
much sluggishness, whereas on a platinum surface it is readily established giving
a large value of exchange current density. A platinized platinum surface, because
of its large projected surface area, gives a still higher value of exchange current
density. Presence of trace impurities such as arsenic, sulfur, and antimony com-
pounds reduces the exchange current density for hydrogen equilibrium drasti-
cally, whereas an increase in temperature raises exchange current density.
2.3.2 Polarization
As the electrodes of a cell, such as in Fig. 2.2, are short-circuited, current starts
flowing through the circuit, indicating that net oxidation and reduction reactions
are taking place at the electrodes. The potentials of these electrodes start deviating
from their equilibrium potential values. This deviation from equilibrium potential
is called
polarization
and the extent of deviation is termed
overvoltage
which is
expressed by the Greek letter,
.
Let us consider the hydrogen evolution reaction at equilibrium:
η
2H
2e
i H
2
,E
H
/H
2
0 volt
(2.22)
Actual hydrogen liberation will not take place unless more electrons are supplied
to the electrode, i.e., the electrode is made more negative. Similarly, the oxidation
reaction will take place only at the potentials more positive than 0 V (SHE).
Metal dissolution reaction is of basic importance in corrosion. Let us consider
the zinc dissolution equilibrium:
Zn s Zn
2
2e
E
Zn/Zn
2
0.76 V(SHE)
(2.20)
