Environmental Engineering Reference
In-Depth Information
There is no electron transfer involved and these reactions are solely dependent
on pH.
The sloping lines represent equilibria involving both electron transfer and pH,
e.g.,
(5) Fe
2
3H
2
O
Fe(OH)
3
3H
e
(2.28)
(6) Fe
2H
2
O
HFeO
2
3H
2e
(2.29)
The hydrogen and oxygen are also shown in the diagram by dotted lines. The
hydrogen line represents the equilibria:
2H
2e
H
2
in acid solutions
(2.30)
or
2H
2
O
2e
H
2
2OH
in neutral or alkaline solutions
(2.31)
These two reactions are equivalent and their pH dependence of single-electrode
potential is represented by:
E
H
/H
2
E
H
/H
2
0.059 pH
(2.32)
At pH
0.059 V. Similarly,
for oxygen equilibrium with water the corresponding reactions at lower and
higher pH are:
0, i.e., for [H
]
1, E
H
/H
2
0 and the slope is
O
2
4H
4e
2H
2
O
(2.33)
and
O
2
2H
2
O
4e
4OH
(2.34)
The pH dependence of single-electrode potential is represented by
E
O
2
/H
2
O
E
O
2
/H
2
O
0.059 pH
(2.35)
At pH
0, E
O
2
/H
2
O
1.226 V and at pH
1, i.e., for [OH
]
1, E
O
2
/H
2
O
0.401 V. Here again the slope of the line is
0.059 V. Water is stable in the
area delineated by these two lines. Below the hydrogen line it is reduced to hydro-
gen gas and above the oxygen line it is oxidized to oxygen.
The potential-pH diagram shows three clear-cut zones:
1.
Immunity zone
. Under these conditions of potential and pH, iron remains in
the metallic form.
2.
Corrosion zone
. Under these conditions of potential and pH, iron corrodes
forming Fe
2
or Fe
3
, or HFeO
2
.
3.
Passive zone
. Under these conditions of potential and pH, protective layers
