Geoscience Reference
In-Depth Information
temperature (referred to 25 degrees).
E
h
is the
oxidation-reduction potential
or
redox potential
, which can determine a current measurable between
two electrodes one of which is dipped in the soil solution and the other
in a reference system (electrode with constant potential compared to a
hydrogen electrode H
2
/H
+
).
When the value of
E
h
increases, the system becomes more oxidizing.
When it is lowered, the system becomes more reducing. The last column
in Table 12.2 corresponds to calculated potentials (or measured if they
are in parentheses).
The Nernst equation shows that
E
h
varies linearly with pH. Figure 12.1
gives the corresponding graph. Each redox couple then corresponds to
a straight line (Poubaix diagram). Above the line, the oxidized form
Ox
predominates over the reduced form
Red
. Below it, the reverse happens.
This diagram was obtained by assigning 10
-5
for activities, a partial
pressure of 0.2 atm. for
N
2
and
CH
4
, and a partial pressure of 0.8 atm.
for
CO
2
and
H
2
(McBride 1994). As for iron, the diagram has been
simplified!
Redox potentials in volts
1.5
MnO
-
Mn
2
+
2
1.25
O
2
-
H
O
2
1.0
-
NO
-
N
3
2
0.75
-
-
NO
-
NO
0.5
3
2
N
-
NH
2
3
0.25
4
-
S
SO
H
-
Fe
+
-
Fe
3
2
+
2
0.0
-0.25
CO
-
CH
2
4
H
2
-
H
O
2
-
0.50
pH
0
2
4
6
8
Fig. 12.1
Eh/pH diagram for some soil compounds (McBride 1994).
It is seen that, for a pH of 5 and
E
h
of 0.75, the oxides of iron are
oxidized, while those of manganese stay reduced.
Under some conditions, if
m
= 1, n =1,
red
=
ox
, T = 298 K, we have
pe
=
E
h
(volts)
_________
0.059
