Geoscience Reference
In-Depth Information
log
K
=
d
log
(
Red
) −
a
log
(
Ox
) +
b
pe
+
c
pH
pe
= [1/
b
log
K
− (
d
/
b
)
log
(
Red
) + (
a
/
b
)
log
(
Ox
)] − (
c
/
b
)
pH
The
pe
varies with
pH
. It decreases when the
pH
rises (slope of
the line is -
c
/
b
). For one electron transferred (
b
= 1) and one proton
consumed (
c
= 1), when
d
= a and (
Red
)
= (
Ox
)
,
pe + pH
= log K; if
pH
= 0,
pe
=
log
K
The
pe
corresponding to all kinds of reduction reactions can be
predicted from thermodynamic calculations. Bartlett and James (1993)
give nearly 90 reactions of this type, some of which are given below
(Table 12.2). Values have been calculated for solution concentrations of
10
-4
M, activity coefficients of unity, partial pressures of the gases of
10
-4
mol l
-1
and concentrations of 0.21 atm. of O
2
, 0.78 atm. of N
2
and
0.00032 atm. of CO
2
.
The higher the
pe
, the greater the tendency to be reduced. Thus O
2
is a powerful oxidizing agent. It is easily reduced and acts as electron
Table 12.2
Principal reduction half reactions in soils; calculated and, (in
parentheses), measured potentials in soils.
Reduction half reactions (= gain of electrons)
log
K
pe
at pH
pe
at Pot. (V)
5
pH 7 at pH 7
¼ O
2
(g)+ H
+
aq
+ e
-
Æ
½ H
2
O
20.8
15.6
13.6
1.2-
(0.682)
1/5 NO
3
-
+ e
-
+ 6/5 H
+
Æ
1/10 N
2
+ 3/5 H
2
O
21.1
14.3
11.9
1.245
(0.420)
½ MnO
2
(s) + 2H
+
+ e
-
Æ
½ Mn
2+
+ H
2
O
20.8
12.8
8.8
1.229
(0.640)
1/3 Fe
3
(OH)
8
(GR*) + 8/3H
+
+ 2/3 e
-
Æ
Fe
2+
+ 8/3H
2
O 15.47 17.7
5.7 0.341
(pH 4)
g
-FeOOH+3H
+
+ e
-
Æ
Fe
2+
+ 2 H
2
O
16.65
11.8
2.7
(pH 4)
a
-FeOOH+3H
+
+ e
-
Æ
Fe
2+
+ 2 H
2
O
14.97
10.0
1.0
(pH 4)
1/8 SO
4
2-
+ 5/4 H
+
+ e
-
Æ
1/8 H
2
S(g) + 1/2 H
2
O
5.13
-1.0
-3.5
-0.220
1/8 CO
2
(g) + H
+
+ e
-
Æ
1/8 CH
4
(g)+ ¼ H
2
O
2.9
-2.1
-4.1
0.169
And, for information:
¼ CO
2
(g) + H
+
+ e
-
Æ
1/24 C
6
H
12
O
6
+ ¼ H
2
O
-0.21
-0.59
-7.9
*GR = green rust
