Agriculture Reference
In-Depth Information
couple are little influenced by the advancement of the reaction towards equi-
librium and the equilibrium electron activity is effectively that of the dominant
redox couple. This is given by the appropriate form of Equation (4.7) for the
dominant couple:
1
n
log
(
Red
)
pe
=
pe
o
−
(
4
.
16
)
(
Ox
)
(Note that this equation can be expressed in terms of the concentrations of Red
and Ox by dividing the activities by the appropriate activity coefficients.) The
pe of a solution will therefore be 'poised' at the pe determined by the dominant
couple until that couple is exhausted. The pe of all other redox couples operating
will tend to adjust to this electron activity.
For example, in oxic natural waters the principal oxidant is O
2
and in agreement
with expectations the pe of such waters is generally poised in the range expected
for the O
2
-H
2
O couple (Morel and Herring, 1993). Thus for water at pH
=
7in
10
−
0
.
7
equilibrium with atmospheric
P
O
2
(
=
atm
)
, the half reaction is
4
O
2
(
g
)
+
H
+
+
e
−
=
2
H
2
Ope
o
1
1
=
20
.
75
therefore
1
P
1
/
4
pe
=
pe
o
−
log
O
2
(
H
+
)
=
13
.
58
,
(
4
.
17
)
which agrees well with the typical range of pe in oxic natural waters; pe
=
12
to 14. Equation (4.17) indicates the pe varies as log
(
H
+
)
but only as
1
4
log
P
O
2
.
Thus the pH has a large effect on pe but the concentration of O
2
has only a minor
effect and small concentrations of O
2
maintain water in an oxidized state.
A further informative example is the organic matter — CO
2
couple, which is the
principal reductant in natural systems. Consider a solution in equilibrium with
atmospheric CO
2
at neutral pH and containing 10
µ
M'CH
2
O', where CH
2
O
represents average organic C in natural systems, whose composition is similar
stoichiometrically to that of carbohydrates. The half reaction is
1
H
+
+
e
−
=
1
1
4
H
2
O e
o
4
CO
2
(
g
)
+
4
CH
2
O
+
=−
1
.
20
therefore
−
log
(
CH
2
O
)
1
/
4
P
1
/
4
pe
=
pe
o
CO
2
(
H
+
)
=−
7
.
83
(
4
.
18
)
Such very low pe values do not generally occur in natural systems because oxidiz-
ing couples such as O
2
-H
2
O or Fe(OH)
3
-Fe
2
+
are usually present in much greater
concentrations. However pe values this low may occur in bacterial cells where
organic matter is being oxidized in the absence of large concentrations of inor-
ganic oxidizing couples, providing strongly reducing microenvironments which
may be linked to redox couples in the external environment via intermediaries
passing across the cell wall.
