Agriculture Reference
In-Depth Information
Figure 4.1(b) shows the diagram for the stable equilibria in the nitrogen system.
According to this diagram N
2
should be largely oxidized to NO
3
−
in most natural
waters. The fact that it is not and N
2
is known to persist in oxic waters indicates
that a complete redox equilibrium does not exist; only a partial equilibrium is
attained under the mediation of microbes. Figure 4.1(c) shows the diagram for
the nitrogen system with N
2
treated as redox-inert and NH
4
+
,
NH
3
,
NO
3
−
and
NO
2
−
as metastable with respect to N
2
. This diagram more correctly represents
conditions in natural systems, with NH
4
+
as the stable species under mildly
reducing conditions and NO
3
−
under oxic conditions. This example illustrates
the difficulty in choosing the correct redox couples to represent real systems in
pe-pH diagrams. Some independent insight into the system is generally required
to choose the correct couples. Figure 4.1(d) and (e) show diagrams for sulfur and
carbon systems.
The situation is further complicated for redox reactions involving several solid
phases. An example is the Fe - CO
2
-H
2
O system shown in Figure 4.2. This
shows that ferrihydrite, Fe(OH)
3
, can be formed over a wide range of pe and pH,
though the pe range is increasingly restricted under increasingly acid conditions,
and Fe
2
+
is then the stable form. Siderite, FeCO
3
, and the hypothetical Fe(II)
hydrous oxide Fe(OH)
2
may be formed under moderately reducing conditions
but only at pH
>
7. Elemental Fe is only stable under very strongly reducing
conditions, outside the range in which water is stable. In real systems the situation
25
FeOH
2
+
20
Fe
3
+
P
O
2
> 1 atm
15
Fe(OH)
4
−
10
Fe
2
+
5
Fe(OH)
3
(amorph, s)
0
P
H
2
> 1
−
5
FeCO
3
(s)
−
10
Fe(s)
Fe(OH)
2
(s)
−
15
0
2
4
6
8
10
12
14
pH
Figure 4.2
pe - pH diagram for the Fe - CO
2
-H
2
O system. [Fe(II)]
=
1 mM, [Fe(III)]
=
0
.
01 mM,
C
T
=
5 mM. Amorphous Fe(OH)
3
is ferrihydrite, FeCO
3
siderite, and Fe(OH)
2
a hypothetical Fe(II) hydrous oxide. The details of the construction of this diagram are
explained in Stumm and Morgan (1996, Chapter 8). Reproduced by permission of Wiley,
New York
