Geoscience Reference
In-Depth Information
Acceptors
Fe
3+
pe
=
13.0
E
H
=
0.77
V
2-
SO
4
Fe
2+
pH
=
2.0
Donors
-
HSO
4
pe
=
0.0
E
0
=
0.0
V
pH
=
0.0
Figure 7.1
Dominance diagram for an acid-base reaction (left) and a redox reaction (right). The acid is the
proton donor and the reductant is the electron donor, while the base and the oxidant are their
respective acceptors. Note the equivalent potential scale used for redox reactions, which is no
longer used for acid-base reactions. The boundaries between the dominance domains are the p
K
,
which for the redox reaction is referred to as pe
0
. The coupling of electron and proton exchange
shifts both boundaries in a linear way.
10
-
HSO
4
8
6
4
2-
SO
4
2
0
-2
-4
H
2
S
-6
-8
HS
-
-10
0
2
4
6
8
10
12
pH
Figure 7.2
A pe-pH dominance diagram for the sulfur system. The double lines represent a change in the
oxidation state of the elements. Note that some boundaries are parallel to the
y
-axis, such as that
separating SO
2
4
and HSO
4
, because they only involve proton exchange. The boundary between
SO
2
4
and HS
−
is a coupled electron and proton exchange and therefore shows as segment of
straight-line with a slope equal to the proton/electron ratio (see
Exercise 1
). To obtain the usual
E
H
-pH diagrams, use E
H
=
0.059 pe at 25
◦
C.
