Geoscience Reference
In-Depth Information
acceptor. Conversely, the reduction of CO
2
to organic carbon (last line in
the table) is extremely difficult. It requires energy (photosynthesis).
Two half reactions constitute a
redox couple
and enable us to see
which of the two elements will be reduced on contact with the other
(ยง 12.1.3).
These data are theoretically derived, hence are only approximate
when we wish to apply them to natural systems, where there is a mixture
of dissolved ions and also precipitated and more or less crystallized
ions. In addition, living organisms also intervene that, with energy
sources available, execute transformations without going in the direction
predicted by the laws of thermodynamics applied to aqueous solutions.
But the considerations regarding
pe
serve to finalize concepts and will
be used in this chapter.
12.1.3 Oxidation-Reduction Potential
The general half reaction of reduction of a substance can be written as
follows:
Oxidized molecule
(
Ox
)
+
m
H
+
+
n
electrons
=
reduced molecule
(
Red
)
The Nernst equation gives the equilibrium potential for the
oxidation-reduction system:
Definition
(
Red
)
+
RT
___
_________
0
E
h
=
E
nF
log
(
Ox
)(
H
+
)
m
h
where:
E
h
is the potential for the given redox couple (in volts),
E
0
the standard potential of the half reaction (in volts),
R
the gas constant (J mol
-1
K
-1
= 8.31),
F
the Faraday constant (96485.309 C mol
-1
),
T
the absolute temperature in Kelvin,
n
the number of moles of electrons,
m
the number of moles of protons.
h
0
only in the case where the activities of the reduced
molecule, the oxidized molecule and the proton is unity (then log 1
= 0). But this never happens in soils! Also, for the given concentrations
of reduced and oxidized molecules, the Nernst equation shows that
E
h
depends on the pH (but not so in exceptional cases) and on the
We get
E
n
=
E
h
