Geology Reference
In-Depth Information
5.4.5 Redox Potential
Redox potential is the measure of the oxidation-reduction state of a soil,
and is determined by redox reactions involving the transfer of electrons
from one chemical species to another. A generalized redox reaction can
be written as
Ox
þ
mH
1
þ
ne
"
Red
(5.19)
where 'Ox' and 'Red' are the oxidized and reduced species, respectively.
Note also that H
1
ions are involved in the reaction, and so pH affects
the redox potential.
The potential produced is a consequence of the ratio of oxidized to
reduced species, and is expressed by the Nernst equation
E
¼
E
0
RT
nF
ln
½
Red
ð
5
:
20
Þ
½
Ox
where
E
¼
potential in volts
E
0
¼
standard electrode potential
R
¼
universal gas constant (8.314 J mol
1
K
1
)
T
¼
absolute temperature in K
n
¼
number of electrons involved
F
¼
Faraday constant 96,487 C mol
1
(the charge when 1 mole of [Ox] is
reduced).
If a temperature of 251C (298 K) is assumed, the values of the
constants R and F are used and natural logs converted to log
10
, the
equation becomes
E
¼
E
0
0
:
0591
n
½
Red
Ox
log
10
ð
5
:
21
Þ
½
Redox potential is measured using an inert platinum electrode, which
acquires the electric potential (E
H
) of the soil when placed into the soil
or a soil suspension. This potential is measured relative to a reference
electrode with a known potential, such as the calomel electrode (E
cal
),
which has a potential of 0.248 V at 251C. This potential has to be added
to the measured value
E
H
¼
E
cal
þ
0.248 V
(5.22)
