Geoscience Reference
In-Depth Information
The redox potential generates a current if we use an apparatus such
as the one represented in Fig. 12.2, the oxidation of iron being taken
as example.
Determination of redox potential
Platinum
electrode
Platinum
electrode
Flow of
electrons
Direction of
current
O
2
/H
2
O
860 mV
Fe
3+
/Fe
2+
107 mV
Fig. 12.2
Current generated by a redox couple.
The redox potential of oxygen being the highest, electrons will flow
in the direction indicated. Iron is spontaneously oxidized on contact
with water and oxygen.
It has been accepted that the normal hydrogen electrode, H
2
/H
+
, will
serve as standard (zero potential). But in the field, the manipulation of
this hydrogen electrode is practically impossible. A calomel Hg/Hg
2
Cl
2
or Ag/AgCl electrode is used instead. This leads to much confusion
(Boivin 2000) and the following equation has to be applied:
E
h
=
E
1
+
E
2
where,
E
h
is the redox potential in volts or millivolts,
E
1
is the potential difference measured in the field between a
platinum electrode and a calomel electrode or even an electrode of the
Ag/AgCl type,
E
2
is a correction factor corresponding to the potential
difference between the electrode used for
E
1
and the standard hydrogen
electrode.
2
= 1000
A
-
B
(
t -
25°)
where
A
= 0.2444 (Hg
2
Cl
2
) or 0.2047 (Ag/AgCl) and
B
= 0.0007588.
The apparatus is standardized using a 0.1 M KCl solution containing
an equimolar (3 mM) mixture of potassium ferricyanide and potassium
ferrocyanide. The potential is then 0.428 V at 25° C.
The redox potential in soil solutions is far from the expected
values. For example, in aerated conditions, it would theoretically
reach or surpass 1000 mV. But only 500 or 600 mV is measured. The
difference is explained by defects in principle or practical difficulties:
