Agriculture Reference
In-Depth Information
thermodynamic properties of common chemical species in natural systems are
given in Stumm and Morgan, 1996 (Appendix 3).
4.1.2
REDOX POTENTIALS
An alternative approach to quantifying redox equilibria is to treat them as elec-
trode reactions and to calculate the electric potential that would exist if the couple
of interest formed a half cell with an inert electrode. The standard reference point
for this is again taken as the hydrogen half reaction, the electrode potential for
which is set at zero, which is equivalent to setting G o equal to zero. This
approach lacks the simplicity of the system based on electron activities and is
not so well suited to equilibrium calculations. However it has historical prece-
dence and is widely used so must be mentioned. Also, the electrode potentials of
many redox couples can be measured directly. An inert electrode, such as plat-
inum metal, is placed in a solution containing the redox couple of interest and
linked via an external circuit to a hydrogen half cell comprising an inert electrode
in a solution in equilibrium with H 2 gas at 1 atm and having unit activity of H +
ions. The potential is measured with a voltmeter.
The standard electrode potential, E H , in which the suffix H indicates that the
potential is on the H 2 -H + scale, is derived as follows. We have
G =− nF E H
( 4 . 11 )
where F is the Faraday. Combining Equation (4.11) with Equation (4.8) gives
2 . 303 RT
F
E H =
pe
( 4 . 12 )
Substituting for pe from Equation (4.6) in Equation (4.12) gives
log K log ( Red )
( Ox )
2 . 303 RT
F
1
n
E H =
( 4 . 13 )
or
2 . 303 RT
nF
log ( Red )
( Ox )
E H = E H
( 4 . 14 )
where E H =
( 2 . 303 RT /nF) log K . This is the Nernst equation.
At 25 C, 2 . 303 RT /F = 0 . 059 V, therefore
E H = 0 . 059 pe
( 4 . 15 )
4.1.3 RELATION BETWEEN pe AND CONCENTRATION
OF REDOX COUPLES
If one redox couple in a redox reaction is present at a much greater concentration
than the other, then the concentration of the reduced and oxidized species in this
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