Page 55 - Soil and water contamination, 2nd edition

42 Soil and Water Contamination

is established on a platinum surface pH = 0.0 and P = 1 atm. The redox potential of a
H2
system relative to the hydrogen electrode is measured in a redox cell, as illustrated in
Figure 2.2. In a solution of pH = 0, H gas is bubbled over the platinum electrode on the
2
left-hand side. In the compartment on the right-hand side, a platinum electrode is present
in the solution whose redox potential is being measured. The redox potential is measured by
a voltmeter connected to both electrodes, and the electrical circuit is closed by a salt bridge.
The conditions at the reference electrode are:

[B ] [H ] 1 mol l 1 (2.65a)
ox
and
[ D red ] P H 2 1 atm (2.65b)

Since both [B ] and [D ] are equal to unity, Equation (2.63) becomes:
ox red
RT [ C ] c
Eh E 0 ln ox (2.66)
nF [ A ] a
red
Equation (2.66) is known as the Nernst equation . At the standard temperature of 25 °C and
with base-10 logarithms instead of natural logarithms, Equation (2.66) becomes

. 0 0592 [ C ] c (2.67)
0
Eh E log ox
n [ A ] a
red
0
where Eh = the redox potential . The standard redox potential E has been tabulated for most
half reaction s. By convention, the sign of the standard redox potential corresponds to the
log K of the reaction written as a reduction reaction (like Equation 2.58). Note that the sign

Volt

H - electrode Pt - electrode
2
H
2
Salt bridge

pH=0 Fe 2+

3+
Fe
6642

Figure 2.2 Schematic representation of a redox cell to measure the redox potential.

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