Geology Reference
In-Depth Information
E
H
is measured at the pH of the sample, but is often expressed corrected
to pH 7 by subtracting 0.0591 V per unit pH up to 7 for samples with a
pH below 7 and adding 0.0591 V per unit pH down to 7 for samples
above pH 7.
E
H
canalsobeexpressedaspe, which is the negative log of the electron
activity (analogous with pH) (see also Chapter 3). This approach considers
electrons as a reactant or product in the reaction. The two are related by
E
H
(in volts)
¼
0.0591 pe
(5.23)
Worked example 5.6 - redox limits in aqueous systems (see also Section
3.2.4.2)
The limits of redox potential in aqueous systems are determined by
the oxidation or reduction of water.
Oxidation of water
2H
2
O
O
2
þ
4H
1
þ
4e
"
log K
¼
83.1
(5.24)
logK
¼
log pO
2
þ
4logH
þ
þ
4log e
83
:
1
¼
log pO
2
þ
4logH
þ
þ
4loge
83
:
1
¼
log pO
2
þ
4pH
þ
4pe
For pO
2
¼
1atm
;
log pO
2
¼
0
83
:
1
¼
4pH
þ
4pe
pe
¼
20
:
78
pH
Reduction of water
2H
1
þ
2e
"
H
2
log K
¼
0
(5.25)
pK
¼
logpH
2
2pH
2pe
For pH
2
¼
1atm
;
logpH
2
¼
0
pe
¼
pH
At pH 7, this gives theoretical limits for the soil environment at E
H
¼
approximately
þ
0.8 V (oxidizing) and approximately
0.4 V (reducing).
In practice soil atmospheres will not contain 1 atm of oxygen or hydrogen.
5.4.6 Reduction Processes in Soil
Reduction is caused by a decrease in oxygen concentration in the soil
atmosphere; for example, by waterlogging or compaction. Oxygen
