3. Soil Eh
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Soil Eh
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Introduction
Electron activity (ae) is an index for expressing the quantity
of electrons in a system. Like the definition for the activity of
other substances, it denotes the contribution of electrons to
the partial molar free energy of the system. It can be
expressed as the negative logarithm of the activity of
electrons (pe), just as pH for the activity of protons:
pe =-logae (1-1)
For the oxidation-reduction reaction
(oxidant)+e (reductant) (1-2)
at equilibrium
K=(reductant)/ (oxidant)(e)
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Soil Eh
(1-3)
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Introduction
Under standard conditions
Log K=pe0
(1-4)
where K
is the equilibrium constant. The equilibrium
constant is related to the change in standard free energy
(∆G0) of a reaction as follows:
∆G0= -RT/nF Ln K (1-5)
hence
E0=RT/nF Ln K
(1-6)
if n=1, at 250C
E0=0.059 log K = 0.059 pe0 (1-7)
Numeral values of pe0 and E0 for some redox systems in
submerged soils are tabulated in Table 1.
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Table 1. pe0 and E0 for some redox systems in submerged soils (Liu, 1985)
pe0 (= log K)
E0 (V)
¼ O2 + H+ + e  ½ H2O
20.8
1.23
½ MnO2 + 2H+ + e  ½ Mn2+ + H2O
20.8
1.23
Fe(OH)3 + 3H+ + e  Fe2+ 3H2O
17.9
1.06
½ NO3- + H+ + e  ½ NO2- + ½ H2O
14.1
0.83
1/8 SO42- + 5/4 H+ + e  1/8 H2S + ½ H2O
5.12
0.30
1/8 CO2 + H+ + e  1/8 CH4 + ¼ H2O
2.86
0.17
0
0
System
H+ + e  ½H2
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Based on the relationship between properties and plant
growth, the oxidation-reduction ststus of soils may classifield
into four categories, namely oxidizing, weakly reducing,
moderately reducing and strongly reducing.
The
corresponding Eh are see Table 2.
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Table 2. Gradiation of redox status of soils (Liu, 1985)
Redox status
Oxidating
Weakly reducing
Moderately
reducing
Strongly reducing
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Eh range
(mV)
Reaction
> 400
O2 predominant, materials
in oxidized form
400 - 200
200 - (-100)
< (-100)
Plant Growth
Benefecial to upland
crops, not suitable for
rice
O2, NO3- and Mn4+ reduced Normal growth of rice,
upland crops affected
Fe3+ reduced, organic
reducting substances
present
Upland crops affected
CO2 and H+ reduced
Rice affected by
reducing substances
Soil Eh
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In Table 3, are tabulated in descending order the theoritical
E0 values and practically measured Eh values of some
oxidation-reduction systems commonly encountered in
paddy soils. Since the pH of most natural systems including
paddy soil is ordinarily around 7, all the Eh are corrected to
this pH.
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Table 3. Sequential reduction of some electron acceptors in paddy soils (in Liu, 1985)
E0 at pH 7 (V)
Actually
meassured Eh (V)
O2 + 4H+ + 4e  2 H2O
0.82
0.65-0.3
NO3- + 2H+ + 2e  NO2- + H2O
0.54
o.5-0.2
MnO2 + 4H+ + 2e  Mn2 + 2H2O
0.43
0.4-0.2
FeOOH + 3H+ + e  Fe2+ 2H2O
0.17
0.3-0.1
-0.03-(-0.2)
0-(-0.2)
SO42- + 9H+ + 6e  HS- + 4H2O
-0.16
0-(-0.15)
H+ + e  ½H2
-0.414
-0.15-(-0.3)
-
-0.2-(-0.3)
Redox reaction
Organic systems
Methane fermentation
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