Agriculture Reference
In-Depth Information
Table 3.7
Henry's law constants for important
gases in submerged soils at 25
◦
C and typical par-
tial pressures in the atmosphere
K
H
Typical partial
pressure
(
MkPa
−
1
)
(kPa)
10
−
6
N
2
6
.
52
×
78
1
.
24
×
10
−
5
O
2
21
3
.
35
×
10
−
4
3
.
5
×
10
−
2
CO
2
CH
4
1
.
27
×
10
−
5
1
.
7
×
10
−
4
5
.
63
×
10
−
1
0
.
1-5
×
10
−
7
NH
3
1
.
04
×
10
−
3
<
2
×
10
−
8
H
2
S
9
.
87
×
10
−
5
1-5
×
10
−
7
NO
2
10
−
5
10
−
8
NO
1
.
88
×
1-5
×
2
.
54
×
10
−
5
3
×
10
−
5
N
2
O
where
R
is the gas constant
(
=
8
.
314 kPa Lmol
−
1
K
−
1
)
and
T
is the temper-
ature (K). Values of
K
H
for important gases in submerged soils are given in
Table 3.7.
For some volatile solutes, slow reactions influence the rate of equilibration
between the gas and liquid phases. Generally the rate of gas transfer across the
liquid-gas interface is the rate-limiting step, as discussed in Section 3.4. But
there may also be slow hydration or other reactions in solution that must be
allowed for. An important example is the hydration of CO
2
,whosehalf-lifemay
be comparable to rates of transfer of CO
2
across the air-water interface.
Kinetics of CO
2
Hydration
The kinetics of the hydration and dehydration reactions are slow in comparison
with some processes in the water. The reactions are
k
f
1
−−−→
CO
2
+
H
2
O
H
2
CO
3
(
3
.
18
)
←−−−
k
b
1
and
k
f
2
−−−→
HCO
3
−
+
H
+
CO
2
+
H
2
O
(
3
.
19
)
←−−−
k
b
2
and the corresponding rate law is
=
k
f
1
[CO
2
]
−
k
b
1
[H
2
CO
3
]
+
k
f
2
[CO
2
][OH
−
]
−
k
b
2
[HCO
3
−
]
(
3
.
20
)
d[CO
2
]
d
t
−
