Agriculture Reference
In-Depth Information
interface and transferred to or from the bulk solution by the diffusion of conjugate
acid-base pairs. Thus
F
GC
=
F
LH
2
CO
3
∗
−
F
LCO
3
2
−
−
F
LOH
−
(
3
.
38
)
which is inherent in the mass and charge balances.
Figure 3.5 shows calculated concentration profiles in the still water layer for
realistic conditions in ricefields. In figure 3.5(a) the CO
2
pressure is large, the pH
in the bulk solution correspondingly low (pH 6.7), and the movement of dissolved
CO
2
to the interface primarily as H
2
CO
3
∗
. The loss of CO
2
raises the pH at the
interface (to pH 8.2), tending to offset the depletion of HCO
3
−
and the gradient
of HCO
3
−
concentration is small. In figure 3.5(b) the CO
2
pressure is small, the
pH in the bulk solution correspondingly high (pH 10.6), and the movement of
dissolved CO
2
away from the interface is primarily as HCO
3
−
. Dissolution of
CO
2
lowers the pH at the interface (to pH 8.3) and there is therefore a gradient of
decreasing OH
−
towards the interface. The gradient of CO
3
2
−
is also negative.
Since the mobility of OH
−
is about five times that of HCO
3
−
and CO
3
2
−
,there
is therefore an excess negative potential at the interface and as a result Ca
2
+
diffuses to the interface and Cl
−
away.
(a)
Concentration (mM)
0.00
0.25
0.50
0.75
1.00
1.25
0.0
Cl
−
Ca
2
+
HCO
3
−
OH
−
H
2
CO
3
*
0.1
0.0
(b)
Cl
−
Ca
2
+
H
2
CO
3
*
HCO
3
−
CO
3
2
−
OH
−
0.1
Figure 3.5
Profiles of CO
2
,
HCO
3
−
,
etc.
across
still water
layer. Still
layer
1000
µ
m, [Ca
2
+
]
L
∞
=
0
.
5mM, [Cl
−
]
L
∞
=
thickness both
=
0
.
15mM,
P
CO
2
L
∞
=
1kPa
(a), 2
.
5
×
10
−
5
kPa (b)
