Agriculture Reference
In-Depth Information
Table 3.2
Equilibrium constants for acid-base equilib-
ria at 25
◦
C
,I
=
0
Equilibrium
−
log
K
H
2
O
=
H
+
+
OH
−
14.0
H
2
CO
3
∗
CO
2
(
g
)
+
=
H
2
O
1.46
H
2
CO
3
∗
=
H
+
+
HCO
3
−
6.35
HCO
3
−
=
H
+
+
CO
3
2
−
10.33
=
H
+
+
H
3
SiO
4
−
H
4
SiO
4
9.86
H
3
SiO
4
−
=
H
+
+
H
2
SiO
4
2
−
13.1
NH
3
(
g
)
=
NH
3
(
aq
)
−
1.76
NH
4
+
=
H
+
+
NH
3
(
aq
)
9.24
H
3
PO
4
=
H
+
+
H
2
PO
4
−
2.15
H
2
PO
4
−
=
H
+
+
HPO
4
2
−
7.20
HPO
4
2
−
=
H
+
+
PO
4
3
−
12.35
H
2
S
(
g
)
=
H
2
S
(
aq
)
0.99
H
+
+
HS
−
H
2
S
(
aq
)
=
7.02
HS
−
=
H
+
+
S
2
−
13.9
CH
2
NH
2
COOH
=
H
+
+
CH
2
NH
2
COO
−
9.78
H
+
+
CH
3
COO
−
CH
3
COOH
=
4.76
with the equilibrium constants in Table 3.4. Similar calculations can be made for
the other dissolved acids.
Table 3.3 gives the equilibria in a closed system in which the total carbonate
concentration,
C
T
, is fixed. In an open system, such as the water on the surface
of a submerged soil,
C
T
is variable and the resulting changes in pH depend
on the balance of charge between the non-carbonate anions and cations present.
Likewise if a quantity of strong acid, HX, or base, MOH, is added to the solution,
the equilibria will adjust so as to neutralize part of the H
+
or OH
−
added and so
buffer the change in pH. The changes in [H
+
] with alkalinity or dissolved CO
2
can be found from (see Equation 9, Table 3.3):
C
B
−
C
A
=
[HCO
3
−
]
+
2[CO
3
2
−
]
+
[OH
−
]
−
[H
+
]
(
3
.
2
)
where
C
A
is the concentration of non-carbonate anions after the addition of
acid HX and
C
B
is the concentration of cations after addition of base MOH. If
C
B
>C
A
, the difference
C
B
−
C
A
is the alkalinity of the solution; if
C
A
>C
B
,
the difference
C
A
−
C
B
is the mineral acidity.
3.1.2 SPECIATION
Many ions and uncharged molecules are present in solution as more than one
species
, depending on the concentrations of ligand ions and molecules and the
