Agriculture Reference
In-Depth Information
Box 4.4
Ion Activity and Ionic Strength
Concentration
refers to the number of molecules or ions per unit volume. An
individual ion experiences weak forces because of its interaction with water
molecules (the formation of a hydration shell) and with other ions of the same or
opposite charge. Effectively, the ability of the ion to engage in chemical reactions
is decreased, relative to what is expected when it is present at a particular
concentration. This effect is accounted for by defining the
activity a
of the ion,
which is related to its concentration
C
by the equation
a
f C
(B4.4.1)
where
f
, with values from 0 to 1, is the activity coefficient of that ion species. In very
dilute solutions where the interaction effects are small,
f
approaches 1, and
a
is ap-
proximately equal to
C
. An extensive theory about how to calculate activity coeffi-
cients exists. A key variable in determining the value of
f
for an ion in solution is
I
,
which incorporates the cumulative effect on
f
of the charge and concentration of all
the ion species (1 to
n
) in solution, through the equation
n
i
0.5
I
C
i
z
i
2
(B4.4.2)
1
i
Equation 4.7 shows that the effective thickness of the
DDL
is inversely pro-
portional to the valency
z
of the counterion and inversely proportional to the
square root of the bulk solution concentration
C
. Some examples of
d
eff
values are
given in table 4.4.
Changes in
DDL
thickness are discussed here in terms of the forces acting on
solute ions. But we should remember that these ions are immersed in a “sea” of
water molecules. A change in the type of ion and its concentration in the
DDL
results also in a change in the activity of water molecules. Water molecules dif-
fuse into, or out of, the
DDL
in response to their own activity gradient across the
DDL
. For example, if the ion concentration in the bulk solution decreases, the
tendency for water molecules to diffuse into the
DDL
is increased. This leads to
a
swelling pressure
, which depends on the type of cations in the
DDL
and the soil
Effective Thickness of the
DDL
at a Clay Surface
in Salt Solutions of Different Concentrations
Table 4.4
Effective Thickness d
eff
of the
DDL
(nm)
a
Salt Solution of Concentration
C
(mol/L)
NaCl
CaCl
2
0.1
1.94
1.0
0.01
6.2
3.2
0.001
19.4
10.1
a
1 nm
10
9
m
