Geology Reference
In-Depth Information
where k
i
is the temperature dependent salting out coecient for mole-
cular species, i.
At higher ionic strength values, an additional dependence on [i]
2
is often
required to fit the observed solubility data.
17
Alternatively, the Pitzer
model can be used with a high degree of accuracy to describe the short-
range binary (neutral-neutral, neutral-cation, neutral-anion) and ternary
(neutral-neutral-neutral, neutral-cation-anion) interactions between ions
and neutral species in single and mixed electrolyte solutions.
17
3.2.1.3 Equilibria and Equilibrium Constants. Many of the important
reactions involving solutes in freshwaters can be described by equilibria.
This approach means that an equilibrium constant, K, relating the
activities of the solutes, can be defined for each stoichiometric equili-
brium expression.
aA
þ
bB
"
cC
þ
d D
(3.13)
K
¼
{C}
c
{D}
d
/ {A}
a
{B}
b
(3.14)
The equilibrium constant can also be defined in terms of the concentra-
tions of the solutes.
K
¼
(g
c
[C])
c
(g
d
[D])
d
/(g
a
[A])
a
(g
b
[B])
b
(3.15)
For infinitely dilute solutions, {}
¼
[ ] and the equilibrium constant can
be written as
K
¼
[C]
c
[D]
d
/[A]
a
[B]
b
(3.16)
For solutions of fixed ionic strength, or, for example, where major ions
in solution, e.g. conservative cations and anions, are present at concen-
trations several orders of magnitude greater than the species involved in
the chemical equilibrium, e.g. A, B, C, and D in Equation (3.13), it can
be assumed that the solute activity coecients are also constants and
can be incorporated into the equilibrium constant. The equilibrium
constant for a fixed ionic strength aqueous solution is termed a constant
ionic strength equilibrium constant,
c
K.
c
K
¼
[C]
c
[D]
d
/[A]
a
[B]
b
where
c
K
¼
(g
a
g
b
/ g
c
g
d
) K
(3.17)
This again enables the use of concentrations rather than activities in
equilibrium calculations.
It is sometimes advantageous to use a mixture of activities and
concentrations and a mixed equilibrium constant, K
0
, is defined as
K
0
¼
{C}
c
[D]
d
/ [A]
a
[B]
b
where K
0
¼
(g
a
g
b
/ g
d
) K
(3.18)
