Chemistry Reference
In-Depth Information
3.6.2 t
HE
s
olubility
-p
roduct
c
onstant
A saturated solution of a slightly soluble salt in contact with undissolved salt involves
an equilibrium like this:
M
x
A
y
(
s
) ⇌ xM
y+
(
aq
) + yA
x−
(
aq
).
(3.71)
In writing the equilibrium constant expression for a heterogeneous equilibrium,
the concentrations of pure liquids and solids are considered to be one. So the equilib-
rium constant expression for the equilibrium above is the following:
K
sp
= [M
y+
]
x
[A
x−
]
y
.
(3.72)
This equilibrium constant is called a
solubility-product constant.
In general, the
solubility-product constant (K
sp
) is the equilibrium constant for the equilibrium that
exists between a solid ionic solute and its ions in a saturated aqueous solution.
Application of the solubility-product constant in QSAR studies started with the
research of Shaw (Shaw 1954a, 1954b) in which he demonstrated that the affinity
of the cations for the sulfhydryl group is directly proportional to the insolubil-
ity of the corresponding metal sulfide. In a similar deduction, the affinity of the
cations for oxygen donor bioligands can be correlated with the solubility of the
corresponding metal hydroxide. Thus, the most used solubility-related param-
eters used in SAR and QSAR studies are the solubility of sulfide (pK
sp
sulfide)
and hydroxide log -K
SO
(MOH) or pK
sp
hydroxide (Shaw 1954a, 1954b; Shaw and
Grushkin 1957; Biesinger and Christensen 1972; Khangarot and Ray 1989). These
and additional studies are discussed in Chapter 5. The values of the solubility-
product constant for some hydroxide and sulfide of metal ions are presented in
3.6.3 lsEr
for
i
inorganic
c
ompounds
The linear solvation energy relationship (LSER) can be a useful predictive tool for
environmental property estimations (Hickey 1999). In LSER, the solution behavior
of a substance (e.g., solubility, bioaccumulation, and toxicity) is directly related to
several aspects of its chemical structure. For example, a LSER equation is depicted
in Equation (3.73):
Log(Property) = mV
i
/100 + sπ* + bβ
m
+ aα
m
(3.73)
where V
i
is the intrinsic (Van der Waals) molecular volume, π* is the solute's abil-
ity to stabilize a neighboring charge or dipole by nonspecific dielectric interactions,
and β
m
and α
m
are the solute's ability to accept or donate a hydrogen in a hydrogen
bond. The equation coefficients m, s, b, and a are constants for a particular set of
conditions, determined by multiple linear regression of the LSER variable values for
a series of inorganic chemicals with the measured value for a particular chemical
property (Hickey 1999).
