Chemistry Reference
In-Depth Information
aqueous solution. The constant for the first hydrolysis (|log K
OH
|) is a parameter often
used in QSAR studies, based on the premise that the metal ions are found as aqua
complexes in biological systems (Newman and McCloskey1996; Tatara et al. 1997;
Enache et al. 2003).
Quantitative correlations between a cation's acidity and electrostatic parameter
can be made (Wulfsberg 1991). A good correlation can be found by plotting the
pK
a
versus the electrostatic parameter (Z
2
/r where r is expressed in pm) for the ele-
ments with an electronegativity of χ ≤ 1.5. The following empirical equation was
obtained;
Z
r
2
.
(3.67)
pK
=
15.14
−
88.16
a
For the more electronegative metals (χ > 1.5 on the Pauling scale), the points deviate
substantially from the line, so Equation (3.67) was corrected with an empirically
derived factor:
2
Z
r
.
(
)
pK
=
15.14
−
88.16
+
0.096
χ
−
1.50
(3.68)
a
Pauling
The constant for the first hydrolysis
(|log K
OH
|
)
was correlated also with the
covalent index
Xr
2
m
in QSAR studies (Newman and McCloskey 1996; Tatara
et al. 1997, 1998).
3.6
SOLUBILITY OF METAL COMPOUNDS
AND METAL ION HYDRATION
3.6.1 t
HErmodynamic
a
spEcts
of
s
olubility
To describe the phenomena that occur during the solubility process, the relationship
between the solubility equilibrium constant and the free enthalpy variation can be
defined first:
Δ
G
o
= −
RT
lnK
eq
.
(3.69)
As for other reactions, K
eq
, and thus solubility, increases as the Δ
G
o
becomes increas-
ingly negative. Free enthalpy variation is related to the variation in both enthalpy and
entropy:
Δ
G
o
= Δ
H
o
−
T
Δ
S
o
.
(3.70)
During the dissolution process, two opposite processes occur—release of ions
from the crystal lattice (endothermic process) and hydration of the liberated ions
(exothermic process).
