Geoscience Reference
In-Depth Information
In the first case, the entropy helps in the dissolution process. In the second case,
it can be both helping and reducing, and in the third case, it reduces the dissolution
process.
In the majority of cases, the solubility of the solid substances in a liquid phase
is limited. Solubility is an important physicochemical and technological parameter
whose estimation determines any dissolution process. It acts as a factor which
strongly influences the kinetics or rate of dissolution. Solubility depends upon the
nature of the substance, its aggregate state, temperature, pressure, and a series of
other factors. Quantitative dependence of the solubility of the solid substance on
the temperature at a given constant pressure is given by:
RT
2
ðδ
ln S
=δ
T
Þ
P
5
ð
Q
2
=
Þðδ
ln a
2
=
ln S
PT
Þ
ð
4
:
21
Þ
where
solubility of the solid substance,
a
2
5
activity of the solid substance,
Q
2
5
heat of dissolution of 1 mol of the substance in the saturated solution,
R
5
gas constant.
The solubility of the solid substance may be expressed as a set of two processes:
(1) Melting of the solid substance and (2) mixing of two liquids.
In the case of ideal sol
ub
ility of the solid substance, the heat of the second pro-
ce
ss is equal to zero and Q
2
is equal to the heat of melting of the solid substance:
Q
2
5
S
5
% melting and does not depend upon the selection of the solvent, where if
δ
ln a
2
/
δ
ln S
1, then
Eq. (4.21)
can be written as:
5
RT
2
ðδ
ln S
=δ
T
Þ
P
5
%melting
=
ð
4
:
22
Þ
such that the right-hand side is
0, i.e., with an increase in the growth temperature,
ideal solubility of the solid substance in an ideal solution increases. The depen-
dence of the solubility of any substance in the solution and the pressure is
expressed as:
.
V
solid
2
ðδ
ln S
=δ
T
Þ
T
5
ð
V
liquid
=
RT
Þðδ
ln a
2
=δ
ln S
Þ
PT
ð
4
:
23
Þ
where V
liquid
5
partial molar volume of the soluble substance in the solution,
V
solid
5
molar volume of that substance in the solid phase. In the case of an ideal
solution, ln a
2
5
δ
δ
ln S,
ln a
2
/
ln S
5
1, and
Eq. (4.23)
becomes:
V
solid
2
ðδ
ln S
=δ
P
Þ=
T
5
ð
V
liquid
Þ=
RT
ð
4
:
24
Þ
Thus, in the case of ideal solutions of the solid substance in the investigated
media, the solubility decreases with an increase in the pressure, if the molar
vo
lume
of the soluble substance increases during its melting. Similarly, if V
solid
2
V
liquid
;
