Handbook of Solvents - George Wypych - ChemTech - Ventech!

9.5 The mixed solvent effect on the chemical equilibrium thermodynamics 557
Figure 9.21. Dependence of stability constants for cadmium
thiocyanate complexes on 1/ε in mixed solvent
water-methanol at 298.15K.
Coordination solvents A and B taking part in
the processes are the same ligands as L.
The second aspect of the solvent effect
is related to the thermodynamic properties of
the complex, such as formation constant.
Discussing the problem of solvent effect on
complex stability, authors often neglect the
change of a complex nature, due to the
change in solvent as follows from equations
[9.123] and [9.123a]. These processes may
also be regarded as resolvation processes.
When the components of mixed
solvate are solvate-inert in relation to the
complex, equation [9.53a] takes the following
form:
ln Kk = a00 + a01
/ ε [9.124]
Coefficient a 00 has a physical meaning
of logarithm of formation constant at ε→∞
related to the standard ionic strengthI=0.In
Figure 9.21, this dependence is presented.
Table 9.14. Complexation process characteristics for system Cd 2+ -thiourea in mixed
solvent water-methanol at 298.15K
Constant
Coefficient of equation
a00 a01 Correlation
coefficient r
cov
σG
Component σG, kJ/mol
-σG el
σε=1 cov
in H2O in MeOH
K2 -0.42 367.7 0.996 -1.0 910 11.6 27.9
K3 1.07 561.9 0.987 2.6 1390 17.7 42.7
K4 7.79 631.3 0.991 19.3 1563 19.9 47.9
K5 2.72 837.0 0.988 6.7 2072 26.4 63.6
The data for cadmium thiourea complexes in water-methanol mixed solvent 70 are presented
in Table 9.14. Dependence of stability constant on the solvent composition is very
complex in the mixed solvents formed by two solvate-active components as defined by the
relative activity, L, of A and B. Dependencies of stability constant for some complexes in
water-B solution (where B is methyl acetate, methanol, etc. 69 ) are presented in Figures 9.21
and 9.22 for complex NiEn 2+ (En=ethylenediamine) in water-non-aqueous solution (the
constants are presented relative to stability constant in water).
9.5 THE MIXED SOLVENT EFFECT ON THE CHEMICAL EQUILIBRIUM
THERMODYNAMICS
In most cases, thermodynamic characteristics of equilibrium processes are determined by
temperature dependence of equilibrium constant K = f(T), according to the classical equa-