Handbook of Solvents - George Wypych - ChemTech - Ventech!

9.4 Mixed solvent influence on the chemical equilibrium 543
Figure 9.13. The composition of solvate shell Ni 2+ (in
molar parts of water) in the mixtures of solvents
formed from water and propanol (1), ethanol (2),
methanol (3) and DMSO (4). Data from Ref. 43.
or
[ − ]
+
In the mixed solvents water - non-aqueous
solvent, in spite of its donor and polar
properties, water is a preferred solvating
agent. This generalization has some exceptions
(solvation in systems Ag + - H 2O -
acetonitrile, Cr 3+ -H 2O - DMSO, F - -H 2O-ethylene
glycol). 45
Solvation energy of proton by donor solvents
is very high. The regularities of the proton
selective solvation and re-solvation
processes were studied in more detail in comparison
with other ions.
Let us consider the changes in the system,
when donor component is added to protic
acid HA in solvent A. Anion solvation can be
neglected, if both solvents have donor character.
The interaction influences the proton
re-solvation process.
+
+ B
+
HAp + B ←⎯→ HAp 1B←⎯→� ←⎯ →HB
p + A
[9.99]
+ +
HA + qB ←⎯→� ←⎯ →HB
+ pA
[9.99a]
p q
The model related to eq. [9.99a] was evaluated, 46 resulting in the supposition that the
+ +
equilibrium of two forms of solvated proton: HA p and HBq is important. Solvation
stoichiometry was not considered. Both proton solvated forms are denoted as HA + and HB + .
If B is the better donor component (it is a necessary requirement for equilibrium [9.99]
shift to the right hand side of equation), the equation from work 46 can be simplified to the
form: 47
{ }
B A ( a a)
[ us B / ( B) ] / [ us B / ( B)
]
A
K = K + K −K K x 1− x 1+ K x 1 −x
[9.100]
a a
where:
Ka ionic association constant
A B
Ka , Ka
the ionic association constants of acid in individual solvents A and B
Kus a constant of resolvation process
xB molar fraction of B
For the calculation of resolvation constant, one must determine the experimental con-
A B
stant of HA association in the mixed solvent and determine independently K a and K a .
When the resolvation process is completed at low concentration of the second component,
the change of permittivity of mixed solvent A-B may be ignored. Thus, one may as-
A B
sume that K and K are constant and calculate Kus from the equation [9.100] in the form:
a
a
{ [ ] }
A
B A
B A
( Ka Ka) ( Ka Ka) [ ( xB) xB] Kus( Ka Ka)
1/ − = 1/ − + 1− / 1/
− [9.101]