Handbook of Solvents - George Wypych - ChemTech - Ventech!

9.4 Mixed solvent influence on the chemical equilibrium 547
state of the theory of individual solvent effects on equilibrium constant of ion association
process (K a) is described in monographs by Izmailov 53 and Barthel. 54
The formation of free (preferably solvated) ions is due to a successive equilibrium
states proceeding in solution:
Kadd
Ki
q+
p− Ka q+
mE + nF ←⎯⎯→EF←⎯→Cat •An←⎯→�←⎯ →pCat
+ qAn p− [9.105]
m n
p
q
where:
Kadd a constant of adduct formation
Ki ionization constant
Ka association constant
The true constant of ion association is a ratio of ionic associate concentration
q+ p-
Cat p •An q to ion concentration product. If ionic associate concentration is unknown (as
is true in many cases), ion association constant is calculated from the analytical concentration
of dissolved substance:
where:
q+ p− q+
p
p−
{ 0 [ ] [ ] } / [ ] [ ]
K = c −pCat −qAn
Cat An
a
c 0
initial concentration of electrolyte
The general theory of ionic equilibrium 53,54 leads to the conclusion that ion association
constant in universal or conditionally universal media, in accordance to the equation
[9.53a], depends exponentially on reciprocal value of permittivity:
where:
ln Ka = a00 + a01
/ ε [9.106]
a 00, a 01
approximation coefficients
Figure 9.15. Dependence K a of (C 2H 5) 4NBr on
permittivity in mixed solvents formed by propylene
carbonate with o-dichlorobenzene (1,2-o), pyridine
(1,2-x) and acetic acid (3) at 298.15K.
q
If ion association process has high energy,
the solvents are solvate-inert because of
large ions, such as (C nH 2n+1) 4N + ,(C nH 2n+1) 4P + ,
etc. Solution of (C 2H 5) 4NBr in mixed solvents
propylene carbonate - o-dichlorobenzene and
propylene carbonate - pyridine 55 serve as an
example. The components of these binary
solvents (with the exception of inert
dichlorobenzene) possess donor properties,
though they do not solvate the large anion
R 4N + because it is solvate-inert component in
relation to anion. That is why lnK a -1/εisotherms
follow the same line (see Figure 9.15).
But the lnK a -1/εisotherm for solution of propylene
carbonate - acetic acid differs from the
other two because acetic acid is a solvate-active
component in relation to anion Br - .