Handbook of Solvents - George Wypych - ChemTech - Ventech!

524 Y. Y. Fialkov, V. L. Chumak
Conductivity of the systems results from the interaction between components. This interaction
proceeds according to the steps outlined in the scheme [9.8]. Conductivity of the
mixed electrolyte solution confirms the interaction between components.
Subtype IIIa isotherms are the most widespread. Such isotherms have one maximum
and bring the conductivity to the origin for pure components.
Subtype IIIb isotherms have a minimum situated between two maxima. The maximum
appears because of the significant increase of the solution viscosity due to the
heteromolecular association process. When the conductivity is corrected for viscosity, the
maximum disappears. Conductivity of the mixed solvent pyrosulfuric acid-acetic acid is an
example of the system.
Subtype IIIb-1 isotherms with a curve is a special case of subtype IIIb isotherms. The
curve is caused by viscosity influence. Isotherm IIIb turns to subtype IIIb-1 at higher temperatures.
A concentration dependence of conductivity for stybium (III) chloride-methanol
is an example of the systems.
Determination of stoichiometry of interaction between the components of the mixed
solvent is discussed elsewhere. 3,12
9.3.3 CHEMICAL PROPERTIES OF MIXED SOLVENTS
Solvating ability of mixed solvent differs from solvating ability of individual components.
In addition to the permittivity change and the correspondent electrostatic interaction energy
change, this is also caused by a number of reasons, the most important of which are discussed
in the chapter.
9.3.3.1 Autoprotolysis constants
Let both components of mixed solvent, AH-BH, to be capable of autoprotolysis process.
HA ↔ H A + A
2 2
2 2
+ −
BH ↔ H B + B
+ −
[9.42a]
[9.42b]
As Aleksandrov demonstrated, 15 the product of the activities of lionium ions sum
(a a H2 A H B
+
2 + + ) on the liate activities sum (a - + a -)
is a constant value in the whole concen-
A B
tration range for a chosen pair of cosolvents. This value is named as ion product of binary
mixed solvent.
( + +
H A H B)( − −
A B )
HA−HB K = a + a a + a
ap
2 2
[9.43]
If components of binary mixed solvent are chemically interacting, then the equilibrium
of protolysis processes [9.42] is significantly shifting to one or another side. In most
such instances, concentrations of lionium ions for one of components and liate ions for another
become so low that correspondent activities can be neglected, that is autoprotolysis
constant can be expressed as:
HA−HB K = a + a
ap
+ −
H2A B
HA−HB K = a + a
ap
+ −
H2B A
[9.44a]
[9.44b]