Handbook of Solvents - George Wypych - ChemTech - Ventech!

9.3 Physical properties of mixed solvents 525
As an example, let us consider mixed solvent water-formic acid. Both components of
the system are subject to autoprotolysis:
2H2O ↔ H3O + +OH - + -
and 2HCOOH ↔ HCOOH2 + HCOO
+
It is obvious, that in the mixed solvent, concentrations of the HCOOH2 ions (strongest
acid from possible lionium ions in the system) and the OH - ions (strongest base from two
possible liate ions) are neglected, because these two ions are mutually neutralized. Equilibrium
constant of the direct reaction is very high:
+ -
HCOOH2 +OH↔HCOOH + H2O
Therefore, in accordance with [9.44a] autoprotolysis constant for this mixed solvent
can be expressed as:
HA− HB
K = a + a
ap
+ −
H 3AHCOO
Figure 9.6. Dependence of pK ap (-logK ap) on the mixed
solvent composition: 1-DMSO-formic acid; 2-
DMSO-acetic acid; 3- formic acid-acetic acid.
Because of lionium and liate ions activities,
both mixed solvent components depend
on solvent composition, as does
autoprotolysis constant value for mixed solvent.
Figure 9.6 demonstrates dependencies
of pK ap values on composition for some
mixed solvents. Using the dependencies, one
can change the mixed solvent neutrality condition
in a wide range. That is, for mixed solvent
composed of DMSO and formic acid,
neutrality condition corresponds to concentration
of H + (naturally solvated) of about
3×10 -2 g-ion*l -1 at 80 mol% formic acid, but
only 10 -4 g-ion*l -1 at 10 mol% acid component.
The method of autoprotolysis constant
value determination based on electromotive
force measurement in galvanic element composed
from Pt,H 2(1 atm)|KOH(m), KBr(m),
solvent|AgBr,Ag was proposed elsewhere. 15a
The method was used for polythermal study
of autoprotolysis constant of binary mixed solvent 2-methoxyetanol - water. On the basis of
ionization constants, polytherms data, and the autoprotolysis process, thermodynamic data
were calculated.
A large data set on the electrophility parameter E T (Reichardt parameter 65 ) for binary
water-non-water mixed solvents was compiled. 66