Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.4 Acid-base equilibria in ionic solvents 633
NaI + 1/ 2O = 1/ 2Na O + 1/ 2I
[10.4.51]
2 2 2 2
are 32.8±2 and 13.9±1 kJ/mole for solid NaI and 30.6±2 and 9.3±2 kJ/mole for liquid one.
These values suggest that NaI has a greater tendency to oxidation than to hydrolysis. This
conclusions cannot be adopted without stipulation, because it is based on values ΔH and not
ΔG, although only the latter values determine the direction of a reversible reactions at constant
p and T.
A simple but unquestionable method has been used to investigate high-temperature
hydrolysis of molten KCl-NaCl. 104 A mixture of HCl and H 2O obtained by passing an inert
gas through aqueous solutions at a definite concentration was passed into the melt. Measurements
of the equilibrium O 2- concentration by potentiometric method allowed to calculate
the equilibrium constants of [10.4.13] as pK=55.3×10 3 T -1 -40.2. At 1000, pK=15.1, it
means that equilibrium [10.4.13] in molten KCl-NaCl is displaced to the left.
A similar method has been used to study the hydrolysis of the eutectic KCl-LiCl at
500 o C (pK=9.77±0.4). 105 The hydrolysis of the chloride melt is thermodynamically unfavorable
and is completely suppressed in the presence of bases (even at a concentration of
~10 -3 mole/kg).
The potentiometric cells:
( H O + H ) Au NMeOH + ( 1−N)
MeX MeX C( X )
| || | [10.4.52]
2 2 2
were used to investigate the hydrolysis of individual alkaline halide melts (with the exception
of the lithium salts). 106
The e.m.f. of cell [10.4.52] is related to the concentration of the reactants by the expression:
0
E = E +
RT
F
P P
ln
P
12 / 12 /
H2 X2
HO 2
a
MeOH
[10.4.53]
1/ 2 1/ 2
where PH and P
2 X are partial pressures of hydrogen and halogen over the melt, respec-
2
tively.
The logarithms of the equilibrium constants
F
lnK ln
RT E
G P P
= − +
RT P
/ /
Δ HX H X
0 12 12
2 2
HO 2
a
MeOH
[10.4.54]
are negative for all the melts studied, which indicates that the hydrolysis of alkali metal halide
melts is thermodynamically unfavorable. The trend towards hydrolysis of melts of individual
alkaline halides diminishes with the increase in the radii of both cation and anion.
According to the results of Smirnov et al. 106 the solutions of alkali metal hydroxides in the
corresponding individual halides are close to ideality.
10.4.5.2 The processes of removal of oxide admixtures from melts
The studies of the elimination of oxygen containing impurities from ionic melts are of an
applied character and are performed in melts having industrial applications. Various