Handbook of Solvents - George Wypych - ChemTech - Ventech!

632 Victor Cherginets
and, hence, pyrophosphate acidity should be lower, since in pair
“metaphosphate-pyrophosphate” the latter is the conjugated base.
The absence of correlation between pH in aqueous solutions and pO in melts for the
same substance is often noted , this conclusion is made mainly from data for oxocompounds
of P V . This fact is due to the following reason. The stages of phosphoric acid neutralisation
in aqueous solutions are:
− − −
OH OH 2 OH
HPO⎯⎯⎯→HPO ⎯⎯⎯→HPO⎯⎯⎯→PO − − 3−
3 4 2 4 4
4
In melts, a similar sequence can be presented as
1
2− 2− 12
1
2−
/ O − 12 / O 4− 12 / O
PO 2 5 ⎯⎯⎯→PO3⎯⎯⎯→ PO 2 7 ⎯⎯⎯→PO 2
2
3−
4
[10.4.47]
[10.4.48]
Hence, correlations should be found between the anion in aqueous solution and corresponding
anhydro-acid in melt.
NaPO3 and Na4P2O7 may be easily obtained by the calcination of the corresponding
acidic salts, but reverse processes do not take place in aqueous media and pyro- and
metaphosphate exist in water as salts of stable acids. For arsenates which are more prone to
hydrolysis similar correlation should take place. The absence of the correlation is caused
mainly by kinetic limitations.
10.4.5 REACTIONS OF MELTS WITH GASEOUS ACIDS AND BASES
The gases present in the atmosphere over molten salts can react both with their principal
components and with impurities. In reactions of the first kind, which include, in particular,
hydrolysis, the gas (water) behaves as a Lux base since its action results in the increase of
O 2- concentration. In reactions of the second type, which are usually employed for the purification
of melts, the gas has acidic properties.
10.4.5.1 High-temperature hydrolysis of molten halides
These reactions were investigated mainly for alkaline halides and their mixtures. Hanf and
Sole 102 studied the hydrolysis of solid and molten NaCl in the temperature range 600-950 o C
by so-called “dynamic method” which consisted of passing inert gas (N2) containing water
vapor through a layer of solid or fused NaCl. The developed routine determined the equilibrium
constant of the following reaction:
NaCl + H O = NaOH + HCl
[10.4.49]
s, 1 2 gas s, 1
gas
The values of the equilibrium constants (log K) of [10.4.49] for solid and liquid sodium
chloride were in the ranges -8 to -6 (650-800 o C) and -6 to -5 (800-900 o C).
The hydrolysis of NaCl occurs to a slight extent, although the rate of HCl evolution exceeded
the expected value because of dissolution of NaOH formed in NaCl.
An evident disadvantage of the above method 102 is assumption that the partial pressure
of HCl in the gas phase is equal to the equilibrium pressure. The same method was used to
study the high-temperature hydrolysis and oxidation of sodium iodide. 103 According to the
estimates, the heats of the reactions:
NaI + H2 O = NaOH + HI
[10.4.50]