Handbook of Solvents - George Wypych - ChemTech - Ventech!

21.3 Oxide solubilities in ionic melts 1485
from melt took place not only because of the cation acidity but owing to formation of new
phase - the precipitate of the oxide. Therefore, works in which the estimations of cation
acidities were based on measurements of changes in oxide concentration before and after
cation addition 3 or by interactions between, e.g., carbonates with Lux acids 4,5 contained distorted
results. The latter works 4,5 contain the additional error - insoluble metal carbonates
MeCO 3 after interaction with acid K 2Cr 2O 7 4 or NaPO3 5 were transformed into insoluble
chromates or phosphates - all reactions were heterogeneous. The above may be also referred
to works of Slama 6,7 where cation acidities were estimated on the base of reactions
2+
− +
Me + NO = MeO ↓ + NO
[21.3.6]
3 2
+ −
NO + NO = 2NO
↑ + O
[21.3.7]
2 3 2
1 2 2
there was no evidence of homogeneity of reaction [21.3.6].
The mentioned method seems to have no wide usage for studying behavior of oxides
in molten salts, solubility studies by isothermal saturation and potentiometric titration
methods are more precise and informative.
From the listed above scheme of equilibria [21.3.1-21.3.4] it follows that the fixation
of oxide ions by metal cations is in common case a heterogeneous process resulting in the
formation of a new phase and cation acidity may have no connection with the completeness
of interaction Me 2+ -O 2- in molten salts.
From eq. [21.3.1] it is clear that molecular oxide concentration in the saturated solution
is not dependent on the constituent ion concentrations but connected with the precipitate
properties, mainly with the molar surface of the deposit, σ.
Since the concentration of the non-dissociated oxide in the saturated solution, as a
rule, is hardly determined, for practical purposes the solubility product, the magnitudes of
PMeO (used below pP≡-logP),
P = a a ≈m
m
MeO 2+ 2− 2+ 2 −
[21.3.8]
Me O Me O
are usually employed for the description of the saturated solutions. The known values of
P MeO and K MeO give possibility to estimate the concentration of the non-dissociated oxide,
s MeO, in the saturated solution:
s
MeO
PMeO
= [21.3.9]
K
MeO
It should be noted, however, that there was no reliable method for determining dissociation
constants and sMeO. Let us consider two generally accepted methods for oxide solubility
determinations.
21.3.1.1 Isothermal saturation method
This method is simple enough and includes some modifications:
1. placing baked sample of the oxide in the melt-solvent and regular tests of the metal
concentration in the melt up to the saturation. Known test routines are either radiochemical 8
or complexonometric 9 analysis of cooled samples of the saturated solutions;