Handbook of Solvents - George Wypych - ChemTech - Ventech!

1486 Victor Cherginets
2. the addition of known weights of oxide to the melt up to the saturation detected by a
potentiometric technique. 10,11
The sum of molecular and ionic form concentrations in the saturated solution, Σs MeO,is
obtained as the main result of such investigations, this magnitude may be expressed by the
following equation:
∑
P
sMeO = PMeO
+
K
MeO
MeO
[21.3.10]
Since Σs MeO is the main result of this method, there exists a possibility to determine the
dissociation constant according to routine 2 if an oxygen electrode was preliminary calibrated
by known additions of the completely dissociated Lux base. But there is no information
about such studies.
Studies connected with the analysis of cooled samples allow to obtain thermal dependencies
of oxide solubilities by analysis of the saturated melt heated to the definite temperature.
Main disadvantages of isothermal saturation method are:
• the inclusion of suspended particles of oxide in the sample for analysis and
following overvaluing of the results;
• the magnitude determined is the concentration but not its logarithm; range of oxides
available for this routine is essentially narrowed and this method is unsuitable for
studies of practically insoluble substances. The presence of oxygen impurities in the
melt studied leads to lowering the calculated solubilities, especially, if oxide
solubility is of the same order as initial oxide concentration since oxide admixtures
suppress the oxide dissolution.
21.3.1.2 Potentiometric titration method
This method is more frequently used than the previously described one. It may be employed
for oxide solubility studies in a wide range of concentrations of saturated solutions. This
method allows to determine solubility products of oxides and in some cases (the existence
of the non-saturated solution region) dissociation constants may be calculated.
As compared with isothermal saturation a potentiometric method possesses the following
advantages:
• e.m.f. values are linearly dependent on logarithm of O 2- concentration; it is possible
to measure the latter even in solutions with extremely low potential-determining ion
concentrations;
• simplicity of e.m.f. measurements and good reproducibility of the experimental
values;
• possibility to made studies in situ, i.e., without any effect on the process studied.
Main limitations of the potentiometric titration are due to the potential of liquid junction,
however, the latter is negligible in molten salts, 10 and the absence of reliable and generally
accepted methods for calculation of activity coefficients for oxide ions and cations.
As for determination of solubility and dissociation parameters it should be noted that
this method allows to determine values KMeO only if the potentiometric curve contains the
non-saturated solution region. Studies in the wide temperature range are impeded as compared
with the isothermal saturation method.