Handbook of Solvents - George Wypych - ChemTech - Ventech!

21.3 Oxide solubilities in ionic melts 1489
Li 2O, and 1.2×10 -2 for BaO, these values showed that all studied cations had considerable
acidic properties. 26,27
Ovsyannikova and Rybkin 3 developed the cation acidity scale in molten KCl-NaCl at
700 o C on the basis of e.m.f. shifts after addition of metal sulfates. Acidic properties of the
main subgroup elements decreased with the increase of their atomic numbers and there was
no similar relationships for side groups and transition metals. Quantitative data elucidation
according to acid-base equilibrium [21.3.2] in this melt is affected by additional reaction:
2−
2−
SO = SO ↑ + O
[21.3.12]
4
3
caused by additions to chloride melts of the corresponding sulfates. The use of sulfates may
cause SO 3 formation from highly acidic cation solutions. In particular, Na + is the most
acidic cation of KCl-NaCl melt, therefore, additions of more basic oxides than Na 2O should
result in oxide ion exchange:
I
+ +
Me O + 2Na→ 2Me
+ Na O
2 2
[21.3.13]
which is shifted to the right. Similar considerations have been made for K + -Na + -O 2- system
in molten KCl-NaCl. 28 From [21.3.13] it follows that any solution of cations in
Na-based melts cannot create oxide ion concentration exceeding that of equimolar addition
of corresponding salt of the most acidic cation of the melt (Na + ). However, the oxide ion
concentration in BaSO 4 solution has been shown 3 to be 10 times higher than in K 2SO 4 and
Na 2SO 4 (ΔE=0.11 V). Similar deviations
for other cations are
smaller, e.g., Cs - 0.49 V, Sr - 0.30
V, Rb - 0.1 V.
A potentiometric study of
ZnO, MgO, NiO and SrO solubilities
at 700 o C have been
conducted 29 by direct and reverse
titrations of cation by KOH. These
measurements have resulted in a
set of characteristics correspond-
Figure 21.3.1. Alkaline oxide solubilities in KCl-NaCl: 1 11,28-32 at
1000K; 2; 29 3; 8 4; 9 5; 33,38 6; 40,41 7; 42 at 700 o C.
ing (by the calculation formula) to
solubility products and dissociation
constants. The averaging or
another procedure of data treat-
ment were not included. Oxide solubilities have been found to increase in sequence:
MgO