Handbook of Solvents - George Wypych - ChemTech - Ventech!

21.3 Oxide solubilities in ionic melts 1487
21.3.2 OXYGEN-CONTAINING MELTS
Frederics and Temple studied CuO, MgO, PbO and ZnO solubilities in molten equimolar
mixture KNO3-NaNO3 in the range 290-320 o C. 12 The solubilities were determined by isothermal
saturation method with the potentiometric control of solubilities. Solubility products
were 2.24×10 -13 for CuO, 2.2×10 -13 for ZnO, 2.16×10 -14 for MgO and 4.34×10 -12 for PbO.
The increase of the temperature to 320 o C led to the increase of the solubilities by 6-10
times. 12
Individual Lux acids derived from B2O3 (borax) and P2O5 (sodium metaphosphate) are
interesting since they are often used as acidic components of different fluxes. Delimarsky
and Andreeva 10 determined PbO solubility in molten NaPO3 at 720 o C by isothermal saturation
method with the potentiometric control of saturation. The concentration cell with oxygen
electrodes Pt(O2) was used. PbO solubility was estimated as 31.6 mol%.
Nan and Delimarsky investigated solubilities of acidic metal oxides (MoO3,WO3, TiO2) in molten borax at 900 o C by a similar method. 13 The control of saturation was performed
by a gravimetric analysis. Solubilities of the oxides deviated from those predicted
by Shreder’s equation, because of chemical interactions between the substances dissolved
and the solvent. Oxide solubilities increased with the reduction of melting point of oxide.
Solubilities were 66.1 mol% for MoO3, 63.2 mol% for WO3 and 21.2 mol% for TiO3. 21.3.3 HALIDE MELTS
21.3.3.1 The eutectic mixture KCl-LiCl (0.41:0.59)
Delarue 14,15 performed qualitative studies on oxide solubilities in the melt at t~500 o Cbyvisual
method. The experiments consisted of addition of potassium hydroxide, KOH, to the
metal chloride solution in KCl-LiCl. If there were no formation of the oxide deposit then the
oxide was considered soluble, in other cases, oxide was referred to slightly soluble or insoluble
depending on the amount of oxide deposited. CdO, PbO, BaO, CaO and Ag2O belonged
to the group of soluble oxides, CoO, NiO, ZnO were examples of slightly soluble
oxides and MgO, BeO and Al2O3 were insoluble. Copper (II) oxide was unstable in molten
chlorides since its dissolution was accompanied by the reduction of Cu II to Cu I . This transformation
has been demonstrated 16 to be substantial even under the partial pressure of chlorine
equal to 1 atm. The similar behavior is typical of other transition cations in the highest
degree of oxidation, e.g., Tl III ,Fe III , and Au III . The addition of corresponding chlorides to the
saturated solutions formed by non-oxidizing cations resulted in dissolution of latter due to
reactions similar to:
2CuCl + ZnO↓ → 2CuCl
+ ZnCl + O ↑ [21.3.11]
2 2
1 2 2
An important feature of oxide solubilities 14,15 was the essential difference of solubility
values of “powdered” (added as powder) and prepared in situ oxide solubilities. Solubilities
of latter samples have been found to be considerably greater (CaO, CdO), although there
was no explanation to this observation. Quantitative characteristics of oxides solubilities in
KCl-LiCl were not presented. 14,15
Laitinen and Bhitia 17 determined solubility products for certain oxides at 450 o Cinorder
to evaluate possibility to use corresponding metal-oxide electrodes for oxoacidity measurements
in molten salts. Oxide solubilities were as the following (mole/kg): NiO -
3.3×10 -4 , BiOCl - 6.8×10 -4 , PdO - 9.4×10 -3 , PtO - 3.32×10 -2 ,Cu 2O - 3.8×10 -2 .