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Abstract
A knowledge of the thermodynamic stabilities of the lanthanum tungstates measured as the Gibbs energies of formation from the free oxides is important as lanthanum is used as a powerful desulphurizing agent and tungsten as a special alloying element in the production of clean steels and special steels. These data will also be useful as the system La2O3 WO3 can be a model since it exhibits many compounds between the end members. Excepting one EMF work, no information is available in the literature on the thermodynamics of this system. This system exhibits the compounds 5 La2O3·22 WO3, La2 O3 · 3 WO3, La2O · 2 WO3, 7 La2O3 · 8 WO3,La2O · WO3, 3 La2O3 · 2 WO3 and 5 La2O3 · 2 WO3. The compounds were prepared from reagent grade free oxides by solid state sintering at high temperature followed by X-ray diffraction. The standard Gibbs energies of formation of the compounds from the free oxides were determined by operating the following galvanic cells with calcium fluoride as solid electrolyte.
O2(g), Pt/ La2O3 · LaOF// CaF2/ / 5 La2O3 · 22 WO3, WO3, LaOF/Pt, O2(g)
O2(g), Pt/La2O3 · LaOF//CaF2//5 La2O3 · 22 WO3, La2O3 · 3 WO3, LaOF/Pt, O2(g)
O2(g), Pt/La2O3 · LaOF//CaF2// La2O3 · 3 WO3, La2O3 · 2 WO3 LaOF/Pt, O2(g)
O2(g), Pt/La2O3 · LaOF//CaF2// La2O3 · 2 WO3, 7 La2O3 · 8 WO3, LaOF/Pt, O2(g)
O2(g), Pt/La2O3 · LaOF//CaF2//La2O3 · WO3, 3La2O3 · 2 WO3 LaOF/Pt, O2(g)
O2(g), Pt/La2O3 · LaOF//CaF2//3 La2O3 · 2 WO3, 5La2O3 · 2 WO3 LaOF/Pt, O2(g)
O2(g), Pt/ La2O3 LaOF// CaF2/ / 3 La2O32 WO3, La2O3, 2 WO3 LaOF/Pt, O2(g)
Stable, reproducible EMFs were obtained in the temperature range 1123 to 1273 K. The net cell reactions are respectively as follows:
5 La2O3 + 22 WO3 = 5 La2O3 · 22 WO3
7 La2O3 + 3 (5 La2O3 · 22 WO3) = 22 (La2O3 · 3 WO3)
La2O3 + 2 (La2O3 · 3 WO3) = 3 (La2O3 · 2 WO3)
3 La2O3 + 4 (La2O3) = 7 La2O3 · 8 WO3
La2O3 + 7 La2O3 · 8 WO3 = 8 (La2O3 · WO3)
La2O3 + 2 (La2O3 · WO3) = 3 La2O3 · 2 WO3
2 La2O3 + 3 La2O3 · 2 WO3 = 5 La2O3 · 2 WO3
The standard Gibbs energies of formation of the different lanthanum tungstates are then calculated by appropriately combining from the above cell reactions and their Gibbs energy changes obtained from the experimental EMF data. The present data are discussed in the light of EMF data reported in the literature for WO3-rich compounds in the system.