Activity of lead oxide in the system sodium oxide-lead oxide-silica
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lumina crucible. The cell was then assembled and the experiment was performed as described e l s e where. 1,2 During each run the emf was almost constant at a constant temperature and was reproducible t o +3 m y . The reversibility of the cell during the runs was proved by the following t e s t s : i) the emf of the cell was unaffected by temperature cycling, the emf was independent of oxygen flow rate in the r e f e r ence electrode; and ii) the cell emf was unaffected by an externally imposed emf. After every run which lasted from 10 t o 12 h including heating t i m e , the crucible and the electrodes were examined for corrosion. No appreciable corrosion was detected in any of the r u n s . Also the slag samples at the beginning and end of each run were taken and chemically analyzed to check the slag composition. RESULTS AND DISCUSSION Experimentally determined emf values are plotted against temperature for different slag compositions in Figs. 1(a) and (b). As the solubilities of oxygen, silicon and sodium in molten lead are very small and the free energy of formation of sodium oxide, silica and their binary solutions are strongly negative, one can assume the metal phase to consist of pure lead and represent the cell reaction by the equation Pb(/) + 1/202(g) = PbO (liquid slag).
[I]
Accordingly the emf of the cell is given by E = E°
-
RT ~
In apbO
[2]
where E and E° are the emfs of the ceils containing a given slag and the one containing pure lead oxide respectively (in volts), R is the universal gas constant (in J/mob/K), T is the absolute temperature in K, n is the valency of oxygen ion, F is Faraday's constant (in J/equivalent/volt) and apbO is the activity of lead oxide in the slag phase. Using Eq. [2] the activity of lead oxide in various s l a g s is calculated at 1000°C and is plotted as a function of mole fraction of lead oxide for various sodium oxide t o s i l i c a ratios in Fig. 2. In Fig. 3 isoactivity curves for lead oxide a r e plotted in the ternary system sodium oxide-lead oxides i l i c a at 1000°C. The data obtained by the extrapolation of these isoactivity curves t o the lead oxides i l i c a line yield activity values of lead oxide in the system lead oxide-silica. These values are in good agreement with those of e a r l i e r workers, x'3-6 This fact is considered as a proof for the satisfactory working of the cell. The results of the present inVOLUME 8B,MARCH 1977-15
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F i g . 2--Activity of lead oxide vs m o l e fraction of lead oxide at 1000°C for various Na20/SiO2 ratios.
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¢b) F i g . 1--(a) E m f as a function of temperature for different slags; (b) E m f as a function of temperature for different slags. 16-VOLUME 8B, MARCH 1977
vestigation are vastly different from t h o s e reported by E s i
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