Measurement of the Activity of Na₂O in Na₂O-SiO₂Melts by Chemical Equilibration Method

Abstract

The activity of Na₂O in the Na₂O-SiO₂ system was measured at 1 100°C, 1 200°C and 1 300°C for compositions ranging from 33% to 60% of Na₂O. Na₂O-SiO₂ melts were equilibrated with molten leadsodium alloy to maintain the sodium chemical potential in a graphite crucible under one atmosphere of carbon monoxide. The activity of Na₂O was found to vary from 1×10^-7 at 40% Na₂O to 5×10^-5 at 60% Na₂O at 1 200°C. The results agree well with those measured by the electrochemical method.
Since the activity coefficient of sodium in lead-sodium alloy at elevated temperatures which was necessary for the calculation of the Na₂O activity was not available in the literature, it was also measured by equilibrating Na₂O· SiO₂ and Na₂O·2SiO₂ melts, thermodynamic properties of which are known, with carbon-saturated iron and lead-sodium alloy. The activity coefficients of sodium in lead-sodium alloy were found as 0.315 at 1 200°C and 0.582 at 1 300°C.
The activity of SiO₂ in the Na₂O-SiO₂ system was calculated from the activity of Na₂O, using the GIBBS-DUHEM relationship.