Na2O activity and thermodynamic mixing properties of SiO2–Na2O–CaO melt

Abstract Electro-motive force (EMF) measurement using concentration cell with direct contact between sample and reference melts has been carried out to determine the Na 2 O activity of silicate melt. Thermodynamic mixing properties of silicate melts have been examined based on the temperature and compositional dependences of the Na 2 O activity. The Na 2 O activities for soda silicate melt and soda aluminosilicate melt measured by the method of this study are in agreement with previously reported values. The Na 2 O activities for (100 − x − y)SiO 2 –xNa 2 O–yCaO (x = 12–30; y = 5–22, mol%) melts have been measured between 1273 K and 1573 K. The Na 2 O activity increases with increasing Na 2 O and CaO content when the melt is a depolymerized composition. The effect of Na 2 O and CaO contents on the Na 2 O activity decreases with increasing SiO 2 content. They can be interpreted in terms of Gibbs energy of mixing as a concave function of composition. The comparison of partial molar entropy of mixing for soda-lime silicate and sodium silicate melts suggests the existence of local maximum of the entropy of mixing in depolymerized composition of SiO 2 –Na 2 O–CaO system. The effect of CaO on redox reaction of iron in silicate melt calculated from the Na 2 O activity is quantitatively consistent with that measured by voltammetry and wet-chemical analysis.