Determination of Na2O activities in silicate melts by EMF measurements

An EMF cell using a Na-β″-alumina electrolyte has been designed for the quantification of the thermodynamic activity of Na₂O (aNa₂O) in a series of sodium-bearing silicate liquids at high temperature. Initial experiments have been performed using Na₂O–0.663WO₃ and Na₂O–0.555MoO₃ as reference liquids. Values of aNa₂O derived for Na₂O–2SiO₂ binary melt are found to be in excellent agreement with data from the literature, confirming the validity of the method. To extend use of this experimental set-up to higher temperature, the aNa₂O of industrial C-glass has been calibrated as a reference liquid at temperatures up to 1263 °C. The influence of additions of CaO, Al₂O₃ and B₂O₃ on the Na₂O activity of binary sodium-silicates has been quantified. For each glass composition, measured values of aNa₂O are a function of temperature, log(aNa₂O) varying as a function of inverse absolute temperature. Activation energies derived from these data are all generally similar with the exception of industrial E-glass, which is rich in Al and poor in Na. At constant temperature, additions of network forming Al₂O₃ and B₂O₃ to a Na₂O–SiO₂ binary melt yield a decrease of the activity of Na₂O, while addition of network modifying CaO results in an increase in (aNa₂O). These changes are qualitatively consistent with predictions based upon expected modifications of melt structure. However, measured values of log(aNa₂O) do not correlate perfectly with theoretical models of glass basicity, suggesting that either sodium activity is decoupled from melt basicity, or that current models are insufficient to calculate that parameter, in particular for the case of liquids poor in Na and rich in Al.