ENTHALPY OF SODIUM-SILICATE GLASSES AND LIQUIDS

The enthalpy data of Hummel and Schwiete (1) for sodium silicate glasses at 298.15 K have been fitted by power series in the mole fraction. It is shown that four coefficients provide an excellent fit of the data (X1 = .1818 to .6667) but five are required for a reasonable extrapolation to pure NaO.5(X1 = 1). If estimates for the partial molal enthalpy of SiO2 are included in the least squares, the equation also extrapolates well to pure SiO2. Combining these values with the expressions for the excess heat capacity of mixing (2) and known solubility data (3,4) for the binary system, one obtains a consistent set of coefficients for the calculation of enthalpies and activities at any temperature and composition. The NaO.5-SiO2 phase diagram calculated from these values is shown. © 1979.