Evaluation of thermodynamic properties of H4SiO4 in the ideal gas state from experimental data

Abstract

Thermodynamic properties of gaseous silicon hydroxides are necessary for analyzing the silicate condensation phenomena in the primordial solar nebula [Hashimoto, 1992], the silica precipitation in steam-rich geothermal systems and recent concerns with the corrosion or deposition of SiO2-containing alloys and ceramics in high-temperature moist environments. Experimental transpiration studies (i.e. studies of the partial pressures of Si over solid phases in the presence of steam) of the reaction of water vapor with cristobalite [Hashimoto, 1992; Jacobson, et al., 2005] agree that at 1100-1650 K and water pressures close to 0.1 MPa the vapor phase concentration of Si is determined by the reaction SiO2(s) + 2H2O(g) = H4SiO4(g). (1) Mass spectrometric data on the volatile species formed from SiO2(s) and water at 1473–1773 K and water vapor pressure between 0.018 and 0.094 MPa [Opila, et al., 1997] also confirm that H4SiO4 is the primary reaction product. However, the literature data on the thermodynamic properties of gaseous H4SiO4 [Allendorf, et al., 1995; Jacobson, et al., 2005; Rutz and Bockhorn, 2005] differ up to 18 kJmol and 9 JKmol in the value of enthalpy of formation and the entropy at Tr=298.15 K and the standard pressure   P 0.1 MPa. Therefore, it was decided employ all available data for the reaction (1) to determine the optimal values of ΔfG and S of H4SiO4(g) at 298.15 K and 0.1 MPa. Selection of experimental data In addition to the transpiration data we accepted data on the solubility of quartz and amorphous silica in water vapor of density below 15 kg·m. The accepted data set of data to calculate the values of equilibrium constant of reaction (1) is given in Table.