Thermodynamics of gas phase species in the Si-O-H system

The transpiration method was used to study the reaction of SiO2 (cristobalite) with water vapor. Measurements were made between T = (1073 and 1728) K with water vapor contents from (0.05 to 0.6) by volume fraction in argon. From the results, the thermodynamic parameters for Si(OH)4(g) and SiO(OH)2(g) were determined. At T = (1073 to 1373) K, Si(OH)4 is the primary vapor species and second law measurements lead to ΔfH∘(1200 K) = (−1344.6 ± 2.7) kJ · mol−1 and S∘(1200 K) = (544.4 ± 2.1) J · mol−1 · K−1. Third law measurements lead to ΔfH∘(298 K) = (−1351.3 ± 1.7) kJ · mol−1. These are in very good agreement with previous measurements and previous ab initio calculations. At T = (1673 and 1728) K, there is evidence of a second vapor species, SiO(OH)2(g). By subtracting the extrapolated Si(OH)4(g) pressure, it was possible to do a third law analysis and obtain a ΔfH∘(298 K) for SiO(OH)2(g) of (−836 ± 40) kJ · mol−1. Previous measurements and ab initio calculations are within this range.