THE SIO2-H2O SYSTEM ALONG THE LOWER THREE-PHASE CURVE AND APPROXIMATE VALUES OF CRITICAL END-POINT TEMPERATURE

Abstract

The review and recalculation of all experimental data on the solubility of quartz and amorphous silica in liquid water and saturated vapour along the lower three-phase curve up to the critical end-points give the relation: in MSiO2(1 or v) = -[a1 + a2 In (MH2O(1)/MH2O(V)) + 1/(a3 In (MH2O(1)/MH2O(V)) + a4)] (± σ), where MSiO2 is a molar concentration (mol/dm3) of SiO2 (1 or v) in liquid water or vapour, respectively; (MH2O(1)/MH2O(V)) is a ratio of molar concentrations of H2O in liquid water and vapour, respectively. Constants: a1, a2, a3, a4 and standard deviation (± σ) are (1) for solubility of quartz in liquid water: 0.8958, 0.7081, 0.1283, 0.1217 and ± 0.218; (2) for solubility of quartz in vapour: 0.055, 3.8851, 0.1283, 0.1217 and ± 0.286; (3) for solubility of amorphous silica in liquid water: 0.4565, 0.4083, 0.0474, 0.2911 and ± 0.117; (4) for solubility of amorphous silica in vapour: -0.3843, 3.5853, 0.0474, 0.2911 and ± 0. 185, respectively. Distribution of silica between liquid water and saturated vapour fits the equation: In (MSiO2(1)/MSiO2(v)) = 3.177 In (MH2O(1)/MH2O(V)) - 0.8408 (± 0.068). The equations result from all the experimental data on silica solubility along the lower three-phase curve within the accuracy of the experimental measurements and provide evidence that the SiO2-H2O system must have a critical end-point temperature lower than that of pure water. The values of the critical temperature are in the interval 646.384-647.071 K (Tc of H2O is 647.096 K). The simple model of silica distribution, with Kd = (MSiO2(1)/MSiO2(v))/[MH2O(1)/M H2O(v))]n=const is valid along a three-phase line from 500 K to 646.23 K. The hydration number n of silica in liquid water and vapour is equal to 3.177 H2O molecules on average. The equation: In (MH2O(1)/MH2O(v)) + 1/[exp (0.1011 + 0.08182 In θ + 0.009746 (In θ)2.078) - 1], where θ = T/(T - 647.096), can be used for quick calculations since it has a maximum deviation measured from Wagner & Pruss's (1993) data that is less than 0.01 for the temperature (T) interval 338.15-647.096 K).