The solubility of amorphous silica in water at high temperatures and high pressures was investigated, using commercial gel and vitreous silica (fused quartz) as starting materials. The experiments were carried out in an Inconel X bomb at temperatures from 180° to 382°C and pressures from 200 to 1379 bars. Periodically, small amounts of solution were withdrawn from the bottom of the bomb through a stainless steel filter into high-pressure stainless steel capillary sampling line, quenched in an ice bath, and immediately diluted with previously weighed silica-free water. The diluted samples were analyzed for colorimetrically-reactive and total silica. The silica gel gave the same colorimetric silica as the vitreous silica.

The solubility of amorphous silica at the vapor pressure of the solution, from 0° to 250° C, is given by the equation log С = −731/T + 4.52, where С is the silica concentration in mg/kg and T is absolute temperature. The maximum solubility at the vapor pressure of solution is 1660 mg/kg at 340°C, and the extrapolated solubility at the critical point is 890 mg/kg. At a constant pressure of 1034 bars, the solubility of amorphous silica from 0° to 380°C is given by the equation log С = −810T + 4.82. The differential heat of solution, ΔH, is 3.71 ± 0.05 kcal/mole, and the differential entropy of solution, ΔS, is 13.9 ± 0.05 cal/mole.

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