The solubility of amorphous silica at 25°C

Solutions in contact with solid amorphous silica at room temperature tend to become supersaturated. Experiments carried out over long periods of time show that steady states of supersaturation, 120 to 170 ppm, may persist for months. Eventually, however, the colorimetrically detectable dissolved silica in contact with silica gel decreases to about 115 ppm. Dissolved silica in waters collected from hot springs (p;H range 6.0 to 8.5) also slowly polymerizes and approaches 115 ppm. Extrapolation to 25°C of solubility data taken at high temperatures also yields a value of about 115 ppm for the solubility of amorphous silica. It is concluded that in nearly neutral solutions 115 ppm is the solubility of amorphous silica at 25°C, the differential heat of solution, ΔH, is 3.35 kcal/mole, and the standard free energy of solution, ΔF°25°C, is approximately 3.7 kcal/mole.