Quartz Dissolution as Influenced by pH and the Presence of a Disturbed Surface Layer

Quartz particles (10 to 2 microns in diameter, produced by grinding) released 45 ppm of SiO 2 to a buffer solution of pH 7 during a 48 hr period. The grinding of quartz particles produced a disturbed surface layer with solubility properties intermediate between crystalline quartz (6 ppm SiO 2 ) and amorphous silica (120 ppm SiO 2 ). Removal of the disturbed layer by treatment with 1 N HF lowered the SiO 2 dissolution from 45 to 3 ppm. Use of 12 N HF decreased the amount of SiO 2 dissolved to 2 ppm, a result found to coincide with a preferential dissolution of the finer quartz particles in addition to the removal of the disturbed surface layer. The rate of dissolution of SiO 2 from quartz particles, subsequent to treatment with 1 N HF, increased with decrease in particle size, from 1 to 3 ppm for quartz particles 50 to 20 microns in diameter to 30 to 44 ppm for particles 2 to 0.2 microns in diameter, during each of three consecutive 14‐day extractions with pH 7 buffer solution. The amount of SiO 2 dissolved from quartz in buffer solutions increased in the pH range of 4 to 10, from 0.6 ppm of SiO 2 at pH 4 to 19 ppm at pH 10 during a 14‐day period of extraction from quartz particles, 10 to 2 microns in diameter. Quartz particles, subsequent to the removal of the disturbed surface layer, removed SiO 2 from pH 4 and pH 8 buffer solutions prepared with more SiO 2 in solution than would be released from quartz in the respective solution.