Effect of synthesis parameters on the composition and properties of the product silica

The properties of two silica samples were studied; one sample precipitated by ammonia from a saturated (NH4)(2)SiF6 solution and the other washed out from the sublimate obtained by joint evaporation of (NH4)(2)SiF6 and SiO2. These silicas are fundamentally different compounds. Their chemical composition was determined. Evolution of samples during heating to 1000 degrees C was interpreted using chemical analysis, IR spectroscopy, and X-ray powder diffraction. A possibility of removing fluorine and ammonia from test samples by heat and chemical treatment is demonstrated. Fluorine impurities in the form of fluoroammonium salts are removed completely during heating to 300-400 degrees C; surface fluoride ions are removed only upon heating to 800 degrees C.