Retarding and accelerating effects of aluminum on the growth of particles of polysilicic acids were studied in the pH range 7-9 at 50°C by gel chromatography, spectrophotometry, and 27Al NMR. The effects depended on the pH and aluminum concentration. The greater retarding effect was observed at lower pH, while the greater accelerating effect at higher pH. It was concluded that the retarding effect is due to the formation of a negatively charged aluminosilicate site by substitution of aluminum for 4-coordinated silicon on polysilicic acid, and that the accelerating effect is due to the formation of unidentified aluminum species, which could not be detected by 27Al NMR, on the surface of polysilicic acid.
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