Instability of SiO2 Colloids and Sorption of Ca2+ Ions

Sook Peng Chan1, David S. Fraser1, Stephen Y. S. Cheng1, Yingnian Xu1 and Yoshikata Koga1, 1
1 Department of Chemistry, The University of British Columbia Vancouver, British Columbia, Canada V6T 1Z1
2 Center for Ceramics Research, Research Laboratory of Engineering Materials Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama, 227 Japan

Abstract: SiO2 sols were made unstable by addition of Ca2+ ions. The resulting states of instability were classified as gelation, flocculation, and precipitation by means of observation, by checking the Tyndall effects on the supernatant or suspending solution, as appropriate, and by measuring the apparent densities of flocculated mass. The concentrations of free Ca2+ ions left in solution were measured by means of a Ca2+ ion selective electrode. The amounts sorbed onto SiO2 particles were then calculated by material balance. It was found that while the amount sorbed dictates the limit of stability, the SiO2 concentration in the mixture is an important factor deciding the state of instability. Depending on the SiO2 concentration, there were two distinct flocs with the apparent floc density of 6 ± 1 and 12 ± 1 mg SiO2/ml.

Key Words: Ca2+ induced instability • Ca2+ sorption • Flocculation • Gelation • Precipitation • Silica colloids • Two distinct flocculates

Clays and Clay Minerals; August 1995 v. 43; no. 4; p. 478-481; DOI: 10.1346/CCMN.1995.0430412
© 1995, The Clay Minerals Society
Clay Minerals Society (www.clays.org)