In view of the discordant theories for the mechanism of sedimentation of silica in natural waters, chemical properties of silica have been studied further. The solubility of amorphous silica has been measured over a wide range of pH and temperature. The study on the depolymerization of colloidal silica indicated that sea-water is not saturated with respect to molecularly dispersed silica. The presence of a small amount of aluminium greatly influences the state of silica in water. Thus colloidal silica can be precipitated even by a trace amount of aluminium, while a larger amount of the latter is required for the precipitation of molecularly dispersed silica. The optimum pH for the precipitation of colloidal silica is about 4·5 in contrast to that of about 8 for molecularly dispersed silica.
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