Preparation of Polysilicic Acid

Solutions containing up to 12% SiOj can be prepared by adding a thin stream of sodium silicate solution with Si0, Na]0 ratio of 3.25 1.0 into the vortex of a violently stirred solution of H,S04 kept at 0-5°C, stopping the addition when the pH rises to about 1.7. A 6% solution can be made continuously by bringing together solutions of sodium silicate and acid in a zone of intense turbulence and in such proportions that the mixture has a pH about 1.5-2.0. 

Solutions free from the sodium salt can be obtained by hydrolyzing methyl or ethyl silicate in water at pH 2 with a strong acid as a catalyst for hydrolysis and temporary stabilizer for the silicic acid. 

Aqueous solutions of polysilicic acid are always unstable since even at low concentration the polysilicic acid continues to polymerize, as has already been discussed in detail. Nevertheless, if the silica concentration is not too high the polymerization can be followed so that the solution can be used at a reproducible stage. 

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The preferred method for the preparation of polysilicic acid is by deionization of a sodium silicate solution with an ion exchange resin at room temperature. In this way the polysilicic acid solution is substantially free of electrolytes and, therefore, is more stable. 

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