Overall Effect of pH on Gelling

At low pH both sols gel, and salt has little effect. In the neutral region the pH of minimum stability is higher ( ) when salt is present. 

Monomeric silicic acid, Si(OHL has never been isolated. It is a very weak acid and exists only in dilute aqueous solution, since it polymerizes when it is concentrated. It 

Monomer can be removed from solution by strong-base ion-exchange resin, presumably because it is ionized to HSiOj by the OH ions at the resin surface, and is then adsorbed. 

The various methods of preparing monosilicic acid may be summarized as follows. A saturated solution of monosilicic acid, SifOH), containing about 0.01% SiOj, is obtained when pure amorphous silica is equilibrated with water at room temperature. A more concentrated (supersaturated) solution can be obtained only indirectly by liberating monosilicic acid from its compounds under carefully controlled conditions at low temperature and low pH, dilute solutions remain supersaturated with respect to amorphous silica for appreciable periods. For example, at pH 3 and 0°C, solutions of monosilicic acid up to O.l (0.6% SiOj) can be prepared by spontaneous hydrolysis of monomeric silicon compounds, sich as silicon tetrachloride or methyl orthosilicate, and also by reacting monomeric silicates, such as sodium or magnesium orthosilicates or hydrated crystalline sodium metasilicate, with dilute acid. 

Jander and Heukeshoven (8) reported that amorphous silica gel gives a true solution of silica in water later Jander and Jahr (9) found that the silica in solution had a diffusion coefficient of 0.53, indicating a molecular size about equivalent to Si(OH),. 

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