Depolymerizing Colloidal Silica before Analysis

Clear solutions of silica may contain polysilicic acids or small colloidal particles that will not react completely with molybdic acid. Thus before determining total silica, depolymerization to monomer is necessary. 

Okura (296) recommended the following procedure to 50 ml of the hot sample in a platinum dish add 4 drops concentrated H SO, 1 ml 10% ammonium molybdate solution, and 20 mg NaF, in that order cool and measure the color. On the other hand, Tarutani (308) stated that F ion interferes with the silicomolybdic method and must be removed by complexing with boric acid or The difference 

Govett (304) recommended that solid samples be fused with NaOH (3 g per 0.1 g sample) in a nickel crucible for 10 min. leached into a plastic beaker, and acidified with 1 N H2SO4 to pH 1.5. (This should be done quickly.) Aqueous samples should likewise be boiled with solid NaOH in a nickel beaker and then diluted. 

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