Sodium Silicate → Sodium orthosilicate

An aqueous solution (c-2) of silica particles was prepared in accordance the paragraphs [0012] to [0031] of the description of JP 06-16414 A. To be specific, sodium water glass JIS No. 3 was dissolved in an aqueous sodium nitrate solution, thereby producing an aqueous sodium silicate solution. The aqueous sodium silicate solution was passed through a hydrogen-type cation exchange resin column, a hydroxyl group-type anion exchange resin column, and again, the hydrogen-type cation exchange resin column in the stated order. Thereafter, an aqueous sodium hydroxide solution was added, thereby obtaining an aqueous silicic acid solution. Subsequently, the aqueous silicic acid solution in an amount equivalent to 20% was subjected to vacuum distillation for removing evaporated water, and simultaneously, the remaining aqueous silicic acid solution was continuously and gradually supplied for successively performing the vacuum distillation, thereby producing a colloidal silica sol. The colloidal silica sol was passed through a hydrogen-type cation exchange resin column, a hydroxyl group-type anion exchange resin column, and again, the hydrogen-type cation exchange resin column in the stated order. Immediately thereafter, a special grade aqueous ammonia was added, thereby obtaining an aqueous silica sol (c-2) in which the pH was 9, the average particle diameter was 4 nm, and the concentration of each of various metal oxides was lower than 500 ppm (reactive functional group: silanol group; proportion: 100 mol %).