NOVELTY - Preparing an aqueous colloidal silica sol comprises (a) preparing an aqueous solution of a water-soluble alkali metal silicate by mixing the water-soluble alkali metal silicate or an aqueous alkali metal silicate solution with an acidifying agent; (b) contacting the acidic alkali metal silicate solution with e.g. a basic anion exchange resin of hydroxyl type; and (c) forming a stable colloidal silica sol by adjusting one/one of the nucleation and particle growth to appropriate temperature, concentration and pH value of the aqueous silica solution obtained from the step (b).    USE - The aqueous colloidal silica sol is useful for polishing purpose and the preparation of silica. Silica is useful for: the preparation of elemental silicon or high-purity raw material for the production of high purity quartz glass for optical fibers or glass devices for laboratory and electronics; producing glass blanks e.g. boules, glass moldings e.g. casing pipes, preferably overcladding tubes or core rods, nuclear materials in planar waveguides, crucibles optical lenses, prisms, photomasks, diffraction gratings, electrical, thermal and magnetic insulators, containers and apparatus for the chemical, pharmaceutical, semiconductor and solar industries, glass rods, glass tubes and lamps; and coating metals, plastics, ceramics or glass. Silica is useful as intra-cladding material in optical fibers, a filler in metals, glasses, polymers, a polishing agent for the semiconductor material and electric circuits and as support material in the manufacture of solar cells and for catalysts (all claimed).    ADVANTAGE - The process provides aqueous colloidal silica sol with high purity.    DETAILED DESCRIPTION - Preparing an aqueous colloidal silica sol comprises (a) preparing an aqueous solution of a water-soluble alkali metal silicate with a pH of less than 2 by mixing the water-soluble alkali metal silicate or an aqueous alkali metal silicate solution with an acidifying agent; (b) contacting the acidic alkali metal silicate solution with a basic anion exchange resin of hydroxyl type and an acidic cation exchange resin of hydrogen type in an arbitrary order, where the anion exchange resin and cation exchange resin are spatial separated or mixed thoroughly; and (c) forming a stable colloidal silica sol by adjusting one/one of the nucleation and particle growth to appropriate temperature, concentration and pH value of the aqueous silica solution obtained from the step (b). INDEPENDENT CLAIMS are included for:    (1) the aqueous colloidal silica sol comprising less than or equal to 5 ppm of aluminum, less than or equal to 5 ppm of calcium, less than or equal to 20 ppm of iron, less than or equal to 5 ppm of barium, less than or equal to 7 ppm of titanium, less than or equal to 5 ppm of zirconium, less than or equal to 5 ppm of magnesium and less than or equal to 5 ppm of phosphorus, where the sum of the above impurities is less than 50 ppm;    (2) the aqueous solution of silicic acid as an intermediate product obtained after the step (b) comprising the above impurities; and    (3) silica obtained by the dehydration of the aqueous colloidal silica sol or aqueous silicic acid solution comprising the impurities.