SiO2xH2O Anhydrous 60.08
Silicon Dioxide is obtained by insolubilizing the dissolved silica in sodium silicate solution. Where obtained by the addition of sodium silicate to a mineral acid, the product is termed silica gel; where obtained by the destabilization of a solution of sodium silicate in such manner as to yield very fine particles, the product is termed precipitated silica. After ignition at 1000C for not less than 1 hour, it contains not less than 99.0 percent of SiO2.
Packaging and storage: Preserve in tight containers, protected from moisture. Labeling: Label it to state whether it is silica gel or precipitated silica. Identification: Transfer about 5 mg to a platinum crucible, mix with about 200 mg of anhydrous potassium carbonate, ignite at a red heat over a burner for 10 minutes and cool. Dissolve the melt in 2 mL of recently distilled water, warming if necessary, and slowly add 2 mL of ammonium molybdate: a deep yellow color is produced. pH: between 4 and 8, in a slurry (1 in 20). Loss on drying: Dry it at 145 for 4 hours: it loses not more than 5.0% of its weight. Loss on ignition: Ignite about 1 g of it, previously dried and accurately weighed, at 1000C for not less than 1 hour: it loses not more than 8.5% of its weight. Chloride: Boil 5 g in 50 mL of water under a reflux condenser for 2 hours, cool, and filter. A 7-mL portion of the filtrate shows no more chloride than corresponds to 1.0 mL of 0.020 N hydrochloric acid (0.1%). Sulfate: A 10-mL portion of the filtrate obtained in the test for Chloride shows no more sulfate than corresponds to 5.0 mL of 0.020 N sulfuric acid (0.5%). Arsenic: To pass the test. --- The limit is 3 ppm. Heavy metals: To pass the test. --- The limit is 0.003%. Assay: Transfer about 1 g of Silica Gel to a tared platinum dish, ignite at 1000 for 1 hour, cool in a desiccator, and weigh. Carefully wet with water, and add about 10 mL of hydrofluoric acid, in small increments. Evaporate on a steam bath to dryness, and cool. Add about 10 mL of hydrofluoric acid and about 0.5 mL of sulfuric acid and evaporate to dryness. Slowly increase the temperature until all of the acids have been volatilized and ignite at 1000C. Cool in a desiccator, and weigh. The difference between the final weight and the weight of the initially ignited portion represents the weight of SiO2.