» Dental-Type Silica is obtained from sodium silicate solution by destabilizing with acid in such a way as to yield very fine particles. The sum of the Assay value and the Sodium Sulfate content is not less than 98.0 percent.
Packaging and storage Preserve in tight containers.
Labeling Label it to indicate the maximum percentage of Loss on drying.
pH 791: between 4.0 and 8.5 in a slurry (1 in 20).
Loss on drying 731 Dry it at 105 for 2 hours: it loses not more than the maximum percentage of its weight, as indicated in the labeling.
Sodium sulfate Transfer about 1 g of Dental-Type Silica, accurately weighed, to a platinum dish, wet with a few drops of water, add 15 mL of perchloric acid, and place the dish on a hot plate. Add 10 mL of hydrofluoric acid. Heat until copious fumes are evolved. Add 5 mL of hydrofluoric acid, and again heat to copious fumes. Add about 5 mL of boric acid solution (1 in 25), and heat to fumes. Cool, and transfer the residue to a 400-mL beaker with the aid of 10 mL of hydrochloric acid. Adjust the volume with water to about 300 mL, and bring to boiling on a hot plate. Add 20 mL of hot barium chloride TS. Keep the beaker on the hot plate for 2 hours, maintaining the volume above 200 mL. After cooling, transfer the precipitate and solution to a dried, tared 0.8-µm porosity filter crucible. Wash the filter and precipitate 8 times with hot water, dry the crucible at 105 for 1 hour, and weigh. The weight, multiplied by 0.6085, is the Sodium sulfate content of the amount of specimen taken. Not more than 4.0% is found.