Colloidal Silicon Dioxide
Colloidal Silicon Dioxide is a submicroscopic fumed silica prepared by the vapor-phase hydrolysis of a silicon compound. When ignited at 1000 for 2 h, it contains NLT 99.0% and NMT 100.5% of SiO2.
• A. Procedure
Analysis: Transfer 5 mg to a platinum crucible, and mix with 200 mg of anhydrous potassium carbonate. Ignite at a red heat over a burner for 10 min, and cool. Dissolve the melt in 2 mL of freshly distilled water, warming if necessary, and slowly add 2 mL of ammonium molybdate TS to the solution.
Acceptance criteria: A deep yellow color is produced.
• B. Procedure
[NoteAvoid contact with o-tolidine when performing this test, and conduct the test in a well-ventilated hood. ]
Analysis: Place 1 drop of the yellow silicomolybdate solution from Identification test A on a filter paper, and evaporate the solvent. Add 1 drop of a saturated solution of o-tolidine in glacial acetic acid to reduce the silicomolybdate to molybdenum blue, and place the paper over ammonium hydroxide.
Acceptance criteria: A greenish blue spot is produced.
Sample: 500 mg
Analysis: Ignite the Sample in a tared platinum crucible at 1000 ± 25 for 2 h, cool in a desiccator, and weigh. Add 3 drops of sulfuric acid, and add enough alcohol to just moisten the sample completely. Add 15 mL of hydrofluoric acid, and in a well-ventilated hood evaporate on a hot plate to dryness, using medium heat (95105) and taking care that the sample does not spatter as dryness is approached. Heat the crucible to a red color with the aid of a Bunsen burner. Ignite the residue at 1000 ± 25 for 30 min, cool in a desiccator, and weigh. If a residue remains, repeat the Analysis, beginning with Add 15 mL of hydrofluoric acid. The weight lost by the assay specimen, previously ignited at 1000 ± 25, represents the weight of SiO2 in the portion taken.
Acceptance criteria: 99.0%100.5% on the previously ignited basis
• Loss on Ignition 733: Ignite the portion of Colloidal Silicon Dioxide, retained from the test for Loss on Drying, at 1000 ± 25 to constant weight: the previously dried Colloidal Silicon Dioxide loses NMT 2.0% of its weight.
• Arsenic, Method 1 211
Sample solution: To 2.5 g add 50 mL of 3 N hydrochloric acid, and reflux for 30 min using a water condenser. Cool, filter with the aid of suction, and transfer the filtrate to a 100-mL volumetric flask. Wash the filter and flask with several portions of hot water, and add the washings to the flask. Cool, and dilute with water to volume.
Analysis: A 15.0-mL portion of Sample solution, to which 3 mL of hydrochloric acid has been added, meets the requirements of the test, the addition of the 7 N sulfuric acid being omitted.
Acceptance criteria: NMT 8 ppm
• pH 791: 3.55.5, in a (1 in 25) dispersion
• Loss on Drying 731: Dry in a tared platinum crucible at 105 for 2 h: it loses NMT 2.5% of its weight. Retain the dried specimen in the crucible for the test for Loss on Ignition.
• Packaging and Storage: Preserve in well-closed containers.