- Description: White coloured amorphous powder.
- Solubility: Practically insoluble in water and in mineral acids.
- Identification:
A: Transfer about 5 mg to a platinum crucible, and mix with about 200 mg of anhydrous potassium carbonate. Ignite at a red heat over a burner for about 10 minutes, 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: a deep yellow color is produced.
B: [Caution—Avoid contact with o-tolidine when performing this test, and conduct the test in a well-ventilated hood.] Place 1 drop of the yellow silicomolybdate solution obtained in 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: a greenish blue spot is produced.
Tests
-
- pH
between 3.5 and 5.5, in a 1 in 25 dispersion.
- Loss on drying
Dry it in a tared platinum crucible at 105 for 2 hours: it loses not more than 2.5% of its weight.
Retain the dried specimen, in the crucible, for the test for Loss on ignition.
Loss on ignition
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 not more than 2.0% of its weight.
Arsenic
Prepare the Test Preparation as follows. Transfer 2.5 g to a flask, add 50 mL of 3 N hydrochloric acid, and reflux for 30 minutes 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 , dilute to volume with water and mix a 15.0 ml portion of this solution , to which 3 ml of hydrochloric acid has been added meets the requirement of the test, the addition of the 7 N sulfuric acid being omitted. The limit is 8µg per g .
- Assay
Transfer about 500 mg of Colloidal Silicon Dioxide to a tared platinum crucible, ignite at 1000 ± 25 for 2 hours, 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 (95 to 105 ) 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 minutes, cool in a desiccator, and weigh. If a residue remains, repeat the procedure, 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.
