(Ph. Eur. monograph 0738)
63231-67-4
Action and use
Excipient.
DEFINITION
Colloidal hydrated silica produced by precipitation or gelation process.
Content
98.0 per cent to 100.5 per cent of SiO2 (Mr 60.1) (ignited substance).
CHARACTERS
Appearance
White or almost white, light, fine, amorphous powder.
Solubility
Practically insoluble in water and in mineral acids, with the exception of hydrofluoric acid. It dissolves in hot solutions of alkali hydroxides.
IDENTIFICATION
About 20 mg gives the reaction of silicates (2.3.1).
TESTS
Solution S
To 2.5 g add 50 mL of hydrochloric acid R and mix. Heat on a water-bath for 30 min, stirring from time to time. Maintain the original volume by adding dilute hydrochloric acid R. Evaporate to dryness. Add to the
residue a mixture of 8 mL of dilute hydrochloric acid R and 24 mL of water R. Heat to boiling and filter under reduced pressure through a sintered-glass filter (16) (2.1.2). Wash the residue on the filter with a hot mixture of 3 mL of dilute hydrochloric acid R and 9 mL of water R. Wash with small quantities of water R, combine the filtrate and washings and dilute to 50 mL with water R.
pH (2.2.3)
4.0 to 7.0.
Suspend 1.0 g in 30 mL of a 75 g/L solution of potassium chloride R.
Water-absorption capacity
In a mortar, triturate 5 g with 5 mL of water R, added drop by drop. The mixture remains powdery.
Substances soluble in hydrochloric acid
Maximum 2.0 per cent.
In a platinum dish, evaporate to dryness 10.0 mL of solution S and dry to constant mass at 100-105 °C. The mass of the residue is not more than 10 mg.
Chlorides (2.4.4)
Maximum 0.1 per cent.
Heat 0.5 g with 50 mL of water R on a water-bath for 15 min. Dilute to 100 mL with water R and centrifuge at 1500 g for 5 min. Dilute 10 mL of the supernatant to 15 mL with water R.
Sulfates (2.4.13)
Maximum 1 per cent.
Dilute 2 mL of solution S to 100 mL with distilled water R.
Iron (2.4.9)
Maximum 300 ppm.
To 2 mL of solution S add 28 mL of water R.
Loss on ignition
Maximum 20.0 per cent, determined on 0.200 g in a platinum crucible by heating at 100-105 °C for 1 h and then at 900 ± 50 °C for 2 h.
ASSAY
To the residue obtained in the test for loss on ignition add 0.2 mL of sulfuric acid R and a quantity of ethanol (96 per cent) R sufficient to moisten the residue completely. Add 6 mL of hydrofluoric acid R and evaporate to dryness at 95-105 °C, taking care to avoid loss from sputtering. Wash the inside of the dish with 6 mL of hydrofluoric acid R and evaporate to dryness again. Ignite at 900 ± 50 °C, allow to cool in a desiccator and weigh. The difference between the mass of the final residue and that of the mass obtained in the test for loss on ignition corresponds to the mass of SiO2 in the test sample.
