Purified Siliceous Earth

This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition

Issued and maintained by the United States Pharmacopeial Convention (USP)

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1 DEFINITION

Purified Siliceous Earth is a form of silica (SiO₂) consisting of the frustules and fragments of diatoms, purified by calcining.

2 IMPURITIES

2.1 Loss on Ignition 〈733〉

Sample: 1 g, previously dried

Analysis: Ignite the Sample at 980 ± 25° for 1 h in a tared platinum or porcelain crucible.

Acceptance criteria: NMT 2.0%

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2.2 Leachable Arsenic

Sample solution: To 10.0 g in a 250-mL beaker add 50 mL of 0.5 N hydrochloric acid, cover with a watch glass, and heat at 70° for 15 min.

Cool, and decant through a Whatman No. 3 filter paper into a 100-mL volumetric flask. Wash the slurry with three 10-mL portions of water, preheated to 70°, and dilute with water to volume.

Analysis: A 3.0-mL portion of the Sample solution meets the requirements in Arsenic 〈211〉, Procedures, Procedure 1.

Acceptance criteria: NMT 10 µg/g

2.3 Leachable Lead

Sample: A 10.0-mL portion of the Sample solution prepared in the test for Leachable Arsenic

Control: 10 mL of Diluted Standard Lead Solution in Lead 〈251〉, Procedures, Procedure 1

Analysis: The Sample meets the requirements in Lead 〈251〉, Procedures, Procedure 1.

Acceptance criteria: NMT 10 µg/g

2.4 Limit of Nonsiliceous Substances

Sample: 200 mg

Analysis: Transfer the Sample to a tared platinum crucible, add 5 mL of hydrofluoric acid and 2 drops of dilute sulfuric acid (1 in 2), and evaporate gently to dryness. Cool, add 5 mL of hydrofluoric acid, evaporate again to dryness, and ignite to constant weight.

Acceptance criteria: The weight of the residue is NMT 50 mg.

3 SPECIFIC TESTS

3.1 Loss on Drying 〈731〉

Analysis: Dry a sample at 105° for 2 h.

Acceptance criteria: NMT 0.5%

3.2 Acid-Soluble Substances

Sample: 10.0 g

Analysis: Digest the Sample with 50 mL of 0.5 N hydrochloric acid at 70° for 15 min, and filter. Wash the residue, adding the washings to the filtrate to obtain a total volume of 100 mL. Evaporate at 110° in a tared porcelain dish to dryness.

Acceptance criteria: NMT 2.0% (weight of the dried residue is NMT 200 mg)

3.3 Water-Soluble Substances

Sample: 12.5 g

Analysis: Place the Sample in a 500-mL conical flask, add 250 mL of water, and shake for 2 h at room temperature. Filter with the aid of vacuum, and again filter if necessary to obtain a clear filtrate. Evaporate in a tared platinum or porcelain dish, and dry at 110°.

Acceptance criteria: NMT 0.2% (weight of the residue is NMT 25 mg)

4 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in well-closed containers.