Calcium Hydroxide

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

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

DOWNLOAD PDF HERE

Ca(OH)₂ 74.09

Calcium hydroxide CAS RN: 1305-62-0; UNII: PF5DZW74VN.

1 DEFINITION

Calcium Hydroxide contains NLT 95.0% and NMT 100.5% of calcium hydroxide [Ca(OH)₂].

2 IDENTIFICATION

A.Sample solution: Mix with three to four times its weight of water.

Acceptance criteria: It forms a smooth magma. The clear supernatant from the magma is alkaline to litmus.

B. IDENTIFICATION TESTS-GENERAL, Calcium (191)

Sample solution: Mix 1 g with 20 mL of water, and add sufficient 6 N acetic acid to effect the solution.

Acceptance criteria: Meets the requirements

3 ASSAY

PROCEDURE

Sample solution: To 1.5 g of Calcium Hydroxide in a beaker, gradually add 30 mL of 3 N hydrochloric acid. When dissolved, transfer the solution to a 500-mL volumetric flask, and rinse the beaker thoroughly, adding the rinsings to the flask. Dilute with water to volume, and mix. Transfer 50 mL of the solution into a suitable container, and add 100 mL of water, 15 mL of 1 N sodium hydroxide, and 300 mg of hydroxy naphthol blue.

Titrimetric system

Mode: Direct titration

Titrant: 0.05 M edetate disodium VS

Endpoint detection: Visual

FFICIA Analysis: Titrate the Sample solution with Titrant to a blue endpoint. Each mL of Titrant is equivalent to 3.705 mg of calcium hydroxide Ca(OH)₂

Acceptance criteria: 95.0%-100.5%

4 IMPURITIES

4.1 LIMIT OF MAGNESIUM AND ALKALI SALTS

Sample solution: Dissolve 0.50 g in a mixture of 30 mL of water and 10 mL of 3 N hydrochloric acid.

Analysis: Heat the solution, and boil for 1 min. Rapidly add 40 mL of oxalic acid TS, and stir vigorously until precipitation is well established. Add immediately to the warm mixture 2 drops of methyl red TS and then 6 N ammonium hydroxide, dropwise, until the mixture is just alkaline. Cool to room temperature, transfer to a 100-mL graduated cylinder, dilute with water to 100 mL, mix, and allow to stand for 4 h or overnight. Filter, and to 50 mL of the clear filtrate in a platinum dish add 0.5 mL of sulfuric acid, and evaporate the mixture on a steam bath to a small volume. Carefully heat over a free flame to dryness, and continue heating to complete decomposition and volatilization of ammonium salts. Finally, ignite the residue to constant weight.

Acceptance criteria: NMT 12 mg (4.8%)

4.2 LIMIT OF ACID-INSOLUBLE SUBSTANCES

Sample solution: Dissolve 2.0 g in 30 mL of 4 N hydrochloric acid, and heat to boiling.

Analysis: Filter the mixture, wash the residue with hot water, and ignite.

Acceptance criteria: 0.5%; the weight of the residue is NMT 10 mg.

4.3 CARBONATE

Sample solution: Mix 2 g with 50 mL of water.

Analysis: Add an excess of 3 N hydrochloric acid to the Sample solution.

Acceptance criteria: The Analysis does not cause more than a slight effervescence.

5 ADDITIONAL REQUIREMENTS

PACKAGING AND STORAGE: Preserve in tight containers.