Litmus Paper, Blue
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This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition
Issued and maintained by the United States Pharmacopeial Convention (USP)
—Usually about 6 x 50 mm in size. It meets the requirements of the following tests.
Phosphate (Reagent test): Cut 5 strips into small pieces, mix with 500 mg of magnesium nitrate in a porcelain crucible, and ignite. To the residue add 5 mL of nitric acid, and evaporate to dryness: the residue shows not more than 0.02 mg of PO4.
Residue on Ignition: Ignite carefully 10 strips of the paper to constant weight: the weight of the residue corresponds to not more than 0.4 mg per strip of about 3 square cm.
Rosin Acids: Immerse a strip of the blue paper in a solution of 100 mg of silver nitrate in 50 mL of water: the color of the paper does not change in 30 seconds.
Sensitiveness: Drop a 10-to 12-mm strip into 100 mL of 0.0005 N acid contained in a beaker, and stir continuously: the color of the paper is changed within 45 seconds. The 0.0005 N acid is prepared by diluting 1 mL of 0.1 N hydrochloric acid with freshly boiled and cooled purified water to 200 mL.
