Calamine BP

Action and use: Antipruritic.
DEFINITION
Calamine is a basic zinc carbonate suitably coloured with iron(III) oxide.
CHARACTERISTICS
An amorphous, impalpable, pink or reddish brown powder, the colour depending on the variety and amount of iron(III) oxide present and the process by which it is incorporated. Practically insoluble in water. It dissolves with effervescence in hydrochloric acid .
IDENTIFICATION
A. Yields the reactions characteristic of carbonates.
B. To 2 g add 5 ml of hydrochloric acid and heat to boiling; if necessary, add hydrochloric acid drop wise until a bright yellow solution is obtained. Cool and add 13.5 M ammonia until the first sign of precipitate (solution A). The solution yields characteristic of iron salts. Dilute 1 ml of solution A to 5 ml with water; the solution yields the reaction characteristic of zinc salts.
TESTS
Calcium: Dissolve 0.50 g in a mixture of 10 ml of water and 2.5 ml of glacial acetic acid and filter. To 0.5 ml of the filtrate add 15 ml of 5 M ammonia and 2 ml of a 2.5% w/v solution of ammonium oxalate and allow to stand for 2 minutes. The solution remains clear.
Soluble barium salts: To the remainder of the filtrate obtained in the test for Calcium add 2 ml of 1 M sulphuric acid and allow to stand for 5 minutes. The solution remains clear.
Lead: Not more than 150 ppm when determined by atomic absorption spectrophotometry ,
Chloride: Dissolve 0.15 g in water with the addition of 1 ml of nitric acid , filter and dilute to 30 ml with water. The resulting solution complies with the limit test for chlorides (0.07%).
Sulphate: Dissolve 0.1 g in water with the addition of 3 ml of 2 M hydrochloric acid , filter and dilute to 60 ml with water. The resulting solution complies with the limit test for sulphates, (0.6%).
Ethanol-soluble dyes: Shake 1.0 g with 10 ml of ethanol (90%) and filter. The filtrate is colourless,.
Matter insoluble in hydrochloric acid: Dissolve 1 g in 20 ml of warm 2 M hydrochloric acid and filter. The residue, when washed with water and dried to constant weight at 105C, weighs not more than 10 mg.
Water-soluble dyes: Shake 1.0 g with 10 ml of water and filter. The filtrate is colourless.
Residue on ignition: 68.0 to 74.0%, when ignited at a temperature not lower than 900C until, after further ignition, two successive weighing do not differ by more than 0.2% of the weight of the residue. Use 1 g.