Al2(SO4)3,nH2O
Relative molecular mass. 342.1 (anhydrous).
Chemical name. Aluminium sulfate (2:3); CAS Reg. No. 10043-01-3 (anhydrous). Aluminium sulfate (2:3) hydrate; CAS Reg. No. 17927-65-0 (hydrate).
Description. Colourless, lustrous crystals or crystalline masses, or a white, crystalline powder; odourless.
Solubility. Soluble in cold water; freely soluble in hot water; practically insoluble in ethanol (~750 g/l) TS.
Category. Astringent drug; used in the preparation of aluminium acetate solution.
Storage. Aluminium sulfate should be kept in a well-closed container.
Aluminium sulfate contains a variable quantity of water of crystallization.
Requirements
Aluminium sulfate contains not less than 51.0% and not more than the equivalent of 59.0% of Al2(SO4)3.
Identity tests
A. Dissolve 0.2 g in 2 ml of water and add 0.5 ml of hydrochloric acid (~70 g/l) TS and 0.5 ml of alkaline thioacetamide TS; no precipitate is formed. Add drop by drop sodium hydroxide (~80 g/l) TS; a white, gelatinous precipitate is formed which dissolves on further addition of sodium hydroxide. Gradually add ammonium chloride (100 g/l) TS; a white, gelatinous precipitate reappears.
B. A 0.1 g/ml solution yields reaction A described under 2.1 General identification tests as characteristic of sulfates.
Heavy metals. Use 1.0 g for the preparation of the test solution as described under 2.2.3 Limit test for heavy metals, Procedure 1; determine the heavy metals content according to Method A; not more than 50 μg/g.
Ammonium salts. Heat 1 g with 10 ml of sodium hydroxide (1 mol/l) VS on a water-bath for 1 minute; no odour of ammonia is perceptible.
Iron. Use 0.4 g; the solution complies with the 2.2.4 Limit test for iron; not more than 100 μg/g.
Alkali and alkaline-earth metals. Dissolve 1.0 g in 100 ml of water, heat, and add 0.1 ml of methyl red/ethanol TS and sufficient ammonia (~100 g/l) TS until the colour of the solution changes to yellow. Dilute to 150 ml with water, heat to boiling, and filter. Evaporate 75 ml of the filtrate to dryness on a water-bath and ignite to constant mass; the residue weighs not more than 2 mg (0.4%).
Colour and clarity of solution. A solution of 0.50 g in 10 ml of water is colourless and not more opalescent than opalescence standard TS2.
pH value. pH of a 20 mg/ml solution in carbon-dioxide-free water R, 2.5-4.0.
Assay. Dissolve about 0.5 g, accurately weighed, in 20 ml of water, and proceed as described under 2.5 Complexometric titrations for aluminium.
Each ml of disodium edetate (0.05 mol/l) VS is equivalent to 8.554 mg of Al2(SO4)3.
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