Potassium Alum
AlK(SO4)2·12H2O 474.39

Sulfuric acid, aluminum potassium salt (2:1:1), dodecahydrate.
Aluminum potassium sulfate (1:1:2) dodecahydrate [7784-24-9].

Anhydrous 258.21 [10043-67-1].
» Potassium Alum contains not less than 99.0 percent and not more than 100.5 percent of AlK(SO4)2, calculated on the dried basis.
Packaging and storage— Preserve in tight containers, and store at room temperature.
A: Add 1 N sodium hydroxide dropwise to a solution of it (1 in 20): a precipitate is formed that dissolves in an excess of the reagent. Ammonia is not evolved (distinction from Ammonium Alum).
B: Hold it in a nonluminous flame: a violet color is imparted to the flame.
C: Add 10 mL of sodium bitartrate TS to 5 mL of a saturated solution of it: a white, crystalline precipitate is formed within 30 minutes.
D: A solution (1 in 20) responds to the tests for Aluminum 191 and for Sulfate 191.
Loss on drying 731 Transfer 2.0 g, in a tared porcelain crucible, to a muffle furnace at 200. Increase the temperature to 400, and dry at 400 to constant weight. Cool in a desiccator, and weigh: it loses between 43.0% and 46.0% of its weight.
Heavy metals, Method I 231 Dissolve 1 g in water to make 20 mL, and add 5 mL of 0.1 N hydrochloric acid. Evaporate the solution in a porcelain evaporating dish to dryness. Treat the residue with 20 mL of water, and add 50 mg of hydroxylamine hydrochloride. Heat the solution on a steam bath for 10 minutes, cool, dilute with water to 25 mL, and proceed as directed, except to add 50 mg of hydroxylamine hydrochloride to the Standard Preparation: the limit is 0.002%.
Iron— Add 5 drops of potassium ferrocyanide TS to 20 mL of a solution (1 in 150): no blue color is produced immediately.
Edetate disodium titrant— Prepare and standardize as directed in the Assay under Ammonium Alum.
Procedure— Transfer about 800 mg of Potassium Alum, accurately weighed, to a 400-mL beaker, moisten with 1 mL of glacial acetic acid, and add 50 mL of water, 50.0 mL of Edetate disodium titrant, and 20 mL of acetic acid–ammonium acetate buffer TS. Warm on a steam bath until solution is complete, and boil gently for 5 minutes. Cool, add 50 mL of alcohol and 2 mL of dithizone TS, and titrate 0.05 M zinc sulfate VS to a bright rose-pink color. Perform a blank determination, and make any necessary correction. Each mL of 0.05 M Edetate disodium titrant is equivalent to 12.91 mg of AlK(SO4)2.
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