Ammonium Alum
AlNH4(SO4)2·12H2O 453.33

Sulfuric acid, aluminum ammonium salt (2:1:1), dodecahydrate.
Aluminum ammonium sulfate (1:1:2) dodecahydrate [7784-26-1].

Anhydrous 237.15 [7784-25-0].
» Ammonium Alum contains not less than 99.0 percent and not more than 100.5 percent of AlNH4(SO4)2, calculated on the dried basis.
A: Add 1 N sodium hydroxide dropwise to a solution of it (1 in 20): a precipitate is formed, and it dissolves in an excess of the reagent with the evolution of ammonia, recognizable by its odor and by its alkaline effect upon moistened red litmus paper exposed to the vapor.
B: 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 300, and dry at 300 to constant weight. Cool in a desiccator, and weigh: it loses between 45.0% and 48.0% of its weight.
Limit of alkalies and alkaline earths— Completely precipitate the aluminum from a boiling solution of 1 g of it in 100 mL of water by the addition of sufficient 6 N ammonium hydroxide to render the solution distinctly alkaline to methyl red TS, and filter. Evaporate the filtrate to dryness, and ignite: the weight of the residue does not exceed 5 mg (0.5%).
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— Dissolve 18.6 g of edetate disodium in water to make 1000 mL, and standardize the solution as follows. Weigh accurately about 2 g of aluminum wire, transfer to a 1000-mL volumetric flask, and add 50 mL of a mixture of hydrochloric acid and water (1:1). Swirl the flask to ensure contact of the aluminum and the acid, and allow the reaction to proceed until all of the aluminum has dissolved. Dilute with water to volume, and mix. Pipet 10 mL of this solution into a 250-mL beaker, add, in the order named and with continuous stirring, 25.0 mL of Edetate disodium titrant and 20 mL of acetic acid–ammonium acetate buffer TS, and boil gently for 5 minutes. Cool, and add 50 mL of alcohol, and 2 mL of dithizone TS. Titrate with 0.05 M zinc sulfate VS to a bright rose-pink color. Perform a blank determination, substituting 10 mL of water for the aluminum solution, and make any necessary correction. Calculate the molarity of the solution taken by the formula:
W / 26.98V
in which W is the weight, in mg, of aluminum in the portion of solution taken; and V is the volume, in mL, of Edetate disodium titrant consumed.
Procedure— Transfer about 800 mg of Ammonium 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 with 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 11.86 mg of AlNH4(SO4)2.
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(SM32010) Monographs - Small Molecules 3
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