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 99.0percent and not more than 100.5percent of AlK(SO4)2,calculated on the dried basis.
Packaging and storage— Preserve in tight containers,and store at room temperature.
Identification—
A: Add 1Nsodium hydroxide dropwise to a solution of it (1in 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 10mLof sodium bitartrate TSto 5mLof a saturated solution of it:a white,crystalline precipitate is formed within 30minutes.
D: Asolution (1in 20)responds to the tests for Aluminum á191ñand for Sulfate á191ñ.
Loss on drying á731ñ— Transfer 2.0g,in a tared porcelain crucible,to a muffle furnace at 200.Increase the temperature to 400,and dry at 400to constant weight.Cool in a desiccator,and weigh:it loses between 43.0%and 46.0%of its weight.
Heavy metals,Method Iá231ñ— Dissolve 1g in water to make 20mL,and add 5mLof 0.1Nhydrochloric acid.Evaporate the solution in a porcelain evaporating dish to dryness.Treat the residue with 20mLof water,and add 50mg of hydroxylamine hydrochloride.Heat the solution on a steam bath for 10minutes,cool,dilute with water to 25mL,and proceed as directed,except to add 50mg of hydroxylamine hydrochloride to the Standard Preparation:the limit is 0.002%.
Iron— Add 5drops of potassium ferrocyanide TSto 20mLof a solution (1in 150):no blue color is produced immediately.
Assay—
Edetate disodium titrant— Prepare and standardize as directed in the Assayunder Ammonium Alum.
Procedure— Transfer about 800mg of Potassium Alum,accurately weighed,to a 400-mLbeaker,moisten with 1mLof glacial acetic acid,and add 50mLof water,50.0mLof Edetate disodium titrant,and 20mLof acetic acid–ammonium acetate buffer TS.Warm on a steam bath until solution is complete,and boil gently for 5minutes.Cool,add 50mLof alcohol and 2mLof dithizone TS,and titrate 0.05Mzinc sulfate VSto a bright rose-pink color.Perform a blank determination,and make any necessary correction.Each mLof 0.05MEdetate disodium titrantis equivalent to 12.91mg of AlK(SO4)2.
Auxiliary Information— Staff Liaison:Lawrence Evans,III,Ph.D.,Scientist
Expert Committee:(PA6)Pharmaceutical Analysis 6
USP28–NF23Page 77
Pharmacopeial Forum:Volume No.27(3)Page 2515
Phone Number:1-301-816-8389