Potassium Alum
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This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition
Issued and maintained by the United States Pharmacopeial Convention (USP)
Potassium Alum
AIK(SO4)2 · 12H2O 474.39
AIK(SO4)2 258.21
Sulfuric acid, aluminum potassium salt (2:1:1), dodecahydrate;
Aluminum potassium sulfate (1:1:2) dodecahydrate CAS RN®: 7784-24-9; UNII: 1L24V9R23S.
Anhydrous CAS RN®: 10043-67-1; UNII: 09OXB01F3O.
1 DEFINITION
Potassium Alum contains NLT 99.0% and NMT 100.5% of potassium alum [AIK(SO4)2], calculated on the dried basis.
2 IDENTIFICATION
A.
Sample solution: 50 mg/mL in water
Analysis: Add 1 N sodium hydroxide dropwise to the Sample solution.
Acceptance criteria: A precipitate is formed that dissolves in an excess of the reagent. Ammonia is not evolved (distinction from ammonium alum).
B.
Analysis: Hold it in a nonluminous flame.
Acceptance criteria: A violet color is imparted to the flame.
C.
Sample solution: Saturated solution in water
Analysis: Add 10 mL of sodium bitartrate TS to 5 mL of the Sample solution.
Acceptance criteria: A white, crystalline precipitate is formed within 30 min.
D. Identification Tests—General, Aluminum〈191〉 and Sulfate〈191〉
Sample solution: 50 mg/mL in water
Acceptance criteria: Meets the requirements
3 ASSAY
Procedure
Edetate disodium titrant: Prepare and standardize as directed in Reagents, Indicators, and Solutions—Volumetric Solutions, Edetate Disodium, Twentieth-Molar (0.05 M).
Sample: 800 mg
Analysis: Transfer the Sample 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 min. 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 potassium alum [AIK(SO4)2].
Acceptance criteria: 99.0%–100.5% on the dried basis
4 IMPURITIES
Change to read:
Iron 〈241〉, Procedures, Procedure 1 (CN 1-Jun-2023)
Sample solution: Potassium alum in water (1 in 150)
Analysis: Add 5 drops of potassium ferrocyanide TS to 20 mL of the Sample solution.
Acceptance criteria: No blue color is produced immediately.
5 SPECIFIC TESTS
Loss on Drying 〈731〉
Sample: 2.0 g
Analysis: Transfer the Sample 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.
Acceptance criteria: 43.0%–46.0%
6 ADDITIONAL REQUIREMENTS
Packaging and Storage: Preserve in tight containers, and store at room temperature.
