Milk of Magnesia

This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition

Issued and maintained by the United States Pharmacopeial Convention (USP)

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Mg(OH)₂ 58.32

Magnesium hydroxide CAS RN®: 1309-42-8;  UNII: NBZ3QY004S.

1 DEFINITION

Milk of Magnesia is a suspension of Magnesium Hydroxide. Milk of Magnesia, Double-Strength Milk of Magnesia, and Triple-Strength Milk of Magnesia contain NLT 90.0% and NMT 115.0% of the labeled amount of magnesium hydroxide [Mg(OH)₂], the labeled amount being 80, 160, and 240 mg/mL of magnesium hydroxide [Mg(OH)₂], respectively. It may contain NMT 0.05% of a volatile oil or a blend of volatile oils, suitable for flavoring purposes.

2 IDENTIFICATION

2.1 A. Identification Tests-General 〈191〉, Chemical Identification Tests, Magnesium

Sample: A solution of the equivalent of 1 g of regular-strength Milk of Magnesia in 2 mL of 3 N hydrochloric acid

Acceptance criteria: Meets the requirements

Delete the following:

2.2 B. The retention time of the magnesium peak of the Sample solution corresponds to that of the Standard solution, as obtained in the Assay.

3 ASSAY

Change to read:

3.1 Procedure

Sample: A portion of Milk of Magnesia, previously shaken in its original container, equivalent to about 800 mg of magnesium hydroxide

Titrimetric system

• Mode: Direct titration
• Titrant: 0.05 M edetate disodium VS
• Endpoint detection: Visual

Analysis: Transfer the Sample to a 250-mL volumetric flask. Dissolve in 30 mL of 3 N hydrochloric acid and dilute with water to volume. Filter, if necessary, and transfer 25.0 mL of the filtrate to a beaker containing 75 mL of water. Adjust the reaction of the solution to a pH of 7 (using pH indicator paper; see Reagents, Indicators, and Solutions—Indicator and Test Papers) with 1 N sodium hydroxide, and add 5 mL of ammonia–ammonium chloride buffer TS and 0.15 mL of eriochrome black TS. Titrate with the Titrant to a blue endpoint. Each milliliter of the Titrant is equivalent to 2.916 mg of magnesium hydroxide [Mg(OH)₂].

Acceptance criteria: 90.0%–115.0%

4 IMPURITIES

4.1 Soluble Alkalies

Sample solution: Centrifuge 50 mL of Milk of Magnesia. Dilute 5.0 mL of the clear supernatant with 40 mL of water.

Titrimetric system

• Mode: Direct titration
• Titrant: 0.10 N sulfuric acid
• Endpoint detection: Visual

Analysis: Add 1 drop of methyl red TS, and titrate with Titrant to the production of a persistent pink color.

Acceptance criteria: NMT 1.0 mL of Titrant is required. Where the specimen is Double- or Triple-Strength Milk of Magnesia, NMT 2.0 or 3.0 mL of Titrant is required, respectively.

4.2 Carbonate and Acid-Insoluble Matter

Sample solution: To the equivalent of 1 g of regular-strength Milk of Magnesia, add 2 mL of 3 N hydrochloric acid.

Acceptance criteria: NMT a slight effervescence occurs, and the solution is NMT slightly turbid.

5 SPECIFIC TESTS

5.1 Microbial Enumeration Tests 〈61〉 and Tests for Specified Microorganisms 〈62〉

The total aerobic microbial count does not exceed 10² cfu/mL, and it meets the requirements of the test for absence of Escherichia coli.

5.2 Acid-Neutralizing Capacity 〈301〉

Analysis: Proceed as directed in the chapter.

Acceptance criteria: NLT 5 mEq of acid is consumed by the minimum single dose recommended in the labeling, and NLT the number of mEq calculated as follows:

Result = 0.8 × (F_M × M)

F_M = theoretical acid-neutralizing capacity of magnesium hydroxide [Mg(OH)₂], 0.0343 mEq/mg

M = quantity of magnesium hydroxide [Mg(OH)₂] in the sample tested, based on the labeled quantity (mg)

6 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in tight containers, preferably at a temperature NMT 35°. Avoid freezing.

Labeling: Double- or Triple-Strength Milk of Magnesia is so labeled, or may be labeled as 2× or 3× Concentrated Milk of Magnesia, respectively.

Delete the following:

USP Reference Standards 〈11〉

USP Calcium Carbonate RS

USP Magnesium Hydroxide RS