Magnesium Citrate

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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1 DEFINITION

Magnesium Citrate contains NLT 14.5% and NMT 16.4% of magnesium (Mg), calculated on the dried basis.

2 IDENTIFICATION

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

B. Identification Tests—General 〈191〉, Chemical Identification Tests, Citrate

Sample solution: 80 mg/mL of Magnesium Citrate in water

Acceptance criteria: Meets the requirements

3 ASSAY

Procedure

Use water with a resistivity of NLT 18 megohm-cm to prepare the solutions.

Mobile phase: 48 mM methanesulfonic acid. [Note—It is recommended to use suitable cation trapping techniques to ensure the Mobile phase is free of all cationic impurities.]

Diluent: 0.02 N hydrochloric acid

System suitability solution: 120 μg/mL of USP Magnesium Citrate RS and 5 μg/mL of USP Calcium Carbonate RS in Diluent

Standard stock solution: 6 mg/mL of USP Magnesium Citrate RS prepared as follows. Transfer an appropriate portion of USP Magnesium

Citrate RS to a suitable volumetric flask. Add about 20% of the final volume of 6 N hydrochloric acid, and dissolve. Dilute with water to volume.

Standard solution: 120 μg/mL of USP Magnesium Citrate RS in water from the Standard stock solution

Sample stock solution: 6 mg/mL of Magnesium Citrate from the previously dried Magnesium Citrate prepared as directed for the Standard stock solution

Sample solution: 120 μg/mL of Magnesium Citrate in water from the Sample stock solution

Chromatographic system

(See Chromatography 〈621〉, System Suitability.)

Mode: LC

Detector: Conductivity with suppression

Columns

Guard: 5-mm × 5-cm; 5.5-μm packing L84

Analytical: 5-mm × 25-cm; 5.5-μm packing L84

Column temperature: 40°

Flow rate: 1.0 mL/min

Injection volume: 10 μL

Run time: NLT 2 times the retention time of magnesium

System suitability

Samples: System suitability solution and Standard solution

[Note—The relative retention times for the magnesium and calcium ions are 1.0 and 1.3, respectively.]

Suitability requirements

Resolution: NLT 3.0 between the magnesium and calcium ions, System suitability solution

Tailing factor: NMT 2.0, Standard solution

Relative standard deviation: NMT 0.73%, Standard solution

Analysis

Samples: Standard solution and Sample solution

Calculate the percentage of magnesium (Mg) in the portion of Magnesium Citrate taken:

Result = (r_U/r_S) × (C_S/C_U) × (A_r/M_r) × F × 100

r_U = peak response of magnesium from the Sample solution

r_S = peak response of magnesium from the Standard solution

C_S = concentration of USP Magnesium Citrate RS in the Standard solution (μg/mL)

C_U = concentration of Magnesium Citrate in the Sample solution (μg/mL)

A_r = atomic weight of magnesium, 24.31

M = molecular weight of magnesium citrate, 451.11

F = number of magnesium ions in one mole of magnesium citrate, 3

Acceptance criteria: 14.5%–16.4% on the dried basis

4 IMPURITIES

Chloride and Sulfate 〈221〉, Chloride

Standard solution: 0.20 mL of 0.020 N hydrochloric acid VS

Sample: 300 mg of Magnesium Citrate

Acceptance criteria: The Sample shows no more chloride than the Standard solution (0.05%).

Chloride and Sulfate 〈221〉, Sulfate

Standard solution: 0.20 mL of 0.020 N sulfuric acid VS

Sample: 100 mg of Magnesium Citrate

Acceptance criteria: The Sample shows no more sulfate than the Standard solution (0.2%).

Change to read:

Arsenic 〈211〉, Procedures, Procedure 1 (CN 1-Jun-2023) : NMT 3 ppm

Change to read:

Iron 〈241〉, Procedures, Procedure 1 (CN 1-Jun-2023)

Sample solution: Boil 50 mg of Magnesium Citrate with 5 mL of 2 N nitric acid for 1 min. Cool, dilute with water to 45 mL, and add 2 mL of hydrochloric acid.

Acceptance criteria: NMT 200 ppm

Limit of Calcium

Mobile phase, Diluent, System suitability solution, Sample solution, and Chromatographic system: Proceed as directed in the Assay.

Standard solution: 3.0 μg/mL of USP Calcium Carbonate RS in Diluent

System suitability

Samples: System suitability solution and Standard solution

[Note—The relative retention times for the magnesium and calcium ions are 1.0 and 1.3, respectively.]

Suitability requirements

Resolution: NLT 3.0 between the magnesium and calcium ions, System suitability solution

Tailing factor: NMT 2.0, Standard solution

Relative standard deviation: NMT 5%, Standard solution

Analysis

Samples: Sample solution and Standard solution

Measure the response for the calcium peak.

Acceptance criteria: NMT 1.0% on the dried basis. The peak response of calcium from the Sample solution does not exceed that of the Standard solution.

5 SPECIFIC TESTS

pH 〈791〉

Sample solution: 50 mg/mL of Magnesium Citrate suspension in water

Acceptance criteria: 5.0–9.0

Loss on Drying 〈731〉

Sample: 1 g of Magnesium Citrate

Analysis: Dry the Sample in a mechanical convection oven at 135° for 16 h, then to constant weight.

Acceptance criteria: NMT 29% for Magnesium Citrate; NMT 2.0% for Magnesium Citrate where it is labeled as anhydrous

6 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in tight containers.

Labeling: Magnesium Citrate that loses NMT 2.0% of its weight in the test for Loss on Drying may be labeled as Anhydrous Magnesium Citrate.

USP Reference Standards 〈11〉

USP Calcium Carbonate RS

USP Magnesium Citrate RS