Magnesium Citrate Oral Solution

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 Oral Solution is a sterilized or pasteurized solution containing NLT 7.59 g of anhydrous citric acid (C₆H₈O₇) and an amount of magnesium citrate equivalent to NLT 1.55 g and NMT 1.9 g of magnesium oxide (MgO) in each 100 mL of Oral Solution.

Prepare Magnesium Citrate Oral Solution as follows.

Dissolve the Anhydrous Citric Acid in 150 mL of hot Purified Water in a suitable dish, slowly add the Magnesium Carbonate previously mixed with 100 mL of Purified Water, and stir until it is dissolved. Add the Syrup, heat the mixed liquids to the boiling point, and immediately add the

Lemon Oil previously triturated with Talc. Filter the mixture, while hot, into a strong bottle of suitable capacity previously rinsed with boiling

Purified Water. Add boiled Purified Water to bring the preparation to final volume. Use Purified Cotton as a stopper for the bottle, and allow to cool. Add the Potassium Bicarbonate, and immediately insert the stopper in the bottle securely. Shake the solution occasionally until the

Potassium Bicarbonate is dissolved, cap the bottle, and sterilize or pasteurize the solution.

Alternatively, 30 g of citric acid containing 1 molecule of water of hydration, equivalent to 27.4 g of Anhydrous Citric Acid, may be used in the foregoing formula. In this process, replace the 2.5 g of Potassium Bicarbonate with 2.1 g of sodium bicarbonate, preferably in tablet form.

The Oral Solution may be further carbonated by the use of carbon dioxide under pressure.

2 IDENTIFICATION

A. Identification Tests—General, Magnesium〈191〉: Meets the requirements

B.

Sample: 5 mL

Analysis: To the Sample add 1 mL of potassium permanganate TS and 5 mL of mercuric sulfate TS, and heat the solution.

Acceptance criteria: A white precipitate is formed.

3 ASSAY

Citric Acid

Mobile phase, Standard preparation 1, and Chromatographic system: Proceed as directed in Assay for Citric Acid/Citrate and Phosphate〈345〉.

Assay preparation: Transfer 10 mL of Oral Solution that has been previously freed from excessive carbon dioxide by repeated pouring to a suitable volumetric flask, and proceed as directed in Assay for Citric Acid/Citrate and Phosphate 〈345〉, Assay Preparation for Citric Acid/Citrate Assay.

Analysis: Proceed as directed in Assay for Citric Acid/Citrate and Phosphate 〈345〉, Procedure.

Calculate the quantity, g, of anhydrous citric acid (C₆H₈O₇) in 100 mL of Oral Solution:

Result = (r_U/r_S) × C_S × V × D × (M_r1/M_r2) × F

r_U = peak area of citrate from the Assay preparation

r_S = peak area of citrate from Standard peparation 1

C_S = concentration of citrate in Standard preparation 1 (μg/mL)

V = final volume of Oral Solution, 100 mL

D = dilution factor

M_r1 = molecular weight of citric acid (C H O ), 192.12

M_r2 = molecular weight of citrate (C H O ), 189.10

F = conversion factor, 10−6 g/μg

Acceptance criteria: NLT 7.59 g in 100 mL of Oral Solution

Magnesium Oxide

Sample: 50.0 mL of Oral Solution that has been previously freed from excessive carbon dioxide by repeated pouring

Analysis: Transfer the Sample to a 100-mL volumetric flask, and dilute with water to volume. Transfer 5.0 mL of the resulting solution to a beaker containing 150 mL of water heated to 70°–80°, and add 1 mL of ammonium chloride TS and 3 mL of ammonium hydroxide. Mix, and slowly add 8 mL of 8-hydroxyquinoline TS with stirring. After standing for 30 min, filter through a sintered-glass crucible, previously dried and weighed, and wash the precipitate with ten 10-mL portions of water. Dry the crucible and contents at 105° for 3 h, cool, and weigh.

Determine the equivalent of magnesium oxide (MgO) in 100 mL of Oral Solution by multiplying the weight of C₁₈H₁₂MgN₂O₂ · 2H₂O so obtained by 4.624.

Acceptance criteria: 1.55–1.9 g in 100 mL of Oral Solution

4 IMPURITIES

Chloride and Sulfate, Chloride 〈221〉

Sample: 2.0 mL

Acceptance criteria: 0.01%; it shows no more chloride than corresponds to 0.30 mL of 0.020 N hydrochloric acid.

Chloride and Sulfate, Sulfate〈221〉

Sample: 2.0 mL

Acceptance criteria: 0.015%; it shows no more sulfate than corresponds to 0.30 mL of 0.020 N sulfuric acid.

Tartaric Acid

Sample: 10 mL

Analysis: Place the Sample in a test tube, add 1 mL of glacial acetic acid and 3 mL of a solution of potassium acetate (1 in 2), shake the mixture vigorously, then gently rub the inner wall of the test tube with a glass rod for a few min, and allow to stand for 1 h.

Acceptance criteria: No white, crystalline precipitate soluble in 6 N ammonium hydroxide is formed.

5 ADDITIONAL REQUIREMENTS

Packaging and Storage: Package in bottles NLT 200 mL in capacity. Store at controlled room temperature or in a cool place.