Mobile Phase and Chromatographic System— Proceed as directed under Assay for Citric Acid/Citrate and Phosphate 345.

Standard preparation— Dissolve USP Citric Acid RS in a freshly prepared 1 mM sodium hydroxide to obtain a solution having a known concentration of about 0.02 mg of anhydrous citric acid per mL.

Assay preparation— Constitute the Oral Solution as directed in the labeling. Transfer the amount of the constituted Oral Solution, equivalent to about 500 mg of magnesium citrate, to a suitable volumetric flask, and dilute quantitatively, and stepwise if necessary, with a freshly prepared 1 mM sodium hydroxide to obtain a solution having a concentration of about 0.02 mg of magnesium citrate per mL, based on the label claim. Pass the resulting solution through a filter having a 0.5-µm or finer porosity, and use the filtrate.

Procedure— Proceed as directed for Procedure under Assay for Citric Acid/Citrate and Phosphate 345, and calculate the content, in g, of anhydrous citric acid (C6H8O7) by the formula: in which 0.001 is the conversion factor from mg to g; CS is the concentration, in mg per mL, of anhydrous citric acid in the Standard preparation; D is the dilution factor for the Assay preparation; VT is the total volume of constituted Oral Solution, as measured, when constituted as directed; V is the volume, in mL, of the constituted Oral Solution taken to prepare the Assay preparation; and rU and rS are the citrate peak areas obtained from the Assay preparation and the Standard preparation, respectively. The content of anhydrous citric acid is between 126.1% and 154.4% of the labeled amount of magnesium citrate.