Lithium Oral Solution
» Lithium Oral Solution is prepared from Lithium Citrate or Lithium Hydroxide to which an excess of Citric Acid has been added. It contains not less than 90.0 percent and not more than 110.0 percent of the labeled amount of lithium (Li).
Packaging and storage Preserve in tight containers.
USP Reference standards 11
USP Lithium Carbonate RS
A: When diluted with an equal volume of 3 N hydrochloric acid, it imparts an intense crimson color to a nonluminous flame.
B: It meets the requirements of the test for Citrate 191.
pH 791: between 4.0 and 5.0.
Standard preparation Prepare as directed in the Assay under Lithium Citrate. Determine its pH.
Assay preparation Transfer an accurately measured volume of Oral Solution, equivalent to about 60 mg of lithium, to a 1000-mL volumetric flask, dilute with water to volume, and mix. Pipet 20 mL of the resulting solution into a 1000-mL volumetric flask, add about 950 mL of water, 2 mL of 1 N hydrochloric acid, and 20 mL of a surfactant solution, and mix. Adjust with 1 N hydrochloric acid or 1 N sodium hydroxide to the same pH (±0.1 pH unit) as that of the Standard preparation, dilute with water to volume, and mix.
Procedure Employ a suitable flame photometer, and adjust the instrument with the surfactant solution. Aspirate into the photometer the Standard preparation and the Assay preparation, and measure the emission at about 671 nm. Calculate the quantity, in mg, of lithium in each mL of the Oral Solution taken by the formula:
(13.88 / 73.89)(50C / V)(A / S)in which 13.88 is twice the atomic weight of lithium; 73.89 is the molecular weight of lithium carbonate; C is the concentration, in µg per mL, of USP Lithium Carbonate RS in the Standard preparation; V is the volume, in mL, of Oral Solution taken; and A and S are the photometer readings of the Assay preparation and the Standard preparation, respectively.
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USP35NF30 Page 3702Pharmacopeial Forum: Volume No. 28(2) Page 314