A: Shake a quantity of the contents of Capsules, equivalent to about 100 mg of quinidine sulfate, with 10 mL of dilute sulfuric acid (1 in 350), and filter: an appropriate dilution of the filtrate exhibits a vivid blue fluorescence. On the addition of a few drops of hydrochloric acid the fluorescence disappears.

B: In the test for Chromatographic purity, the RF value of the principal spot obtained from the Test preparation corresponds to that from the Standard preparation.

C: Shake a quantity of the contents of Capsules, equivalent to about 100 mg of quinidine sulfate, with 10 mL of dilute hydrochloric acid (1 in 100), and filter: the filtrate responds to the tests for Sulfate 191.

Medium: 0.01 N hydrochloric acid; 900 mL.

Apparatus 1: 100 rpm.

Time: 30 minutes.

Procedure— Determine the amount of (C20H24N2O2)2·H2SO4·2H2O dissolved by employing UV absorption at the wavelength of maximum absorbance at about 248 nm on filtered portions of the solution under test, suitably diluted with Dissolution Medium, in comparison with a Standard solution having a known concentration of USP Quinidine Sulfate RS in the same Medium.

Tolerances— Not less than 85% (Q) of the labeled amount of (C20H24N2O2)2·H2SO4·2H2O is dissolved in 30 minutes.

Procedure for content uniformity— Transfer the contents of 1 Capsule to a 250-mL volumetric flask, add about 175 mL of dilute hydrochloric acid (1 in 100), and shake by mechanical means for 30 minutes. Add dilute hydrochloric acid (1 in 100) to volume, and mix. Filter a portion of the mixture, discarding the first 20 mL of the filtrate. Concomitantly determine the absorbances of this solution, quantitatively diluted, if necessary, and a Standard solution of USP Quinidine Sulfate RS in dilute hydrochloric acid (1 in 100) having a known concentration of about 40 µg per mL, in 1-cm cells, at the wavelength of maximum absorbance at about 345 nm, with a suitable spectrophotometer, using water as the blank. Calculate the quantity, in mg, of active ingredients, calculated as quinidine sulfate [(C20H24N2O2)2·H2SO4·2H2O], in the Capsule taken by the formula: in which T is the labeled quantity, in mg, of quinidine sulfate in the Capsule, D is the concentration, in µg per mL, of quinidine sulfate in the solution from the Capsule, based on the labeled quantity per Capsule and the extent of dilution, C is the concentration, in µg per mL, of USP Quinidine Sulfate RS in the Standard solution, and AU and AS are the absorbances of the solution from the Capsule and the Standard solution, respectively.

(Official January 1, 2007)

Methanesulfonic acid solution, Diethylamine solution, Mobile phase, System suitability preparation, and System suitability test— Proceed as directed for Limit of dihydroquinidine sulfate under Quinidine Sulfate.

Standard preparation— Transfer about 20 mg of USP Quinidine Sulfate RS, accurately weighed, to a 100-mL volumetric flask, dissolve in and dilute with Mobile phase to volume, and mix.

Assay preparation— Transfer the contents of not less than 20 Capsules to a container, and mix. Transfer an accurately weighed portion of the powder, equivalent to about 160 mg of quinidine sulfate, to a 100-mL volumetric flask, add 80 mL of methanol, and shake the flask by mechanical means for 30 minutes. Dilute with methanol to volume, mix, and filter, discarding the first 10 mL of the filtrate. Transfer 3.0 mL of the filtrate to a 25-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Procedure (see Chromatography 621)—Inject equal volumes (about 50 µL) of the Standard and Assay preparations into a chromatograph equipped with a 235-nm detector and a 3.9-mm × 30-cm column that contains packing L1. Calculate the quantity, in mg, of the sum of quinidine sulfate and dihydroquinidine sulfate in the portion of Capsules taken by the formula: in which C is the concentration, in mg per mL, of USP Quinidine Sulfate RS in the Standard preparation, rb,U and rb,S are the peak responses of quinidine obtained from the Assay preparation and the Standard preparation, respectively, and rd,U and rd,S are the peak responses of dihydroquinidine obtained from the Assay preparation and the Standard preparation, respectively.