Medium: water; 1000 mL.

Apparatus 2: 50 rpm.

Time: 20 minutes.

Procedure— Filter 15 mL of the solution under test, and transfer 10.0 mL of the filtrate to a 25-mL volumetric flask. Dilute with 2 N sulfuric acid to volume, and mix. Determine the amount of disopyramide (C21H29N3O) dissolved from UV absorbances at the wavelength of maximum absorbance at about 268 nm of this solution, using water as the blank, in comparison with a Standard solution having a known concentration of USP Disopyramide Phosphate RS in the same medium.

Tolerances— Not less than 80% (Q) of the labeled amount of C21H29N3O is dissolved in 20 minutes.

Methanolic sulfuric acid— Cautiously add 5.4 mL of sulfuric acid to about 1800 mL of methanol with stirring, dilute with methanol to 2000 mL, and mix.

Procedure— Weigh the contents of not fewer than 20 Phosphate Capsules, and calculate the average weight per Capsule. Mix the combined contents of the Capsules, and transfer an accurately weighed portion, equivalent to about 125 mg of disopyramide phosphate, to a glass-stoppered, 125-mL flask. Add 50 mL of Methanolic sulfuric acid, and stir for 30 minutes. Filter through a medium-porosity, sintered-glass filter, and rinse thoroughly with Methanolic sulfuric acid. Transfer the combined filtrate and rinsings to a 100-mL volumetric flask, dilute with Methanolic sulfuric acid to volume, and mix. Dilute an accurately measured portion of this solution quantitatively and stepwise with the same solvent to obtain a solution having a concentration of about 40 µg per mL. Dissolve an accurately weighed portion of USP Disopyramide Phosphate RS in Methanolic sulfuric acid, and dilute quantitatively and stepwise with the same solvent to obtain a solution having a known concentration of about 40 µg per mL. Concomitantly determine the absorbances of both solutions in 1-cm cells at the wavelength of maximum absorbance at about 268 nm, with a suitable spectrophotometer, using Methanolic sulfuric acid as the blank. Calculate the quantity, in mg, of C21H29N3O in the portion of Capsules taken by the formula: in which 339.48 and 437.47 are the molecular weights of disopyramide and disopyramide phosphate, respectively; C is the concentration, in µg per mL, of USP Disopyramide Phosphate RS in the Standard solution; and AU and AS are the absorbances of the solution from the Capsules and the Standard solution, respectively.