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 C
21H
29N
3O in the portion of Capsules taken by the formula:
3.125(339.48 / 437.47)C(AU / AS),
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.
621
) coated with a 0.25-mm layer of chromatographic silica gel mixture. Allow the spots to dry, and develop the chromatogram in a solvent system consisting of a mixture of toluene, alcohol, and ammonium hydroxide (170:28:2) until the solvent front has moved about three-fourths of the length of the plate. Remove the plate from the developing chamber, mark the solvent front, and allow the solvent to evaporate. Locate the spots on the plate by viewing under short-wavelength UV light: the RF value of the principal spot obtained from the test solution corresponds to that obtained from the Standard solution.