A: The contents of the Capsules meet the requirements of Identification test A under Benzonatate. If a difference is observed, or if excipients are present, use an amount of the contents of Capsules equivalent to about 100 mg of benzonatate, mixed with 25 mL of 0.01 N hydrochloric acid, and proceed as directed under Identification—Organic Nitrogenous Bases 181, beginning with “Transfer the liquid to a separator.”

B: The contents of the Capsules respond to Identification test B under Benzonatate.

Medium: water; 900 mL.

Apparatus 2: 50 rpm.

Time: 30 minutes.

Mobile phase— Prepare a filtered and degassed mixture of acetonitrile and 0.04 M monobasic potassium phosphate (3:1). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard solution— Transfer 50 mg, accurately weighed, of USP Benzonatate RS to a 100-mL volumetric flask, and add about 50 mL of water. Sonicate for 10 minutes, cool, dilute with water to volume, and mix. Transfer 10.0 mL of this solution to a 50-mL volumetric flask, and dilute with water to volume.

Test solution— Pass a portion of the solution under test through a 0.45-µm filter.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 310-nm detector and a 3.9-mm × 30-cm column that contains packing L1. The flow rate is about 1.5 mL per minute. Chromatograph the Standard solution, and record the peak responses as directed for Procedure: the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 15 µL) of the Standard solution and the Test solution into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the amount of benzonatate dissolved by the formula: in which rU and rS are the peak responses obtained from the Test solution and the Standard solution, respectively; CS is the concentration, in mg per mL, of USP Benzonatate RS in the Standard solution; 900 is the volume, in mL, of Medium; 100 is the conversion factor to percentage; and LC is the Tablet label claim, in mg.

Tolerances— Not less than 80% (Q) of the labeled amount of benzonatate is dissolved in 30 minutes.

Standard preparation— Transfer about 50 mg of USP Benzonatate RS, accurately weighed, to a 100-mL volumetric flask, dilute with water to volume, and mix.

Assay preparation— Mix a number of Capsules, equivalent to about 500 mg of benzonatate, with 40 mL of chloroform in a suitable high-speed blender, and dilute with chloroform to 100.0 mL. Transfer 10.0 mL of this solution, equivalent to about 50 mg of benzonatate, to a 100-mL volumetric flask, and evaporate the chloroform on a steam bath with the aid of a current of air. Dissolve the residue in water, dilute with water to volume, and mix.

Procedure— Transfer 4.0 mL each of the Standard preparation, the Assay preparation, and water to provide the blank, to separate test tubes. To each tube add in succession 1.0 mL of 1 M hydroxylamine hydrochloride and 1.0 mL of 3.5 N sodium hydroxide, mixing after each addition. Allow to stand for 10 minutes, accurately timed, then add 1.0 mL of 3.5 N hydrochloric acid, mix, add 1.0 mL of ferric chloride solution (8 in 100), and mix. Allow to stand for 30 minutes, accurately timed. Gently swirl the tubes for 1 minute to remove any gas bubbles present, then concomitantly determine the absorbances of the solutions in 1-cm cells, at the wavelength of maximum absorbance at about 500 nm, with a suitable spectrophotometer, using the blank to set the instrument. Calculate the quantity, in mg, of benzonatate (C30H53NO11 av.) in the number of Capsules taken by the formula: in which C is the concentration, in µg per mL, of USP Benzonatate RS in the Standard preparation; and AU and AS are the absorbances of the solutions from the Assay preparation and the Standard preparation, respectively.