A: Crush 1 Tablet, boil it with 50 mL of water for 5 minutes, cool, and add 1 or 2 drops of ferric chloride TS: a violet-red color is produced.

B: Infrared Absorption 197K—Prepare the test specimen as follows. Shake a quantity of finely powdered Tablets, equivalent to about 500 mg of aspirin, with 10 mL of alcohol for several minutes. Centrifuge the mixture. Pour off the clear supernatant, and evaporate it to dryness. Dry the residue in vacuum at 60 for 1 hour.

Apparatus 1: 100 rpm.

Time: 90 minutes, for Buffer stage.

Diluent— Prepare a mixture of 0.1 N hydrochloric acid and 0.20 M tribasic sodium phosphate (3:1), and adjust, if necessary, with 2 N hydrochloric acid or 2 N sodium hydroxide to a pH of 6.8 ± 0.05.

Procedure— Determine the amount of C9H8O4 dissolved by determining UV absorbances at the wavelength of the isosbestic point of aspirin and salicylic acid (about 280 nm in the Acid stage, and about 265 nm in the Buffer stage), using a filtered portion of the solution under test, diluted, if necessary, with 0.1 N hydrochloric acid (analyzing the Acid stage) and with Diluent (analyzing the Buffer stage), in comparison with a Standard solution having a known concentration of USP Aspirin RS in the same Medium.

Mobile phase and Diluting solution— Prepare as directed in the Assay.

Standard solution— Dissolve an accurately weighed quantity of USP Salicylic Acid RS in the Standard preparation prepared as directed in the Assay, to obtain a solution having a known concentration of about 0.015 mg of salicylic acid per mL.

Test solution— Use the Stock solution, prepared as directed for Assay preparation in the Assay.

Chromatographic system— Use the Chromatographic system described in the Assay. Chromatograph the Standard solution, and record the peak responses as directed for Procedure: the relative retention times are about 0.7 for salicylic acid and 1.0 for aspirin; the resolution, R, between salicylic acid and aspirin is not less than 2.0; and the relative standard deviation of the salicylic acid peak responses is not more than 4.0%.

Procedure— Proceed as directed for Procedure in the Assay. Calculate the percentage of salicylic acid (C7H6O3) in the portion of Tablets taken by the formula: in which C is the concentration, in mg per mL, of USP Salicylic Acid RS in the Standard solution; QA is the quantity, in mg, of aspirin (C9H8O4) in the portion of Tablets taken, as determined in the Assay; and rU and rS are the peak responses of the salicylic acid peaks obtained from the Test solution and the Standard solution, respectively: not more than 3.0% is found.

Mobile phase— Dissolve 2 g of sodium 1-heptanesulfonate in a mixture of 850 mL of water and 150 mL of acetonitrile, and adjust with glacial acetic acid to a pH of 3.4.

Diluting solution— Prepare a mixture of acetonitrile and formic acid (99:1).

Standard preparation— Dissolve an accurately weighed quantity of USP Aspirin RS in Diluting solution to obtain a solution having a known concentration of about 0.5 mg per mL.

Assay preparation— Weigh and finely powder not fewer than 20 Tablets. Transfer an accurately weighed quantity of the powder, equivalent to about 100 mg of aspirin, to a suitable container. Add 20.0 mL of Diluting solution and about 10 glass beads. Shake vigorously for about 10 minutes, and centrifuge (Stock solution). Quantitatively dilute an accurately measured volume of the Stock solution with 9 volumes of Diluting solution (Assay preparation). Retain the remaining portion of Stock solution for the test for Limit of free salicylic acid.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 280-nm detector and a 4.0-mm × 30-cm column containing packing L1. The flow rate is about 2 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the tailing factor is not greater than 2.0; and the relative standard deviation is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of aspirin (C9H8O4) in the portion of Tablets taken by the formula: in which C is the concentration, in mg per mL, of USP Aspirin RS in the Standard preparation; and rU and rS are the peak responses of the aspirin peaks obtained from the Assay preparation and the Standard preparation, respectively.