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—

Test specimen— Shake a quantity of finely powdered Tablets, equivalent to about 500 mg of aspirin, with 10 mL of chloroform for several minutes. Centrifuge the mixture. Pour off the clear supernatant, and evaporate it to dryness.

Medium: 0.05 M acetate buffer, prepared by mixing 2.99 g of sodium acetate trihydrate and 1.66 mL of glacial acetic acid with water to obtain 1000 mL of solution having a pH of 4.50 ± 0.05; 500 mL.

Apparatus 2: 75 rpm. [note—Where the Tablet is composed of multiple layers, a stainless steel wire helix may be used, if needed, to hold the Tablet in proper orientation in the apparatus.]

Time: 30 minutes.

Procedure— Determine the amount of aspirin (C9H8O4) dissolved by employing UV absorption at the wavelength of the isosbestic point of aspirin and salicylic acid at 265 ± 2 nm on filtered portions of the solution under test, suitably diluted with Medium, if necessary, in comparison with a Standard solution having a known concentration of USP Aspirin RS in the same Medium. [note—Prepare the Standard solution at the time of use. An amount of methanol not to exceed 1% of the total volume of the Standard solution may be used to dissolve the Reference Standard prior to dilution with Medium.]

Tolerances— Not less than 80% (Q) of the labeled amount of C9H8O4 is dissolved in 30 minutes.

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— Prepare as directed 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 determined from salicylic acid is not more than 4.0%.

Procedure— Proceed as directed 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 salicylic acid 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 more 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 aspirin obtained from the Assay preparation and the Standard preparation, respectively.