A: Dissolve 0.25 g in 3 mL of 1 N sodium hydroxide, transfer to a 500-mL volumetric flask, dilute with water to volume, and mix. Transfer a 5-mL aliquot to a 250-mL volumetric flask containing 12.5 mL of pH 7 phosphate buffer (see Buffer Solutions in the section Reagents, Indicators, and Solutions), dilute with water to volume, and mix. This solution, when compared in a suitable spectrophotometer against a blank of the same buffer in the same concentration, exhibits absorbance maxima at 265 ± 2 and 299 ± 2 nm, and the ratio A265/A299 is between 1.50 and 1.56.

B: Place about 1 g in a small, round-bottom flask, and add 10 mL of acetic anhydride. Heat the flask on a steam bath for 30 minutes, add 40 mL of water, mix, filter, cool, and allow to stand until the diacetyl derivative has crystallized. Collect the precipitate on a filter, wash well with water, and dry at 105 for 1 hour: the diacetyl derivative so obtained melts between 191 and 197.

C: Shake 0.1 g with 10 mL of water, and filter. To 5 mL of the filtrate add 1 drop of ferric chloride TS: a violet color is produced.

Mobile phase— Prepare as directed in the Assay.

Internal standard solution— Prepare a solution of sulfanilamide in Mobile phase having a concentration of about 5 µg per mL.

Standard solution— Dissolve an accurately weighed quantity of USP m-Aminophenol RS in Mobile phase to obtain a solution having a known concentration of about 12 µg per mL. Transfer 10.0 mL of this solution and 10.0 mL of Internal standard solution to a 100-mL low-actinic volumetric flask, dilute with Mobile phase to volume, and mix.

Test solution— Transfer about 50 mg of Aminosalicylic Acid, accurately weighed, to a 100-mL low-actinic volumetric flask, add 50 mL of Mobile phase, and swirl to dissolve. Add 10.0 mL of Internal standard solution, dilute with Mobile phase to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 280-nm detector and a 4.6-mm × 25-cm column that contains 10-µm 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 retention times are about 0.66 for sulfanilamide and 1.0 for m-aminophenol; the resolution, R, between m-aminophenol and sulfanilamide is not less than 2.5; and the relative standard deviation for replicate injections is not more than 7%.

Procedure— [note—After use, wash the column for 30 minutes with a filtered and degassed mixture of methanol, water, and phosphoric acid (77:23:0.6), and then wash for 30 minutes with a filtered and degassed mixture of methanol and water (50:50).] Separately inject equal volumes (about 20 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 percentage of m-aminophenol, in relation to the quantity of aminosalicylic acid in the portion of Aminosalicylic Acid taken by the formula: in which C is the concentration, in µg per mL, of USP m-Aminophenol RS in the Standard solution, W is the quantity of aminosalicylic acid, in mg, in the portion of Aminosalicylic Acid taken, as determined in the Assay; and RU and RS are the ratios of the response of the m-aminophenol peak to the response of the sulfanilamide peak obtained from the Test solution and the Standard solution; respectively: not more than 0.25% of m-aminophenol is found.

Mobile phase— Prepare a mixture of 425 mL of 0.05 M dibasic sodium phosphate, 425 mL of 0.05 M monobasic sodium phosphate, and 150 mL of methanol containing 1.9 g of tetrabutylammonium hydroxide. Filter, and degas. Make adjustments if necessary (see System Suitability under Chromatography 621).

Internal standard solution— Prepare a solution of acetaminophen in Mobile phase having a concentration of about 5 mg per mL.

Standard preparation— Transfer about 12.5 mg of USP Aminosalicylic Acid RS, accurately weighed, to a 25-mL low-actinic volumetric flask, add 15 mL of Mobile phase, and swirl to dissolve. Add 2.5 mL of Internal standard solution, dilute with Mobile phase to volume, and mix.

Assay preparation— Prepare as directed for Standard preparation, except to use Aminosalicylic Acid instead of USP Aminosalicylic Acid RS.

Chromatographic system (see Chromatography 621)—The chromatograph is equipped with a 254-nm detector and a 4.6-mm × 25-cm column that contains packing L1. The flow rate is about 1.5 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative retention times are about 0.83 for acetaminophen and 1.0 for aminosalicylic acid; the resolution, R, between aminosalicylic acid and acetaminophen is not less than 1.7; and the relative standard deviation of the ratios of the response of the aminosalicylic acid peak to the response of the acetaminophen peak is not more than 1.0%.

Procedure— [note—After use, wash the column for 30 minutes with a filtered and degassed mixture of methanol, water, and phosphoric acid (77:23:0.6), and then wash for 30 minutes with a filtered and degassed mixture of methanol and water (50:50).] Separately inject equal volumes (about 20 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 C7H7NO3 in the Aminosalicylic Acid taken by the formula: in which C is the concentration, in mg per mL, of USP Aminosalicylic Acid RS in the Standard preparation; and RU and RS are the ratios of the response of the aminosalicylic acid peak to the response of the acetaminophen peak obtained from the Assay preparation and the Standard preparation, respectively.