Sulfamethoxazole
(sul'' fa meth ox' a zole).
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C10H11N3O3S 253.28

Benzenesulfonamide, 4-amino-N-(5-methyl-3-isoxazolyl)-.
N1-(5-Methyl-3-isoxazolyl)sulfanilamide [723-46-6].
» Sulfamethoxazole contains not less than 99.0 percent and not more than 101.0 percent of C10H11N3O3S, calculated on the dried basis.
Packaging and storage— Preserve in well-closed, light-resistant containers. Store at room temperature.
USP Reference standards 11
USP Sulfamethoxazole RS Click to View Structure
USP Sulfanilamide RS Click to View Structure
Identification—
B: Ultraviolet Absorption 197U
Solution: 10 µg per mL.
Medium: sodium hydroxide solution (1 in 250).
Absorptivities at 257 nm, calculated on the dried basis, do not differ by more than 2.0%.
C: Dissolve about 100 mg in 2 mL of hydrochloric acid, and add 3 mL of sodium nitrite solution (1 in 100) and 1 mL of sodium hydroxide solution (1 in 10) containing 10 mg of 2-naphthol: a red-orange precipitate is formed.
Melting range, Class I 741: between 168 and 172.
Loss on drying 731 Dry it at 105 for 4 hours: it loses not more than 0.5% of its weight.
Residue on ignition 281: not more than 0.1%.
Selenium 291: 0.003%, a 200-mg test specimen being used.
Sulfanilamide and sulfanilic acid—
Standard solution— Dissolve 100 mg of USP Sulfamethoxazole RS in 0.10 mL of ammonium hydroxide, dilute with methanol to 10.0 mL, and mix.
Reference solution— Dissolve 20 mg of USP Sulfanilamide RS and 20 mg of sulfanilic acid in 10 mL of ammonium hydroxide, and dilute with methanol to 100 mL. Transfer 2.0 mL of the solution to a 50-mL volumetric flask, add 10 mL of ammonium hydroxide, dilute with methanol to volume, and mix.
Test solution— Dissolve 100 mg in 0.10 mL of ammonium hydroxide, dilute with methanol to 10.0 mL, and mix.
Procedure— Apply 10 µL of the Standard solution, 25 µL of the Reference solution, and 10 µL of the Test solution to a suitable thin-layer chromatographic plate (see Chromatography 621) coated with a 0.25-mm layer of chromatographic silica gel. Allow the spots to dry, and develop the chromatogram in a solvent system consisting of a mixture of alcohol, n-heptane, chloroform, and glacial acetic acid (25:25:25:7) until the solvent front has moved about three-fourths of the length of the plate. Remove the plate from the chamber, and allow it to air-dry. Spray the plate with a solution prepared by dissolving 0.10 g of p-dimethylaminobenzaldehyde in 1 mL of hydrochloric acid and diluting with alcohol to 100 mL. Sulfamethoxazole produces a spot at about RF 0.7, sulfanilamide at about RF 0.5, and sulfanilic acid at about RF 0.1. Any spots produced by sulfanilamide or sulfanilic acid from the Test solution do not exceed in size or intensity similar spots, occurring at the respective RF values, produced by sulfanilamide or sulfanilic acid from the Reference preparation (0.2%).
Assay— Dissolve about 500 mg of Sulfamethoxazole, accurately weighed, in a mixture of 20 mL of glacial acetic acid and 40 mL of water, and add 15 mL of hydrochloric acid. Cool to 15, and immediately titrate with 0.1 M sodium nitrite VS, determining the endpoint potentiometrically using a calomel-platinum electrode system. Each mL of 0.1 M sodium nitrite is equivalent to 25.33 mg of C10H11N3O3S.
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Monograph Leonel M. Santos, Ph.D.
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