Co-trimoxazole Tablets
Action and use
Dihydrofolate reductase inhibitor + sulfonamide antibacterial.
Definition
Co-trimoxazole Tablets contain Trimethoprim and Sulfamethoxazole in the proportions, by weight, 1 part to 5 parts.
Content of trimethoprim, C14H18N4O3
92.5 to 107.5% of the stated amount of trimethoprim.
Content of sulfamethoxazole, C10H11N3O3S
92.5 to 107.5% of the stated amount of sulfamethoxazole.
Identification
5 volumes of dimethylformamide, 10 volumes of methanol and 100 volumes of chloroform.
One of the principal spots in the chromatogram obtained with solution (1) corresponds to the spot in the chromatogram obtained with solution (2) and the other corresponds to the spot in the chromatogram obtained with solution (3).
Assay
Weigh and powder 20 tablets.
For trimethoprim
To a quantity of the powder containing 50 mg of Trimethoprim add 30 mL of 0.1m sodium hydroxide and extract with four 50-mL quantities of chloroform, washing each extract with the same two 10-mL quantities of 0.1m sodium hydroxide. Reserve the aqueous layer for test A for Identification. Combine the chloroform extracts and extract with four 50-mL quantities of 1m acetic acid. Wash the combined extracts with 5 mL of chloroform and dilute the aqueous extracts to 250 mL with 1m acetic acid. To 10 mL of the solution add 10 mL of 1m acetic acid and sufficient water to produce 100 mL and measure the absorbance of the resulting solution at the maximum at 271 nm, Appendix II B. Calculate the content of C14H18N4O3 taking 204 as the value of A(1%, 1 cm) at the maximum at 271 nm.
For sulfamethoxazole
Dissolve, as completely as possible, a quantity of the powder containing 0.5 g of Sulfamethoxazole in 60 mL of water and 10 mL of hydrochloric acid. Add 3 g of potassium bromide, cool in ice and titrate slowly with 0.1m sodium nitrite VS, stirring constantly and determining the end point electrometrically. Each mL of 0.1m sodium nitrite VS is equivalent to 25.33 mg of C10H11N3O3S.