(en sul' i zole).
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C13H10N2O3S 274.30
1H-Benzimidazole-5-sulfonic acid-2-phenyl-;    
2-Phenylbenzimidazole-5-sulfonic acid     [27503-81-7].
Ensulizole contains NLT 98.0% and NMT 102.0% of C13H10N2O3S, calculated on the dried basis.
•  B. Ultraviolet Absorption 197U
Sample solution:  5 µg/mL
[Note—Transfer 100 mg of Ensulizole to a 100-mL volumetric flask, dissolve in 4 mL of 1 N sodium hydroxide, and dilute with water to volume. Transfer 0.5 mL of the solution to a 100-mL volumetric flask, and dilute with water to volume. ]
Acceptance criteria:  The absorptivities, calculated on the dried basis, at the wavelength of maximum absorbance at about 302 nm, do not differ by more than 3.0%.
•  Procedure
Sample solution:  48 mg/mL of Ensulizole in 0.5 N sodium hydroxide
Titrant:  0.5 N hydrochloric acid VS
Analysis:  To the Sample solution, add phenolphthalein TS, and titrate the excess with Titrant. Perform a blank determination, and make any necessary corrections (see Titrimetry 541). Each mL of 0.5 N sodium hydroxide is equivalent to 137.15 mg of C13H10N2O3S.
Calculate the percentage of C13H10N2O3S in the portion of Ensulizole taken:
Result = [(N × (VB VA))/(W × (1 LOD))] × Mr1 × 100
N== actual normality of the Titrant
VB== volume of Titrant used for the blank (mL)
VA== volume of Titrant used for the Sample solution (mL)
W== weight of Ensulizole taken for the Sample solution (mg)
LOD== percentage of loss on drying expressed as a decimal fraction, as determined in the test for Loss on Drying
Mr1== molecular weight of ensulizole (274.30)
Acceptance criteria:  98.0%–102.0% on the dried basis
•  Loss on Drying 731: Dry a sample at 105 for 4 h: it loses NMT 2.0% of its weight.
•  Packaging and Storage: Preserve in tight containers in a cool place.
•  USP Reference Standards 11
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Topic/Question Contact Expert Committee
Monograph Behnam Davani, Ph.D., M.B.A.
Senior Scientific Liaison
(SM12010) Monographs - Small Molecules 1
Reference Standards RS Technical Services
USP35–NF30 Page 3046
Pharmacopeial Forum: Volume No. 35(4) Page 849