A rapid and convenient method for the determination of an immunogenic impurity in acetylsalicylic acid, acetylsalicylic anhydride, is described. The method is based on the spectrophotometric measurement at 362 nm of 4‐benzylidene‐2‐phenyloxazolin‐5‐one, formed in quantitative yield by allowing acetylsalicylic anhydride, pre‐extracted from the acetylsalicylic acid by a benzene‐aqueous phosphate buffer system, to react with a mixture of pyridine and sodium α‐benzamido‐cinnamate in ethanol for 30 min at 20°. The method is reproducible and permits determination of 0ṁ0005% of acetylsalicylic anhydride in acetylsalicylic acid. Applying the method to nine different commercial acetylsalicylic acid preparations, acetylsalicylic anhydride was found to be present in amounts ranging from 0ṁ0012 to 0ṁ024%.

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