Spectrophotometric determination of microgram quantities of acetylsalicylic anhydride in acetylsalicylic acid

Abstract 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%.