Spectrophotometric determination of ampicillin sodium in the presence of its degradation and polymerization products

Abstract

A spectrophotometric method is described for the quantitative determination of ampicillin sodium [sodium salt of 6-(d(–)-phenylacetamido) penicillanic acid]. The method involves acetylation of ampicillin with acetic anhydride in aqueous solution at pH 9 to yield α-acetamido benzylpenicillin and subsequent measurement at 325 nm of α-acetamidobenzylpenicillenic acid mercuric mercaptide, formed in a quantitative yield on heating for 30 min at 60° in a 1·0m imidazole and 8 times 10−4m mercuric chloride solution at pH 6·8. It has been demonstrated that degradation products do not interfere in the method whereas those di- and polymerization products of ampicillin which contain an intact β-lactam ring are capable of reacting with imidazole with the formation of penicillenic acid. A technique involving acid-catalysed opening of the β-lactam ring of these products under conditions where ampicillin is degraded to only a minor extent has been developed, and besides permitting a selective determination of ampicillin the technique permits the determination of the polymeric substances.