A new spectrophotometric method for the selective determination of ampicillin, amoxycillin and cyclacillin in the presence of polymers and other degradation products

A rapid and convenient spectrophotometric method is described for the quantitative determination of ampicillin and other amino-penicillins. The method involves conversion of the penicillins to the corresponding piperazine-2,5-dione derivatives by heating in an alkaline sorbitol—zinc ion solution for 10 min at 60°C and subsequent treatment of these derivatives with 1 M sodium hydroxide to give a highly absorbing product with λ max at 322 nm. Since an intact β-lactam moiety and a free amino group in the side-chain of the penicillin molecules are required for the piperazinedione formation, the method is highly selective. The method was found to be free of interference from polymerization and other degradation products and its application to assess the stability of the amino-penicillins was demonstrated.