Relative Stability of Hetacillin and Ampicillin in Solution

Abstract

Sodium ampicillin in solution loses almost 10% of its activity in 1 hr. at room temperature at a concentration of 250 mg./ml.; but at 30 mg./ml., it requires 8 hr. for the same loss of activity. Hetacillin, an acetone adduct of ampicillin, loses somewhat less than 10% activity in 6 hr. at a concentration of 250 mg./ml. under the same conditions. Hetacillin has been shown to convert to ampicillin with a half-life of only 20-30 min. at room temperature and, therefore, should lose more than 10% activity in 6 hr. An explanation of these inconsistent data is offered herein by adopting a model reaction scheme which incorporates the concentration dependence of both ampicillin degradation and the hetacillin-ampicillin equilibrium. The consistency of the model is shown using literature data, and the properties of the model are discussed.