Effect of pH on the stability of sodium ampicillin solutions

The stability of sodium ampicillin was studied in a series of solutions with pH values in the range of 2.3-9.3. The buffer solutions were prepared from standard formulas and sterilized for use. Sodium ampicillin was added to each of the solutions at a concentration of 10 mg/ml. At time intervals of 0, 4, 8 and 12 hours after admixture, the stability of the drug was evaluated, utilizing spectrophotometric and microbiological assay procedures. Results indicated that under the conditions of the study sodium ampicillin appears to be most stable at a pH of 7.5. The rate of degradation appears to increase as the pH varies above or below that value. It appears that adequate potency is retained at a pH of 7.5 for a period of at least 12 hours.