Effect of Ionic Strength on Chemical Stability of Potassium Penicillin G

Abstract

Ionic strength up to 0.5 does not affect the chemical stability of potassium penicillin G unless one of the ions contributing to the ionic strength is also catalytic. Thus, ionic strength adjusted with NaCl had no effect on the chemical stability of 0.01 M potassium penicillin G solutions in 0.06 M citrate buffer at pH 7.0 where the major buffer species was the noncatalytic citrate ion. However, in penicillin solutions buffered with 0.06 M citrate buffer, the ionic strength had a direct effect at pH 6.5 or lower. The magnitude of the ionic strength effect was inversely related to pH. The monohydrogen citrate and dihydrogen citrate ions were found to be catalytic. At 45°, the catalytic rate constants were 0.272 and 1.92 l. mole−1 hr.−1, respectively. At pH's where either of these catalytic species was present, an increase in the ionic strength by the addition of NaCl caused an increased rate of degradation.