Ionization and solubility of an amphoteric β-lactam antibiotic

Abstract

Solution equilibria of an experimental antibiotic (code named CP-38, 371) were investigated. In the pH range 2–4 the compound has two pK values. These overlapping macroconstants were determined by three methods: potentiometric titration, spectro-metric titration, and curve-fitting of pH—solubility data. Potentiometric values of pK 1 = 2.86 and pK 2 = 3.81 at 25°C ( μ = 0.2) were in agreement ( ± 0.1 unit) with the constants determined by the other methods. CP-38,371 is amphoteric and pK 3 = 7.50 was determined by potentiometric and solubility methods. The complete microionization scheme for the compound was simplified by consideration of the UV spectral characteristics of the ionizing groups. Using absorbance—pH data, 4 microconstants for the important processes were calculated by the method of Edsall et al. (1958), and by a method developed by the authors. The two procedures gave identical results.