Joseph B. Bogardus

Common Ion Equilibria of Hydrochloride Salts and the Setschenow Equation

A simple equation was derived to describe the relationship between the aqueous solubility of sparingly soluble salts (S0) and the empirical Setschenow salting-out constant (k): k = 0.217/S0. This relationship and the Setschenow equation were found to be valid only at low concentrations of added salt. This equation agreed with recently published data when compared for the effect of the chloride ion on the solubility of a series of drug hydrochloride salts. The theoretical treatment also predicts the curvature which has been reported in literature Setschenow plots at higher salt concentrations. As the concentration of added salt increases, the apparent k value is not constant but is dependent on solubility and the rate of change of solubility with added salt concentration. It was concluded that the Setschenow treatment is generally inappropriate for description and analysis of common ion equilibria.

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 pK1 = 2.86 and pK2 = 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 pK3 = 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.