Microionization constants of commercial cephalosporins

The equilibrium constants of five commercial cephalosporins were determined. Two are monoacidic and possess one Ka while three are amphoteric and have four microconstants. Although several assumptions were made in the calculations, good agreement was found between the compounds and with previously reported macroionization constants. By utilizing the microionization constants, the ratios of zwitterion to unchanged species were calculated to be in the 900-50,000 range and to have a maximum concentration between pH 3.5 and 5.