The microscopic ionization constants of amoxicillin were determine by potentiometric and spectrophotometric techniques. By utilizing the microionization constants, the species distribution as a function of pH were calculated At physiol. pH (7.4), the max. concentration in the form of zwitterion is 50.2%, the dianion form only 0.3%.

Key words: SPECTROPHOTOMETRY, BIOLOGICAL ACTIVITY, POTENTIOMETRIC TITRATION, PENICILLIN, HYDROXYLAMINES, IONIZATION EQUILIBRIUM