Abstract:

In this work, a proposal for pKa calculation based on spectroscopic measurements is presented to support undergraduate students. pKa is defined as the negative logarithm of the acidic dissociation equilibrium constant (Ka), thus highly acidic compounds present low pKa values. pKa calculation of organic molecules is a key issue in many fields with important implications in real life: for instance, in pharmaceutical development, medical chemistry, or material sciences. pKa values can be calculated based on a variety of instrumental measurements. The strategy we purpose here is based on spectrophotometric and spectrofluorimetric measurements, since the target molecules possess both absorptive and fluorescent properties. The target molecules are an antibiotic of the family of fluoroquinolones (ciprofloxacin) and a bioactive compound naturally present in grapes and wine (resveratrol-3-O-glucoside). Compounds from such a different families have been selected to show students the possibilities and importance of pKa calculation in different fields. The experimental protocol implies the preparation of solutions of each compound at pH values ranging approximately between 2 and 12. Then, the acid and basic forms of each compound are spectrophotometrically and spectrofluorimetrically characterized by registering the spectra of the solutions with the lowest and the highest pH, respectively. Later, photometric and fluorimetric measurements at the typical wavelengths of the acidic and the basic forms are performed on each solution. Once the experimental part is over, two different calculation methods are proposed, namely “Stenstrom and Goldsmith” and “Wilson and Lester”.

Keywords:

Analytical Chemistry, Laboratory practice, Dissociation equilibrium, Acidity constants.