Multi-wavelength spectrophotometric determination of acid dissociation constants: a validation study Original Research Article

The pKa values of 25 structurally diverse drug compounds including mono-, di- and tri-protic molecules were determined by a recently developed multi-wavelength spectrophotometric technique, called dip-probe absorption spectroscopy (D-PAS), and the traditional spectrophotometric titration. All experiments were performed at 25.0±0.5°C in an ionic strength of 0.15 M KCl in two laboratories (Semmelweis and Sirius), to evaluate the reproducibility and reliability of the spectrophotometric techniques for pKa determination. It was found that the pKa values determined by the D-PAS technique are in excellent agreement with values measured by the traditional spectrophotometric method (average deviation=0.08 units) and are consistent with the literature data where available. We have demonstrated that the D-PAS approach is a simple, precise and easy-to-use method for pKa determination of drug molecules with ionisable group(s) in proximity to chromophore(s). The advantages and limitations of spectrophotometric pKa determination are discussed