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

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 0.15M 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