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

The pK(a) 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. Ah experiments were performed at 25.0 +/- 0.5 degreesC in anionic strength of 0.15 M KCl in two laboratories (Semmelweis and Sirius), to evaluate the reproducibility and reliability of the spectrophotometric techniques for pK(a) determination. It was found that the pK(a) 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 pK(a) determination of drug molecules with ionisable group(s) in proximity to chromophore(s). The advantages and limitations of spectrophotometric pK(a) determination are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.