Macedonian pharmaceutical bulletin, 63 (2) 21 - 27 (2017) ISSN: UDC: 544.362:543.544.5.068.7 DOI: 10.33320/maced.pharm.bull.2017.63.02.003 Review paper Overview on chromatographic and potentiometric based approaches for pKa determination of sparingly soluble substances Natalija Nakov 1 *, Jelena Acevska 1 , Katerina Brezovska 1 , Aneta Dimitrovska 1 1 Institute of Applied Chemistry and Pharmaceutical Analysis, Faculty of Pharmacy, Ss. Cyril and Methodius University, 1000 Skopje, Republic of Macedonia Received: February 2018; Accepted: April 2018 Abstract The dissociation constant (pKa) is one of the most important physicochemical property of the compounds and it is of great importance for the pharmaceutical industry. Many of the active pharmaceutical ingredients are sparingly soluble in water or have no chromophore, thus the conventional UV/Vis spectrometry and potentiometry methods are not applicable. This article presents an overview of several approaches based on RP-HPLC and cosolvent/surfactant potentiometry that have been recently published for the pKa determination of sparingly soluble active pharmaceutical ingredients. Additionally, different calculation methods that have been used for the estimation of the aqueous dissociation constant are summarized. Keywords: dissociation constant, pKa, sparingly soluble substances, determination, reverse-phase liquid chromatography, potentiometry, surfactants Introduction The dissociation constant (pКа) is one of the primary physicochemical property of active pharmaceutical ingredients. The accurate measurement of the pKa value is of great importance to a wide range of research fields. The pharmacokinetics (ADMET: adsorption, distribution, metabolism, excretion and toxicity) of active pharmaceutical ingredients depend on their pKa value. The determination of the pKa value during the early stage studies could provide proper selection of a candidate for active pharmaceutical ingredient among the large number of compounds produced by combinatory chemistry. The knowledge of the pKa value for the analyte and its related impurities is essential for analytical method development; providing ruggedness and faster method optimization (Avdeef, 2001; Kerns & Di, 2004; Manallack, 2007). Several techniques are used for determination of the dissociation constant. Computational methods are valuable tool for pKa calculation, especially for large set of compounds present in the screening stage of the drug candidate. There are several computer programs (such as pKa Pallas program, ACD/pKa, CSpKa Predictor, ADME/Tox WEB, etc.) which are based on different theoretical approaches for predicting the pKa value. It should be noted that the accuracy of the computational methods depends only on the model used to perform the calculations. However, computational methods could give unreliable results for sparingly soluble compounds (Babic et al., 2007; Reijenga et al., 2013; Špirtović-Halilović & Zavrśnik, 2010). Traditionally, UV/Vis spectrometry and potentiometry have been standard methods for the pKa determination (Erdemgli et al., 2007). The UV/Vis __________________ * natalijan@ff.ukim.edu.mk