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CE determination of pKa is new, compared to the other techniques [144 147] It has the advantage of being a rather universal method since different detection systems can be coupled to CE Because it is a separation technique, sample impurities seldom are a problem A fused-silica capillary, with an inner diameter of 50 75 mm and 27 70 cm in length is lled with a dilute aqueous buffer solution (ionic strength
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001 005 M) [144] About 10 nL of a sample solution, whose concentration is $50 mM, is gathered at one end of the capillary, and a 20 30-kV potential is applied between the ends of the capillary dipped into each of two beakers Sample consumption is roughly 02 ng per injection Sample species migrate according to their charge and uid drag Apparent electrophoretic mobility is determined, which is related to the migration time, the length of the capillary, and the applied voltage The mobility of ionizable compounds is dependent on the fraction of the compound in the charged form This, in turn, depends on the pKa The plot of the apparent mobility versus pH has a sigmoidal shape, with the midpoint pH equal to the pKa The practical range for buffer pH in CE is 2 3 at the low end and 11 12 at the high end When UV detection is used, the limit of detection for a molecule having the molar absorptivity of benzoic acid at 220 nm is $2 mM [144] Ishihama et al [145] were able to determine the pKa of multiprotic molecules by CE, one molecule having seven ionization groups They reported a 10 mM limit of detection for verapamil Its reported pKa , 889, compares well to that determined by potentiometry, 907 [ pION] Ishihama et al [147] have describe a rapid screening method for determining pKa values of pharmaceutical samples by pressure-assisted CE, coupled with a photodiode array detector Each CE run was completed in less than 1 min, so a 96-well microtiter plate could be measured in one day Determinations of the pKa values of 82 drugs illustrated this interesting new method Since most drug discovery projects deal with very sparingly soluble compounds, the usual CE sample concentration would lead to precipitation The handling of real drug candidate molecules is poorly developed in CE applications, in comparison to the most robust potentiometric method
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CHROMATOGRAPHIC pKa MEASUREMENT
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Oumada et al [148] described a new chromatographic method for determining the aqueous pKa of drug compounds that are sparingly soluble in water The method uses a rigorous intersolvent pH scale in a mobile phase consisting of a mixture of aqueous buffer and methanol A glass electrode, previously standardized with common aqueous buffers, was used to measure pH online The apparent ionization constants were corrected to a zero-cosolvent pH scale Six sparingly soluble nonsteroidal antiin ammatory weak acids (diclofenac, urbiprofen, naproxen, ibuprofen, butibufen, fenbufen) were used successfully to illustrate the new technique
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In certain types of multiprotic molecules it is possible that chemically different species of the same stoichiometric composition are formed [142,230 244] The pH-metric titration technique cannot distinguish between such tautomeric species In such cases the determined pKa is a composite constant, a macroconstant The
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Figure 34 Microspeciation of cetirizine, a three-pKa molecule [142] The numeric quantities refer to micro-pKa values The asterisks denote the principal species at various pH states [Avdeef, A, Curr Topics Med Chem, 1, 277 351 (2001) Reproduced with permission from Bentham Science Publishers, Ltd]
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thermodynamic experiment is a proton-counting technique It cannot identify the site in the molecule from which the proton comes It can only be said that a proton emerges from somewhere in the molecule On the other hand, microconstants are characteristic of individual species, of which there may be more than one with the same composition Various relationships between macro- and microconstants have been derived in the cited literature The microspecies and microconstants of cetirizine (triprotic molecule with macroconstant pKa values 212, 290, and 798) are shown in Fig 34, based on the impressive work of Tam and Quere [142] The microspecies denoted by an astrisk in Fig 34 are the principal species present in solution As pH increases, the protonated nitrogen nearest the phenyl groups is the rst center to shed charge The corresponding dication ! monocation reaction has the micropKa 232 The next principal center to shed a proton is the carboxylic group, leading to the formation of a zwitterion (micro-pKa 270) The highest-pH principal deprotonation consists of the protonated nitrogen nearest the carboxylate group losing its proton (micro-pKa 798) to form the anionic species on the right side of Fig 34 In cetirizine, the carboxylic group has four different micro-pKa values in the range, 270 547, depending on the neighboring-group charge state The nitrogen nearest the phenyl groups has the micro-pKa values in the range 202 733 The other nitrogen has values in the range 277 798
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