Figure 4.3 (a) Lipophilicity profile of a weak base at two values of background salt and (b) log-log speciation plot at 0.15 M KCl. [Avdeef, A., Curr. Topics Med. Chem., 1, 277-351 (2001). Reproduced with permission from Bentham Science Publishers, Ltd.]

values are 4.45 and 8.47, respectively; in Fig. 4.3a, the two values are 9.24 and 5.51, respectively. Since pK^ct is associated with ion pairing, its value depends on the ionic strength, as discussed above. This is clearly evident in Figs. 4.2a and 4.3a.

It may surprise some that for a diprotic molecule with overlapping pKa values the region of maximum log D (0.76 in Fig. 4.4a) does not equal log P; a displaced horizontal line in Fig. 4.4a indicates the logP to be 0.89 for morphine [161,162].

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