Simultaneous Determination of Acid Dissociation Constants and True Partition Coefficients by Analyses of the Apparent Partition Coefficients. I

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Since the partition coefficient P and the dissociation constant pK_a are important in studies of structure-activity relationships, the pH dependence of the apparent partition coefficient P_a was quantitatively studied for the cases of acids and bases. Linear relations for mono-acids and mono-bases, but parabolic relations for the dissociation of two functional groups in the pH region studied were derived between P_a and proton concentration. The equations, however, are different for acids and bases. These equations make it possible to determine pK_a and the log P simultaneously. Some experimental systems have been measured and compared with the results obtained by other techniques ; the agreement was good. A consideration of the dissociation in the organic phase showed that the correction factor due to ionic dissociation in the organic phase for an octanolbuffer system is negligible in the pH region near the pK_a value, but is not negligible in the general equation connecting the P_a and P values.