| Popis: |
The pH-dependent distribution of weak acids in the octanol-water system was investigated. For a series of six 4-hydroxycoumarins, substituted at position 3, the apparent partition coefficient or the distribution coefficient, P app , was measured in the pH range 1–11. The log P app -pH profiles obtained are in complete agreement with the theory. At high pH the value of P app is independent of pH, but dependent on the concentration of counterions (Na + in this study) in the aqueous phase; in other words ion-pair extraction occurs. The pNa (—log [Na + ])-dependent distribution is analogous to the pH-dependent distribution, except in the case of 4-hydroxy-coumarin. Evidence was found that the sodium ion-pair of 4-hydroxycoumarin dissociates considerably in the octanol phase. A large discrepancy was noted between the experimentally obtained log P values and the theoretical values calculated using the hydrophobic fragmental constant system. Log P app values obtained under conditions where the compounds are ionized in the aqueous phase depend on both the partition coefficient and the dissociation constant of the acid or salt. Partition coefficients of the sodium ion-pairs obtained by extrapolation are just slightly lower than the partition coefficients of the undissociated acids. |
