PH-METRIC LOG-P .1. DIFFERENCE PLOTS FOR DETERMINING ION-PAIR OCTANOL-WATER PARTITION-COEFFICIENTS OF MULTIPROTIC SUBSTANCES

New potentiometrically-based octanol-water partition coefficient (log P) equations for di-, tri- and tetraprotic weak acids are derived. These equations are valid when one or more species partition into the octanol phase as neutral or ion-paired substances. Citric and phthalic acids were selected to demonstrate the log P derivation procedure experimentally. Citric acid appears to participate in weak ion-pair partitioning. Addition of tetra-n-butylammonium bromide to the solutions containing the weak acids, induces the hydrogen phthalate anion, the dihydrogen citrate anion and the hydrogen citrate dianion to partition into octanol, presumably as ion-pairs with the lipophilic cation. Extensive use is made of difference titration curves. The graphically derived models were refined by a general weighted nonlinear least squares procedure.