pH‐Metric log P. Part 1. Difference Plots for Determining Ion‐Pair Octanol‐Water Partition Coefficients of Multiprotic Substances

Abstract 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 Pderivation 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 dianinn 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.