Partition Coefficient by Potentiometric Titration

The partition coefficient is defined as the ratio of unionized molecule distributed between organic phase and aqueous phase at equilibrium.

Partition coefficient =    Concentration of molecule in organic phase

                                    Concentration of molecule in aqueous phase 

In potentiometric determination of partition coefficient, typically, a preacidified solution of a weak acid is alkalimetrically titrated to some approximately high pH; the partition solvent such as octanol is then added, and the dual–solvent mixture is acidimetrically titrated back to the starting pH. Analysis of the two titration curves will yield two pK values.The partition coefficient is calculated from the following.

Calculation of logP_ow:

From the pKa and the volume ratio r between the two phases, logP_ow is calculated:

𝑙𝑜𝑔𝑃_𝑜𝑤= 𝑙𝑜𝑔[(10₀^{|Δ𝑝𝐾_𝑎|} − 1)/𝑟]

where

│ΔpKa │ = absolute value of (pK_a^octanol-pK_a^water)

r = V_octanol/V_water = 3

Partition coefficient is used to measure the efficiency of active ingredients dissolved in biological fluids to get absorbed by e.g. the tissues in human body to achieve successful healing treatment.1-octanol is used a model compound to simulate the tissue boundary in human body.

This application describes the determination of the partition coefficient of acetic acid in 1-octanol/water by potentiometric titration.