This Test Guideline describes the slow-stirring method, which permits the determination of the 1-octanol/water partition coefficient (POW) values up to a log POW of 8.2. The partition coefficient between water and 1-octanol (POW) is defined as the ratio of the equilibrium concentrations of the pure test substance in 1-octanol saturated with water (CO) and water saturated with 1-octanol (CW).

In order to determine the partitioning coefficient, water, 1-octanol, and the test substance are equilibrated with each other at constant temperature in a thermostated stirred reactor at 25 °C and protected from daylight. Exchange between the phases is accelerated by stirring. The concentrations of the test substance in the two phases are determined. Each POW determination has to be performed employing at least three independent slow-stirring experiments with identical conditions. The regression used to demonstrate attainment of equilibrium should be based on the results of at least four determinations of CO / CW at consecutive time points.