The partition coefficient (Pow) is defined as the ratio of the equilibrium concentrations of a dissolved substance in a two-phase system consisting of 2 largely immiscible solvents (n-octanol and water). Being Pow the quotient of 2 concentrations, or the quotient of the fractions of the test substance in the 2 phases multiplied by a fixed volume ratio, is dimensionless and is usually given in the form of its logarithm to base ten.

OECD TG 107: Determination of the partition coefficient n-octanol/water: Shake flasks method (Solubility > 10^-2 g/L)

OECD TG 107: Determination of the partition coefficient n-octanol/water: Shake flasks method (Solubility > 10-² g/L)
Test vessels containing the two-phase system are placed in a mechanical shaker. After a set mixing time, phase separation is carried out usually by centrifugation. The test substance concentrations are determined in both phases and the total quantity of test substance present is calculated and compared to the amount initially added.

STUDY DESIGN
Method validations, to determine the substance content in water phase and in n-octanol phase respectively, are performed according to SANCO/3030/99 rev. 4 guidance document.
Validation of the analytical methods for each phase includes:
Linearity with at least 5 concentrations of the ananlytical standard (2 injections/concentration) in the appropriate range of concentrations.
Precision with 5 sample solutions (2 injections/sample solution)
Accuracy with al least 3 fortification levels with 3 samples for each fortification level (2 injections/sample solution) and 1 blank sample for the interference check and specificity.
Confirmatory analyses/specificity by high specific technique or other proper confirmatory technique.
LOQ and LOD determinations.
The test for the determination of Pow is carried out in temperature range (25±1)°C. 2 sample solutions for each volume ratio of the 2 phases are performed except for the 5 sample solutions related to the precision check.
For ionizable substances, the test will be planned to take into account the effects of pH.

REFERENCES AND GUIDELINES
OECD Guideline for Testing of Chemicals, No. 107 (27^th July 1995) – Partition Coefficient (n-octanol/water): Shake Flask Method.
SANCO/3030/99 rev. 4 (11/07/2000) – Technical Material and Preparations: Guidance for generating and reporting methods of analysis in support of pre- and post-registration data requirements for Annex II (part A, Section 4) and Annex III (part A, Section 5) of Directive 91/414.