Partition coefficient octanol/water

Objective of the study

The objective of the study is to determine the octanol/water partition coefficient (P_OW  or K_ow) of the pesticide active ingredient.

For inert organic substances highly significant relationships have been found between 1-octanol/water partition coefficients (P_OW)and their bioaccumulation in fish.  Moreover, P_OW has been demonstrated to be correlated to fish toxicity as well as to sorption of chemicals to solids such as soils and sediments.

A wide variety of relationships between the 1-octanol/water partition coefficient and other substance properties of relevance to environmental toxicology and chemistry have been established. Therefore, the 1-octanol/water partition coefficient has evolved as a key parameter in the assessment of the environmental risk of chemicals as well as in the prediction of fate of chemicals in the environment.

Circumstances under which the study is recommended to be required

The study is always required.

Principle of the test

In order to determine the partitioning coefficient, water, 1-octanol, and the test substance are equilibrated with each other at constant temperature. Then the concentrations of the test substance in the two phases are determined.

The partition coefficient of a substance between water and a lipophilic solvent (1-octanol) characterizes the equilibrium distribution of the chemical between the two phases. The partition coefficient between water and 1-octanol (POW) is defined as the ratio of the equilibrium concentrations of the test substance in 1-octanol saturated with water (CO) and water saturated with 1-octanol (C_W); so P_OW = C_O / C_W.

Different test procedures are available to determine the P_ow :

• The slow-stirring method is particularly suitable for the determination of POW for substances with expected log P_OW values of 5 and higher, for which the shake-flask method is prone to yield erroneous results.
• The shake-flask method is prone to artifacts due to transfer of octanol microdroplets into the aqueous phase and its use is therefore limited to substances with log P_OW  < 4.
• The HPLC method relies on solid data of directly determined P_OW values to calibrate the relationship between HPLC-retention behavior and measured values of P_OW.

Test substance

Since the presence of substances other than the test substance might influence the activity coefficient of the test substance, the test substance should be tested as a pure active ingredient (PAI). The highest purity commercially available should be employed for the 1-octanol/water partition experiment.

Typical endpoint of the study

Log P_ow  established at equilibrium.

Testing guidelines

The following test guidelines may be used for the determination of octanol/water partition coefficient:

• OECD Guidelines for the Testing of Chemicals. Test No. 123: Partition Coefficient (1-Octanol/Water), Slow-Stirring Method.
• OECD Guidelines for the Testing of Chemicals. Test No. 107: Partition Coefficient (n-octanol/water), Shake Flask Method.
• OECD Guidelines for the Testing of Chemicals. Test No. 117: Partition Coefficient (n-octanol/water), HPLC Method.
• US-EPA Product Properties Test Guidelines: OPPTS 830.7550 Partition Coefficient (n-Octanol/Water), Shake Flask Method.
• US-EPA Product Properties Test Guidelines: OPPTS 830.7560 Partition Coefficient (n-Octanol/Water), Generator Column Method.
• US-EPA Product Properties Test Guidelines: OPPTS 830.7570 Partition Coefficient (n-Octanol/Water), Estimation by Liquid Chromatography.