Estimating the octanol-water partition coefficient for chemical substances: feasibility of extending the log KOW range of OECD test guideline 117: summary
Published 10 January 2025
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1. Chief Scientist’s Group report summary
This project assessed the feasibility of expanding the range of chemical substances that can be assessed in a standardised test method for an important physico-chemical property.
1.1 Background
The way a dissolved substance moves between n-octanol (an organic solvent) and water gives us information about how it might behave in the environment, such as whether it is likely to be taken up by organisms or be attracted to river sediments. This distribution between the two solvents is described in terms of a partition coefficient (KOW), usually expressed as a base ten logarithm (log KOW).
A standardised method for measuring log KOW is described in Organisation for Economic Co-operation and Development Test Guideline 117 (OECD TG 117). It uses an analytical technique called high-performance liquid chromatography (HPLC) to measure the time taken for a test substance in a solvent (the liquid phase) to pass through a synthetic solid material (the stationary phase). To calculate the log KOW, the time taken by the test substance is compared to that of a range of substances of similar structure that have known log KOW values (called reference substances). OECD TG 117 is generally limited to substances with log KOW up to 6 due to a lack of reference substances with higher log KOW values.
1.2 Approach
This project explored the feasibility of expanding the log KOW range of OECD TG 117 by identifying suitable reference substances. A search for substances with laboratory-derived log KOW values above 6 was made using the OECD eChemPortal, which collates information on tests from a wide range of international databases. Specific criteria were applied to decide if the results of the tests were reliable. Substances were also identified from questionnaires sent to contract research organisations (CROs) that conduct OECD TG 117 tests. Corroboration of the reported log KOW values was sought from additional peer reviewed academic data or predictions based on chemical structure. The structural diversity of the identified substances was assessed using publicly available software to understand if there were enough similar structures so that several reference substances were available to compare against any test substance (as required by the guideline), but also enough diversity so that there were reference substances for different chemical types.
1.3 Results
The search found 377 tests where a log KOW above 6 was measured, 74 of which were concluded to be reliable. There were also 22 substances already being used by CROs. Information to verify the log KOW values was obtained for around a third of the substances involved. The assessment of the structures of the verified reference substances showed sufficient diversity but also enough similarity for their use with a range of different test substances while complying with the requirements of guideline.
The report identified 32 substances with log KOW values up to 10.2 that could be used as reference substances for the OECD TG 117 method. It recommends practical work to test whether these substances are suitable for use in a HPLC system, as well as additional testing that could be used to confirm log KOW values for other potential reference substances.
1.4 Publication details
This summary relates to information from the following project:
- Title: Estimating the octanol-water partition coefficient for chemical substances.
- Project manager: Richard Gibson, Chemicals Assessment Unit, Chief Scientist’s Group
- Contractor: WCA Environment Limited, Brunel House, Volunteer Way, Faringdon, Oxfordshire, SN7 7YR
This project was commissioned by the Environment Agency’s Chief Scientist’s Group, which provides scientific knowledge, tools and techniques to enable us to protect and manage the environment as effectively as possible.
Enquiries: research@environment-agency.gov.uk.
© Environment Agency