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n-Octanol/Water Partition Coefficient (Kow/logKow)

By Little Pro on 2016-01-13 Views:  Update:2017-01-18

n-Octanol/Water Partition Coefficient (Kow) is defined as the ratio of the concentration of a chemical in n-octanol and water at equilibrium at a specified temperature.

  • Kow = Concentration in Octanol/ Concentration in water

Values of Kow are unitless and usually expressed as logKow, a relative indicator of the tendency of an organic compound to adsorb to soil and living organism. LogKow are generally inversely related to water solubility and directly proportional to molecular weight of a substance. 

Kow and logKow: Their Meaning for Chemical Risk Assessment

LogKow is a very important parameter for predicting the distribution of a substance in various environmental compartments (water, soil, air, biota, etc). Substances with high logKow values tend to adsorb more readily to organic matter in soils or sediments because of their low affinity for water (see Kd/Koc). Chemicals with very high logKow values (i.e, >4.5) are of greater concern because they may have the potential to bio-concentrate in living organisms. 

For above reason, n-octanol/water partition coefficient (Kow) is used as a screening test for bio-accumulation test. The assumption behind this is that the uptake of an organic substance is driven by its hydrophobicity. For organic substances with a logKow value below 4.5 it is assumed that the affinity for the lipids of an organism is insufficient to exceed the bio-accumulation criterion i.e. a BCF value of 2000.

For some groups of chemicals, such as metals and surface active compounds, logKow is not a valid descriptor for assessing the bioaccumulation potential. Information on bioaccumulation of such substances should therefore take account of other descriptors or mechanisms than hydrophobicity.

Under REACH, Kow or logKow is not required for inorganic substances. Kow or logKow is a key input parameter in environmental modelling tools to estimate environmental exposure levels. Please read:

Testing Guidelines for Measuring Kow/logKow

Reference

More Physicochemical Properties

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