Kow = Concentration in octanol phase / Concentration in aqueous phase ( I -1 )

Values of Kow are thus unitless. The parameter is measured using low solute concentrations, where Kow is a very weak function of solute concentration. Values of Kow are usually measured at room temperature (20 or 25'C . The effect of temperature on Kow is not great - usually on the order of 0.001 to 0.01 Iog Kow units per degree - and may be either positive or negative.

Measured values of Kow for organic chemicals have been found as low as 10^-3 and as high as 10⁷, thus encompassing a range of ten orders of magnitude. In terms of log Kow, this range is from -3 to 7. It is frequently possible to estimate log Kow with an uncertainty (i.e., method error) of no more than 10.1-0.2 Iog Kow units.

The octanol/water partition coefficient is not the same as the ratio of a chemical's solubility in octanol to its solubility in water, because the organic and aqueous phases of the binary octanol/water system are not pure octanol and pure water. At equilibrium, the organic phase contains 2.3 mol/L of water, and the aqueous phase contains 4.5 X 10^-8 mol/L of octanol. Moreover, Kow is often found to be a function of solute concentration. The chemical in question is added to a mixture of octanol and water whose volume ratio is adjusted according to the expected value of Kow. Very pure octanol and water must be used, and the concentration of the solute in the system should be less than 0.01 mol/L. The system is shaken gently until equilibrium is achieved (15 min to I hr). Centrifugation is generally required to separate the two phases, especially if an emulsion has formed. An appropriate analytical technique is then used to determine the solute concentration in each phase. A rapid laboratory estimate of Kow may be obtained by measuring the retention time in a high-pressure liquid chromatography system; the logarithm of the retention time and the logarithm of Kow have been found to be linearly related.

In recent years the octanol/water partition coefficient has become a key parameter in studies of the environmental fate of organic chemicals. It has been found to be related to water solubility, soil/sediment adsorption coefficients, and bioconcentration factors for aquatic life. Because of its increasing use in the estimation of these other properties, Kow is considered a required property in studies of new or problematic chemicals.

Values of Kow can be considered to have some meaning in themselves, since they represent the tendency of the chemical to partition itself between an organic phase (e.g., a fish, a soil) and an aqueous phase. Chemicals with low Kow values (e.g., less than 10) may be considered relatively hydrophilic; they tend to have high water solubilities, small soil/sediment adsorption coefficients, and small bioconcentration factors for aquatic life. Conversely, chemicals with high Kow values (e.g., greater than 10⁴) are very hydrophobic.