Test No. 107: Partition Coefficient (n-octanol/water): Shake Flask Method

This Test Guideline describes a method to determine experimentally Pow values in the range log Pow between -2 and 4 (occasionally up to 5). This method can not be used with surface-active materials. The partition coefficient is defined as the ratio of the equilibrium concentrations of a dissolved substance in a two-phase system consisting of two largely immiscible solvents.

The test should be done at a temperature in the range 20 to 25°C, kept constant at ± 1°. There are three runs with different volumes ratio of n-octanol to water. Duplicate vessels containing accurately measured amounts of the two solvents and stock solution are used in all three runs. After agitation the separation of the two phases, in general, is achieved by centrifugation. It is necessary to determine the concentrations of the test substance in both phases. Analytical methods which may be appropriate are: photometry, gas chromatography and high performance liquid chromatography. The total quantity of substance present in both phases should be calculated and compared with the quantity originally introduced. A Pow value is calculated from the data of each run. The six log Pow values should fall within a range of ± 0.3 units.

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