The formerly used term partition coefficient is not recommended by IUPAC.
Solutions:
(1) 1-octanol (or other solvent immiscible with water)
(2) water
Procedure:
(1) The pH of the water is adjusted so that the compound will not be ionized.
(2) A mixture of 1-octanol and water is prepared,
(3) The compound is added to the mixture.
(4) The mixture is shaken.
(5) The mixture is allowed to separate into its 2 phases (reach equilibrium).
(5) The concentration of the compound is measured in the 1-octanol.
(6) The concentration of the compound is measured in the water.
partition ratio =
= (molar concentration in 1-octanol) / (molar concentration in water)
The ratio is unitless.
Although the 2 solutions are allowed to separate, in actuality there is some water in the octanol and some octanol dissolved in the water.
A high ratio indicates lower aqueous solubility.
A low ratio indicates a higher aqueous solubility.
The partition coefficient may also be expressed as LOG10(ratio).
For a drug that is ionized in the body, then the Kow must be adjusted to that level of ionization. Also the temperature of the analysis versus that of the body must be taken into account.
A drug that has a high partition coefficient (hydrophobic) may accumulate in body fat and may not be cleared for some time.
