Partition Coefficient

Abstract

Place a solute in a solvent and there is little doubt what will be the concentration of the solution. If a second immiscible solvent is added to the mixture, the concentration in the first solvent will become less than originally present. Immediately the question needs to be asked, “Where has it gone?” Analysis of the concentration of the solute in each of the solvents will reveal that the solute has distributed itself between the two solvents. The ability of a compound to penetrate membranes is critical to its ability to be active pharmacologically. If a compound cannot penetrate into membranes, or cannot be transported through membranes, it will be impossible for the compound to get to the site where it needs to act. One property that is used to indicate the ability of a compound to penetrate membranes is its distribution coefficient or partition coefficient. The normal procedure used to determine the partition coefficient is to measure the equilibrium concentrations of the compound in two immiscible liquid phases which are in contact. The two phases are an aqueous solvent and an immiscible organic solvent. The organic solvent is usually n-octanol but occasionally other solvents such as chloroform are used.