The pH of the buffers used in the solution mixture was selected based on the pKa of the compound. This approach was necessary due to the low aqueous solubility of the drug substances in their neutral state, hence it was necessary to determine logD at a pH that the molecules were significantly ionized in the aqueous state to ensure total dissolution of the drug within the bi-phasic solvents. The samples were analyzed using the HPLC analytical instrument, which included a needle-wash for the HPLC injection needle to eliminate cross-contamination, and the distribution coefficient was computed using the equation below (Equation ):