Editing Partition coefficient (section)

== Distribution coefficient and log ''D'' ==
The '''distribution coefficient''', '''log ''D''''', is the ratio of the sum of the concentrations of all forms of the compound (ionized plus un-ionized) in each of the two phases, one essentially always aqueous; as such, it depends on the [[pH]] of the aqueous phase, and log ''D'' = log ''P'' for non-ionizable compounds at any pH.<ref name="Scherrer"/><ref name="Manners_1998"/> For measurements of distribution coefficients, the pH of the aqueous phase is [[Buffer solution|buffered]] to a specific value such that the pH is not significantly perturbed by the introduction of the compound. The value of each '''log ''D''''' is then determined as the logarithm of a ratio—of the sum of the experimentally measured concentrations of the solute's various forms in one solvent, to the sum of such concentrations of its forms in the other solvent; it can be expressed as<ref name = "Comer_2001"/>{{rp|275–8}}

: <math>\log D_\text{oct/wat} = \log_{10}\left(\frac{\big[\text{solute}\big]_\text{octanol}^\text{ionized} + \big[\text{solute}\big]_\text{octanol}^\text{un-ionized}}{\big[\text{solute}\big]_\text{water}^\text{ionized} + \big[\text{solute}\big]_\text{water}^\text{un-ionized}}\right).</math>

In the above formula, the superscripts "ionized" each indicate the sum of concentrations of all ionized species in their respective phases. In addition, since log ''D'' is pH-dependent, the pH at which the log ''D'' was measured must be specified. In areas such as drug discovery—areas involving partition phenomena in biological systems such as the human body—the log ''D'' at the physiologic pH&nbsp;= 7.4 is of particular interest.{{citation needed|date=March 2016}}

It is often convenient to express the log ''D'' in terms of ''P''<sup>I</sup>, defined above (which includes ''P''<sup>0</sup> as state {{math|1=''I'' = 0}}), thus covering both un-ionized and ionized species.<ref name="logp_review" /> For example, in octanol–water:

: <math>\log D_\text{oct/wat} = \log_{10}\left(\sum_{I=0}^M f^I P_\text{oct/wat}^I \right),</math>

which sums the individual partition coefficients (not their logarithms), and where <math>f^I</math> indicates the pH-dependent [[mole fraction]] of the {{mvar|I}}-th form (of the solute) in the aqueous phase, and other variables are defined as previously.<ref name="logp_review" />{{verify source|date=March 2016}}