Ionic partition diagram
Similar to Pourbaix's diagram for the speciation of redox species as a function of the redox potential and the pH, ionic partition diagrams indicate in which an acid or a base are predominantly present in a biphasic system as a function of the Galvani potential difference between the two phases and the pH of the aqueous solution
Ionic partition diagram of an hydrophilic acid AH in a biphasic water/organic solvent system.
At a high aqueous pH, the acid is in the anionic form and can exist in both phases according to the Galvani potential difference. The Nernst equation for the distribution of the anion, ignoring the activity coefficients is written.
Thus, the separation limit between the anionic form in water and the organic solvent () is a horizontal straight line. As in Pourbaix diagrams, the separation limit between the acid and basic forms in water is a vertical line given by .
The line separating the neutral acid in water and the anion Ao– in the organic phase is given by considering the aqueous acidity constant to give
As in the Pourbaix diagrams, we obtain a delimiting line that depends on the pH as shown below.
