Similar to Pourbaix's diagram for the speciation of

redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...

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 Biphasic, meaning having two phases, may refer to: * Phase (matter) In the physical sciences, a phase is a region of space (a thermodynamic system), throughout which all physical properties of a material are essentially uniform. Examples ...

system as a function of the

Galvani potential In electrochemistry, the Galvani potential (also called Galvani potential difference, or inner potential difference, Δφ, delta phi) is the electric potential difference between two points in the bulk of two phases. These phases can be two differ ...

difference between the two phases and the pH of the aqueous solution Ionic partition diagram of an

hydrophilic A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clarendon Press. In contrast, hydrophobes are no ...

acid AH in a biphasic water/

organic solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...

system. At a high aqueous pH, the acid is in the

anionic An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

form and can exist in both phases according to the Galvani potential difference. The

Nernst equation In electrochemistry, the Nernst equation is a chemical thermodynamical relationship that permits the calculation of the reduction potential of a reaction ( half-cell or full cell reaction) from the standard electrode potential, absolute tempe ...

for the distribution of the anion, ignoring the

activity coefficient In thermodynamics, an activity coefficient is a factor used to account for deviation of a mixture of chemical substances from ideal behaviour. In an ideal mixture, the microscopic interactions between each pair of chemical species are the same ( ...

s 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.

See also