That value of pH in an aqueous phase at which the distribution ratio is unity at equilibrium. 50% of the solute is extracted (\(E = 0.5\)) only when the phase ratio is unity.
Source: PAC, 1993, 65, 2373. (Nomenclature for liquid-liquid distribution (solvent extraction) (IUPAC Recommendations 1993)) on page 2385 [Terms] [Paper]
Citation: 'pH0.5 or pH1/2' in IUPAC Compendium of Chemical Terminology, 3rd ed. International Union of Pure and Applied Chemistry; 2006. Online version 3.0.1, 2019. https://doi.org/10.1351/goldbook.P04525 RISBibTexEndNote