## pH0.5 or pH1/2

in solvent extraction
https://doi.org/10.1351/goldbook.P04525
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]