pH0.5 or pH1/2 in solvent extraction

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