A rare metal concentration method from plating rinse water using homogeneous liquid-liquid extraction (HoLLE) with n-perfluorocarboxylic acid (C6
) was developed. After HoLLE, the volume ratio (Va
) of the aqueous phase (Va
) and the sedimented liquid phase (Vs
) was 1,430 (50 mL → 0.035 mL).Of the cobalt, 99.7 % was extracted into the sedimented liquid phase.