What your after is an isotherm. If it is a solvent extraction application for metals you will have one isotherm for extraction and one isotherm for stripping. The isotherm is a plot of metal concentration in the aqueous phase vs metal concentration in the organic.
The results are obviously specific to the conditions and the reagent selected. Your best approach is to discuss the specific duty with a manufacturer of the extractant, e.g. Cognis, Cytec.
The manufacturer will get you to the optimal conditions much quicker and reduce the experimental load.
Cheers,
NB