FTIR analytics was used to estimate the complex stoichiometry when reactively extracting cobalt and nickel. Investigations were carried out at three distinct concentration levels at a Co-to-Ni ratio of 10. Additionally, the nonideal phase behavior in the system 2.4 M NaOH, phosphinic acid and diluent Escaid 120 was studied. Using the derived CoR4 and NiR4 complexes allows modeling of the reactive phase equilibria by applying the Pitzer model and the regular solution theory of Hildebrand and Scott.