Chemically based model for the extraction of Pr(III) and Nd(III) with mixtures of various organophosphorus compounds

Mason et al. have studied the extraction of trivalent lanthanides using mixtures of di(2-ethylhexyl) phosphoric acid (D2EHPA) and TBP or TOPO with small synergistic enhancement. With the higher lanthanides (atomic number Z > 64), Zangen reported antagonistic effects with mixtures of D2EHPA and a series of neutral esters. Hence it is of interest to investigate the synergism, if any, in the extraction of Pr(III) and Nd(III) with mixtures of EHPNA and TBP or TOPO. Due to the difficulty in establishing the reaction mechanisms at high concentrations for the extraction of Pr(III) and Nd(III) with mixtures of EHPNA and TBP or TOPO by the usual slope analysis method, recourse was taken to nonlinear regression analysis using a computer. In this approach, plausible reaction mechanisms are assumed and the extraction constants are deduced by minimizing the root mean square fractional error between the experimental and calculated distribution coefficients.