Centrifugal Partition Chromatographic Separations of Platinum Group Metals by Complexation and Ion Pair Formation

Abstract

Base line separation of the chloroanions of the platinum group metals (Pt, Pd, Ir, Rh) in chloride media has been achieved using centrifugal partition chromatography (CPC) employing a heptane−water phase pair, by both complexation with trioctylphosphine oxide (TOPO) and ion pair formation with protonated TOPO (HTOPO+). Extraction of the chloroanions at low acid concentrations (<0.1 M HCl) occurs as the neutral complex MCl2(TOPO)2 irrespective of the chloride concentration, while at higher acid concentrations the chloroanions are extracted by ion pair formation with HTOPO+, which is especially useful in the separation of the kinetically inert species IrCl62- and PtCl62-. The CPC efficiencies for the extraction of Pt(II) and Pd(II) as their MCl2(TOPO)2 complexes are mainly limited by the slow kinetics of dissociation of this complex. This dissociation proceeds by the rapid dissociation of TOPO in a preequilibrium step yielding MCl2(TOPO), which then reacts with Cl- in the rate-limiting step (rls), yielding MCl3- and TOPO. The preequilibrium constant for the Pt complex is 2 orders of magnitude larger than that for the Pd complex, but the rate constants for the rls for the two complexes differ by less than a factor of 2. As a result, PtCl2(TOPO)2 dissociates more rapidly than PdCl2(TOPO)2, resulting in better CPC efficiency for Pt than for Pd. Further, the chemical kinetic contributions to the reduced plate height, CETPck, for Pt and Pd bear the same direct linear correlation with the half-lives for the dissociation of their respective MCl2(TOPO)2 complexes. The rate constants for the reaction MCl3- + TOPO, the rls in the formation of the MCl2(TOPO)2 complexes, can be derived from the preequilibrium, the dissociation rate, and the extraction equilibrium constants. These agree well with values obtained directly from formation kinetics experiments and are surprisingly similar for Pt(II) and Pd(II).