CENTRIFUGAL PARTITION CHROMATOGRAPHIC SEPARATIONS OF PLATINUM-GROUP METALS BY COMPLEXATION AND ION-PAIR FORMATION

Base line separation of the chloroanions of the platinum group metals (Pt, Pd, Tr, 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 chloroanion s at low acid concentrations (<0.1 M HCL) occurs as the neutral comple x MCl(2)(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 o f the kinetically inert species IrCl62- and PtCl62-. The CPC efficienc ies for the extraction of PI(II) and Pd(II) as their MCl(2)(TOPO)(2) c omplexes are mainly limited by the slow kinetics of dissociation of th is complex, This dissociation proceeds by the rapid dissociation of TO PO in a preequilibrium step yielding MCl(2)(TOPO)2(,) which then react s with Cl- in the rate-limiting step (rls), yielding MCl(3)(-) and TOP O. The preequilibrium constant for the PI complex is 2 orders of magni tude larger than that for the Pd complex, but the rate constants for t he rls for the two complexes differ by less than a factor of 2. As a r esult; PtCl2(TOPO)(2) dissociates more rapidly than PdCl2(TOPO)(2), re sulting in better CPC efficiency for Pt than for Pd. Further, the chem ical kinetic contributions to the reduced plate height, CETP(ck), for Pt and Pd bear the same direct linear correlation with the half-lives for the dissociation of their respective MCl(2)(TOPO)(2) complexes, Th e rate constants for the reaction MCI(3)(-) + TOPO, the rls in the for mation of the MCl(2)(TOPO)(2) complexes, can be derived from the preeq uilibrium, the dissociation rate, and the extraction equilibrium const ants, These agree well with values obtained directly from formation ki netics experiments and are surprisingly similar for Pt(II) and Pd(II).