Palladium(II) sequestration by phytate in aqueous solution - speciation analysis and ionic medium effects

Palladium(II) speciation in the presence of phytate (Phy12-) was studied by ISE-H+ potentiometry at t = 25°C in NaNO3aq and in NaClaq at I = 0.1 mol L-1, in order to evaluate the effect of the ionic medium on the sequestering ability of phytate toward palladium(II). Owing to the discrepancies found in literature on both the nature and the stability of hydrolytic species formed by this cation, Pd2+ hydrolysis was studied under the same experimental conditions. Since palladium(II) forms stable complex species with chloride ion, the stability constants of various Pd2+ - Cl- species were also calculated, as well as those of weak species formed by nitrate. The stability constants of six palladium(II) - phytate species were determined, namely PdPhyOH11-, PdPhy10-, PdPhyH9-, PdPhyH28-, PdPhyH37-, and PdPhyH46-. The sequestering ability of this ligand toward Pd2+ was evaluated by the calculation of various pL50 values in different conditions. Phytate sequestering ability toward palladium(II) was then compared with that toward other divalent cations, in various conditions. Finally, the dependence of pL50 on pH was modeled by a simple empirical relationship.