Environmental context. In the last 20 years, the demand for palladium and other platinum-group elements has intensified, causing a significant increase in their concentration in the environment, with particular accumulation in urban areas. Knowledge about Pd²⁺ speciation in aqueous media is fundamental for the understanding of its biological and environmental activity in contaminated areas. Phytic acid appears to be a good sequestering agent towards Pd²⁺ under various conditions, indicating its potential use in the remediation of contaminated sites.

Abstract.  Palladium(II) speciation in the presence of phytate (Phy^12–) was studied by H⁺ ion selective electrode (ISE) potentiometry at 25°C in NaNO_3(aq) and in NaCl_(aq) at ionic strength I = 0.1 mol L^–1, in order to evaluate the effect of the ionic medium on the sequestering ability of phytate towards palladium(II). Owing to the discrepancies found in the literature on both the nature and the stability of hydrolytic species formed by this cation, Pd²⁺ hydrolysis was studied under the same experimental conditions as phytate/Pd^II measurements. As palladium(II) forms stable complex species with the chloride ion, the stability constants of various Pd²⁺–Cl^– species were also calculated, as well as those of weak species formed with nitrate. The stability constants of six palladium(II)–phytate species, namely PdPhyOH^11–, PdPhy^10–, PdPhyH^9–, PdPhyH₂^8–, PdPhyH₃^7– and PdPhyH₄^6–, were determined. The sequestering ability of this ligand towards Pd²⁺ was evaluated by the calculation of various pL₅₀ values (total ligand concentrations, as antilogarithm, necessary to bind the 50% of the metal ion as a trace present in the solution) under different conditions. Phytate sequestering ability towards palladium(II) was then compared with that towards other divalent cations under various conditions. Finally, the dependence of pL₅₀ on pH was modelled by a simple empirical relationship.