The hydrolysis and precipitation of Pd(II) in 0.6 mol kg-1 NaCl:: A potentiometric, spectrophotometric, and EXAFS study

The soluble and insoluble hydrolysis products of palladium were investigated in aqueous solutions of 0.6 mol kg(-1) NaCl at 298.2 K. Potentiometric titrations of millimolal palladium(II) solutions were used to monitor hydrolysis reactions of the mononuclear PdCl3OH2- and PdCl2OH22- species. Spectrophotometric titrations were also used to corroborate the speciation change and to extract the correlative molar absorption coefficients for the PdCl3OH2- species in the 210-320 nm range. Longer-term potentiometric titrations systematically yielded precipitates which matured over a period of 6 weeks and resulted in a more extensive release of protons to the solution. Precipitation experiments in the 3-11 pH range showed the dominant precipitating phase to be Pd(OH)(1.72)Cl-0.28. EXAFS measurements yielded an average of 3.50 O and 0.50 Cl atoms per Pd atom with a Pd-O distance of 2.012 angstrom and a Pd-Cl distance of 2.185 angstrom. Speciation modeling of proton and palladium mass balance data of experiments for palladium concentrations ranging from 0.047 to 10.0 mmol kg(-1) required the presence of polynuclear complexes containing 3-9 palladium atoms. The existence of such complexes is moreover supported by previous investigations of palladium hydroxide chains of the type [Pd(OH)(1.72)Cl-0.28](n), that are coiled and/or aggregated into nanometer-sized (15-40 angstrom) spheroids. Published by Elsevier Ltd.