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

The hydrolysis of palladium was investigated in 0.6 mol kg-1 NaCl at 298.2 K. Potentiometric titrations of solutions at various total concentrations of palladium(II) revealed that dilute (millimolar) conditions can be used to monitor the proton release due to hydrolysis reactions up to 2 protons per palladium(II) as long as the equilibration time is kept small. Spectrophotometric titrations were used to corroborate the homogeneous changes in speciation for the PdCl3OH2- species and to extract its correlative molar absorption coefficients in the 210-320 nm range. The molar absorption coefficients are similar to those of PdCl42- but exhibit a broader distribution of excitation energies resulting from the blue shift of the dominant charge transfer bands due to the presence of OH-. The longer-term potentiometric titrations systematically yielded, on the other hand, precipitates which matured over a period of 6 weeks and resulted in a more extensive release of protons to the solution. Precipitation experiments at six different total palladium(II) concentrations in the 3-11 pH range showed the dominant precipitating phase as Pd(OH)1.72Cl0.28. The coordination environment of Pd in this solid was investigated by extended X-ray absorption fine structure spectroscopy (EXAFS) and yielded an average 1.75 O and 0.25 Cl per Pd atoms with a Pd-O distance of 2.0 (angstrom) and Pd-Cl of 2.1 (angstrom). Finally, the precipitation experiments showed the final products to be of larger solubility than a literature Pd(OH)2 solubility study in which the KCl media induced a solid phase transformation to Pd(OH)1.72Cl0.28. Polynuclear complexes Pdq(OH)r2q-r with q=r=[3,9] explain the combined precipitation and hydrolysis data and may represent subsets of [Pd(OH)2]n and/or [Pd(OH)1.72Cl0.28]n chains coiled into nanometer-sized spheroids previously described in the literature