Experimental determination of the hydrolysis constants of Pt 2+ and Pd 2+ at 25°C from the solubility of Pt and Pd in aqueous hydroxide solutions

Abstract

The solubilities of Pt and Pd metal were measured at 25°C in 10 −4 to 10.0 molal NaOH solutions under a reduced oxygen atmosphere in order to determine the stoichiometry and stability constants for Pt and Pd hydroxide complexes. Equilibration times of over one year were employed. The Pd data are consistent with the existence of Pd(OH) 2 0(aq) from pH 9 to 12 and Pd(OH) 3 − from pH 12 to 15.5. No conclusive evidence for a Pd(OH) 4 2− complex was obtained, but the data do not preclude its existence at high pH. For Pt, the data are consistent with a single complex for pH = 9 to 15.5, I.E., Pt(OH) 2 0(aq). A graphical treatment of the data yields the following cumulative stability constants: log β 2 = 18.9 ± 1.0 and log β 3 = 20.9 ± 1.0 for Pd and log β 2 = 29.9 ± 1.0 for Pt. The stepwise stability constant for Pd(OH) 3 − log K 3 = 2.0 is in relatively good agreement with that derived from data in the literature ( log K 3 = 1.8 ). However, the cumulative stability constants for Pd measured in this work are considerably smaller than those reported in the literature. The log β 2 = 29.9 ± 1.0 value measured for Pt compares relatively well with a theoretically estimated value of 28.3. The data suggest that the predominant inorganic form of Pt and Pd in freshwaters may be the neutral hydroxide species. In seawater, the hydroxide complex of Pt is also predicted to predominate over the chloride complex, but, in the case of Pd, the hydroxide complex appears to be less stable and it is presently not clear whether the chloride or the hydroxide complex will predominate. In fluids responsible for serpentinization, Pt and Pd may also be mobilized as hydroxide complexes.