Comparative Geochemistries of PdsupII/sup and PtsupII/sup:: Formation of Mixed Hydroxychloro and Chlorocarbonato-complexes in Seawater

Comparative observations of Pd ^II and Pt ^II hydrolysis in chloride solutions indicate that [PdCl ₃ OH ²⁻ ]/[PdCl42−] and [PtCl ₃ OH ²⁻ ]/[PtCl42−] concentration ratios in salinity 35 seawater (S = 35) are smaller than one at a typical surface ocean pH (∼8.2), and are larger than one at pH = 8.2 when S < 10. The hydrolysis behaviors of PdCl42− and PtCl42− are very similar. In 0.5 M NaCl at 25°C the hydrolysis constant for both elements, written in the form β ₁ = [MCl ₃ OH ²⁻ ][Cl ⁻ ][H ⁺ ][MCl42−] ⁻¹ , is logβ1*=−8.97. Between ionic strengths 0.3 M and 1.0 M for Pd ^II and between 0.1 M and 1.0 M for Pt ^II , log β ₁ is within approximately 0.1 units of the value appropriate to 0.5 M NaCl. This small dependence of PdCl42− and PtCl42− hydrolysis constants on ionic strength is consistent with predictions based on expected activity coefficient behavior.

Carbonate is observed to complex PdCl42− significantly, but to a smaller extent than OH ⁻ under conditions appropriate to seawater. Complexation of PtCl42− by CO32− was observed in this work but the rate of complexation was too slow to allow equilibrium observations. The principal dissimilarity between the chemistries of Pd ^II and Pt ^II in our investigation was the sharp contrast in observed Pd ^II and Pt ^II reaction rates. Differences in reaction kinetics may cause fractionation of Pd ^II and Pt ^II in the environment. The speciation of Pt ^II , unlike Pd ^II , is likely to be based on chemical environments experienced by Pt ^II over a period of days, and perhaps weeks.