Abstract The solubility of vysotskite PdS (cr) was measured in aqueous sulfide solutions from 4. 5 to 490°C at 1–1000 bar as a function of pH. The total reduced sulfur concentrations varied between 0. 03 and 106 m mol (kg H 2 O) –1. The solubility data were described in terms of the aqueous complexes Pd (HS) 2 (aq), Pd (HS) ₃^ -, and PdS (HS) ₂^{2 - }. The 1st complex dominates in acidic to near-neutral solutions, the 2nd complex in neutral to alkaline pH, and the 3rd one under alkaline pH conditions. The PdS (cr) solubility constants show a minimum at 200°C (Pd (HS) 2 (aq), PdS (HS) ₂^{2 - }) and 350°C (Pd (HS) ₃^ -). The experimental values pooled with reliable literature data, were fitted to the HKF (Helgeson–Kirkham–Flowers) model equation of state (EoS). Literature data on the solubility of cooperite PtS (cr) were regressed to calculate of the HKF EoS parameters of Pt (HS) 2 (aq) and Pt (HS) ₃^ -, the 3rd complex was not detected for Pt. The PtS (cr) solubility constant to form the main Pt hydrosulfide complex, Pt (HS) 2 (aq), shows a pronounced minimum at 100°C and then increases sharply, whereas the Pt (HS) ₃^ - formation constant has a maximum at 250°C. At ambient temperature, the solubility of PdS (cr) exceeds that of PtS (cr) by 2–3 log units, depending on pH. The temperature of ca. 250°C is a borderline one where the solubilities of Pd and Pt sulfides are equal. At temperature above 300°C, due to the sharp increase in the Pt (HS) 2 (aq) formation constant, the dissolved Pt concentration exceeds that of Pd in the whole pH region from acidic to alkaline. At 700°C and 2000 bar, the solubility of PtS (cr) reaches a few ppm, whereas the solubility of PdS (cr) comprises about 40 ppb in a solution containing 0. 1 m total reduced sulfur. These concentrations are high enough to sustain hydrothermal mobility of these metals in high-temperature magmatic-hydrothermal fluids, whereas the differences in the stability of Pt (HS) 2 (aq) and Pd (HS) 2 (aq) can result in separation of these metals in hydrothermal systems.
Rubtsova et al. (Mon,) studied this question.