Hydrosulfide/sulfide complexes of copper(I): Experimental confirmation of the stoichiometry and stability of Cu(HS)2− to elevated temperatures

The solubility of chalcocite has been measured over the temperature range 35-95°C at pH 6.5-7.5 in aqueous hydrosulfide solutions in order to determine the stability constants of the Cu(HS)₂⁻ complex. A heated flow-through system was used in which solutions are collected at temperature to avoid the problem of copper precipitation due to quenching. The quality of the data was sufficient to resolve a 0.1 log unit increase of the dissolution/complexation equilibrium constant with each 10°C increase in temperature. The equilibrium constants were fit using previously published methods to obtain the values of thermodynamic parameters for the Cu(HS)₂⁻ complexation reaction. To compare results with predictive techniques, one-term and two-term isocoulombic extrapolation methods were applied to the stability constants measured below 100°C. The two-term extrapolation to 350°C showed excellent agreement with the derived constants proving its applicability to soft metal-soft ligand interactions. The one-term method gave a reasonable agreement but deviated about one logarithmic unit at 350°C. This is attributed to differences in energetic, volumetric, and structural properties of the reactants and products. Speciation calculations show that at low temperatures (<150°C), the hydrosulfide complexes of copper will dominate over chloride complexes at low salinites (<0.1 mol kg⁻¹) while at higher temperatures, chloride complexes will be dominant under most geological conditions. Only in solutions with high reduced sulfur content and alkaline pH values will hydrosulfide complexes predominate and may play a role in the generation of economic copper mineralization.