Solubility product constants of covellite and a poorly crystalline copper sulfide precipitate at 298 K

Details Solubility product constants of covellite and a poorly crystalline copper sulfide precipitate at 298 K Solubility product constants of covellite and a poorly crystalline copper sulfide precipitate at 298 K

Feb 1989

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989gecoa..53..229s&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 53, Issue 2, pp.229-236

Computer Science

2

Scientific paper

The equilibrium constant at 25°C for the following reaction has been measured in NaCl media by an indirect method: CuS(cov) + H + (aq) Cu 2+ (aq) + HS - (qa), K sp = M Cu 2+ M HS - (10 +pH ) where CuS(cov) designates synthetic covellite. Values of pK sp are 21.39, 21.04 and 20.95 at NaCl = 0.2, 0.7 and 1.0 M, respectively; the uncertainty in these K sp values is ±0.15. The free energy of formation of covellite, for which published values are discordant, is calculated to be -11.83±0.4 kcal/mole at 298 K (-49.50 ± 1.7 kJ/mole). This value is obtained by extrapolating the meaured pK sp values to infinite dilution with corrections for Cl - complexing. Applying similar Cl - complexing corrections, based on recent measurements by Seward, to previously published solubility data for galena yields a revised pK sp 0 for galena of 12.78. A poorly crystalline precipitate, obtained by mixing Cu 2+ and HS - solutions, yielded a reversible solubility product 3 orders of magnitude greater than that of covellite but about 3 orders of magnitude less than that of a truly amorphous phase, super-cooled liquid CuS. The poorly crystalline phase has not been studied previously. Its bulk composition was Cu 1.18 S, but microprobe analysis revealed that it was a partially exsolved mixture of roughly Cu 1.11 S and Cu 1.32 S (similar to known blaubleibender covellites). It was kinetically unstable, and converted to covellite when thermally annealed or when exposed to polysulfide solutions. Because of its instability, a material of this nature is unlikely to account for the amorphous copper sulfide alleged to occur in the Red Sea Brine deposits. However, it is possible that on short time scales dissolved Cu in sulfidic waters is controlled by metastable, rather than stable phases, as is known to be the case with dissolved Fe.

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