The solubility of amorphous As2S3 from 25 to 90°C

Abstract The solubility of amorphous arsenic sulfide [As 2 S 3 (am)] has been experimentally determined from 25 to 90°C. In acidic, sulfide-deficient solutions, H 3 AsO 0 3 is the dominant product ofAs 2 S 3 (am) dissolution: 1 2 As 2 S 3 ( am ) + 3H 2 O = H 3 AsO 0 3 + 3 2 H 2 S 0 . The respective logarithms of equilibrium constants for this reaction are −11.9 ± 0.3 at 25°C, −11.2 ± 0.2 at 40°C, −10.2 ± 0.1 at 60°C, and −9.0 ± 0.2 at 90°C, all at 0.1 m ionic strength. In acidic solutions with excess sulfide, experimental data are consistent with H 2 As 3 S − 6 being the dominant dissolved As species: 3 2 As 2 S 3 ( am ) + 3 2 H 2 S 0 = H 2 As 3 S − 6 + H + The respective logarithms of equilibrium constants for this reaction are −5.0 ± 0.3 at 25°C, −5.1 ± 0.3 at 40°C, −4.9 ± 0.2 at 60°C, and −4.1 ± 0.1 at 90°C, all at 0.1 m ionic strength. Speciation calculations for a number of acidic hot springs indicate that many have As and H 2 S 0 concentrations that are bracketed by the solubilities of As 2 S 3 (am) and orpiment and that H 3 AsO 0 3 is generally the dominant dissolved As species. The solubility data indicate that lowering of temperature and pH are the most likely mechanisms of As 2 S 3 (am) precipitation in hot spring systems.