Reactivity of Ag+ ion with thiol ligands in the presence of iron sulfide

Dissolved silver (Ag) in the presence of amorphous iron sulfide(FeS) rapidly equilibrates to ultra-trace levels ( ∼5 ng/L), which agrees with equilibrium calculations based on Ag2S solubility. This finding, along with the authors' discovery that crystalline Ag2S (acanthite) precipitates rapidly under supersaturated conditions, suggests that a discrete Ag-S phase is formed. Furthermore, potential interferences (Cl−, Cu2+, and dissolved organic carbon) show no effect on the Ag-FeS system. Only the presence of organosulfur (thiol) ligands at 10−3 to 10−4 M result in increased dissolved silver. Agreement between calculations and measured values were good for 1.8 × 10−4 M 3-mercaptopropanoic acid; however, calculations predicted higher dissolved silver than measured values for higher thiol levels, possibly due to Ag-thiol binding to FeS.