The stability of hydrosulphido- and sulphido-complexes of Au(I) and Ag(I) at 25°C

Abstract

The solubility of Au 2S at 25°C in aqueous H 2SHS − solutions were measured from pH 2 to 12 and over a range of reduced sulphur concentrations from 0.005 to 0.4 molal. The species AuHS 0, Au(HS) 2 − and Au 2S 2 2− were identified and thermodynamic solubility constants were derived for the reaction Au 2S + 2(n+z) x −1 HS − = 2 x AU x( HS) n S (x−n−2z) z+ 2z x − H + using a nonlinear least squares procedure: log K Au,110=−6.68 (0.03) for AuHS 0; log K Au,120= +4.52 (0.01) for Au(HS) 2 −; and log K Au,202=−14.63 (0.03) for Au 2S 2 2−. The quoted uncertainty corresponds to one standard deviation. The first dissociation constant for H 2S obtained from the Au 2S solubility measurements was log K a1 = −6.98 (0.01). From published high temperature and pressure values of K r120 for the reaction Au (c)+H 2S+HS−=Au(HS) − 2+H 2( g) the standard partial molar volume of Au(HS) 2 − was obtained ( T°C, V 0 cm mol −1): 175°, −36; 200°, −25; 225°, −10; and 250°, −15. Published solubility data for Ag 2S is solutions at I = 1.0 and 20°C have been reinterpreted to give the following thermodynamic solubility constants at infinite dilution: log K Ag, 110 = −8.94 (0.01) for AgHS 0, log K Ag, 120 = −1.02 (0.01) for Ag(HS) 2 −; and log K Ag, 221 = −4.44 (0.01) for Ag 2(HS) 2S 2−.