The stability of aqueous silver bromide and iodide complexes at 25–300°C: Experiments, theory and geologic applications

Abstract

The solubilities of AgBr (s) in NaBr solutions and AgI (s) in NaI solutions were measured at elevated temperature. Referring to the following general reactions (where X = Cl, Br, or I): AgX (s) ⇋ Ag + + X − (1) AgX (s) ⇋ AgX (aq) (2) AgX (s) + X − ⇋ AgX 2 − (3) AgX (s) + 2X − ⇋ AgX 3 2− (4) the following equilibrium constants were obtained: log K 2,Br = −4.01 ± 0.20 (200°C) and −3.12 ± 0.20 (300°C); log K 3,Br = −1.81 ± 0.10 (200°C) and −1.01 ± 0.10 (300°C); log K 3,I = −2.46 ± 0.20 (150°C), −1.92 ± 0.20 (200°C) and −1.47 ± 0.10 (250°C); and log K 4,I = −1.9 ± 0.4 (150°C) and −2.2 ± 0.4 (200°C). These results were combined with previously published data and our own extrapolations to obtain smoothed equilibrium constants at 25–300°C for reactions (1)–(4) at 25–300°C. Values of log K 1 and log K 2 at any given temperature decrease in the order X = Cl > Br > I, whereas the opposite trend is shown for log K 4 at 18°C. Values of log K 3 are similar for X = Cl, Br, I at all temperature, with Δ r H° = +40.7 to + 45.9 kJ mol −1. The enthalpies of reactions 2 Br and 2 Cl are also similar ( Δ r H° = +56.0 and +54.1 kJ mol −1, respectively). Conversion of the above data to cumulative formation constants ( β i , i = 1, 2, 3) indicates that the bromide and iodide complexes of silver are much stronger than their chloride counterparts. However, chloride complexes will dominate silver transport in most cases, due to the low Br Cl and I Cl ligand ratios of natural waters. Exceptions include certain oil field brines with I Cl > 10 −3 , in which case iodide or mixed chloride-iodide complexes become the dominant silver species. Our calculations indicate that AgI (s) is many orders of magnitude less soluble than AgCl (s) in connate brines, and may be an important solubility-limiting phase in sedimentary basins. AgI (s) may also form in the weathering environment, especially in the supergene zones of silver-rich hydrothermal mineral deposits, although AgCl (s) is more stable at Eh conditions above the aqueous I − IO 3 − boundary. Except in very unusual circumstances, silver halide minerals with be too soluble to precipitate directly from high-temperature hydrothermal fluids.