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⁻ ⇋ AgX2⁻ (3) AgX(s) + 2X⁻ ⇋ AgX3²⁻ (4) the following equilibrium constants were obtained: log K2,Br = −4.01 ± 0.20 (200°C) and −3.12 ± 0.20 (300°C); log K3,Br = −1.81 ± 0.10 (200°C) and −1.01 ± 0.10 (300°C); log K3,I = −2.46 ± 0.20 (150°C), −1.92 ± 0.20 (200°C) and −1.47 ± 0.10 (250°C); and log K4,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 K1 and log K2 at any given temperature decrease in the order X = Cl > Br > I, whereas the opposite trend is shown for log K4 at 18°C. Values of log K3 are similar for X = Cl, Br, I at all temperature, with ΔrH° = +40.7 to + 45.9 kJ mol⁻¹. The enthalpies of reactions 2Br and 2Cl are also similar (ΔrH° = +56.0 and +54.1 kJ mol⁻¹, respectively).