THE STABILITY OF AQUEOUS SILVER BROMIDE AND IODIDE COMPLEXES AT 25-300-DEGREES-C - EXPERIMENTS, THEORY AND GEOLOGIC APPLICATIONS

The solubilities of AgBr(s) in NaBr solutions and AgI(s) in NaI soluti ons 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) ASX((s)) + X- reversible arrow AgX2- (3) AgX(s) + 2X( -) reversible arrow AgX32- (4) the following equilibrium constants wer e obtained: log K-2,K-Br = -4.01 +/- 0.20 (200 degrees C) and - 3.12 /- 0.20 (300 degrees C); log K-3,K-Br = - 1.81 +/- 0.10 (200 degrees C ) and - 1.01 +/- 0.10 (300 degrees C); log K-3,K-1 = - 2.46 +/- 0.20 ( 150 degrees C), - 1.92 +/- 0.20 (200 degrees C) and -1.47 +/- 0.10 (25 0 degrees C); and log K-4,K-1 = - 1.9 +/- 0.4 (150 degrees C) and -2.2 +/- 0.4 (200 degrees C). These results were combined with previously published data and our own extrapolations to obtain smoothed equilibri um constants at 25-300 degrees C for reactions (1)-(4) at 25-300 degre es 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 degrees C. Values of log K-3 are similar for X = Cl, Br, I at all temperatures, with Delta(r)H degrees = + 40.7 to + 45.9 kJ m ol(-1). The enthalpies of reactions 2(Br) and 2(Cl) are also similar ( Delta(r) H degrees = + 56.0 and + 54.1 kJ mol(-1), respectively).Conve rsion of the above data to cumulative formation constants (beta(i), i = 1, 2, 3) indicates that the bromide and iodide complexes of silver a re much stronger than their chloride counterparts. However, chloride c omplexes will dominate silver transport in most cases, due to the low Br/Cl and I/Cl ligand ratios of natural waters. Exceptions include cer tain oil field brines with I/Cl > 10(-3), in which case iodide or mixe d chloride-iodide complexes become the dominant silver species. Our ca lculations indicate that AgI(s) is many orders of magnitude less solub le than AgCl(s) in connate brines, and may be an important solubilit)i -limiting phase in sedimentary basins. AgI(s) may also form in the wea thering environment, especially in the supergene zones of silver-rich hydrothermal mineral deposits, although AgC(s) is more stable at Eh co nditions above the aqueous I-/IO3- boundary. Except in very unusual ci rcumstances, silver halide minerals will be too soluble to precipitate directly from high-temperature hydrothermal fluids.