The disproportionation of gold(I) chloride complexes at 25 to 200°C

Abstract

The disproportionation of aqueous Au(I) chloride complexes at elevated temperature has been investigated experimentally using the solubility method. At 300°C, the dominant gold species in aqueous HCl solutions is AuCl 2 −. Upon cooling, this aurous complex partially decomposes according to the following reaction: 3 AuC 1 2 − = 2 Au ( s ) + AuC 1 4 − + 2 C 1 − (A1) Log K A1 values were obtained at 100°C (4.42 ± 0.22), 150°C (2.86 ± 0.12), and 200°C (1.45 ± 0.19). The results are in excellent agreement with the earlier potentiometric study of Nikolaeva et al. (1972) at 25–80°C. The combined data were used to obtain the following polynomial: log K A1 = −13.55 + 8593/ T−700610/ T 2 ( T =Kelvin, valid from 25 to 200°C). The rate of reaction (A1) at 25°C was investigated by monitoring the production of AuCl 4 − after quench using a UV spectrophotometer. The rates were very slow for the first 5–10 min, but then rapidly increased to values that remained approximately constant with further reaction progress. The measured reaction rates fell in the range 2.1 · 10 −8 to 3.7 · 10 −6 moles AuCl 4 − · kg H 2O −1 · minute −1. In general, faster rates were obtained for samples with high initial AuCl 2 − concentrations. Addition of gold foils caused an abrupt increase in rate, indicating that the reaction is catalyzed by the native metal. Gold crystals formed during the disproportionation reaction at 25°C show a variety of morphologies, including examples with anomalous fivefold symmetry. Our results indicate that the stability of AuCl 2 − relative to AuCl 4 − increases quickly with temperature. At 25°C, AuCl 4 − is unlikely to be of geochemical importance, with the possible exception of oxidized, acidic solutions that are also rich in chloride ion. In contrast, AuCl 2 − may be the dominant form of dissolved gold in brines with near-neutral pH (e.g., seawater), as well as hydrothermal fluids that are both saline and oxidized. Cooling or dilution of solutions saturated with AuCl 2 − could result in deposition of Au via a disproportionation reaction, as in our experiments.