Ch. Gammons et al., THE DISPROPORTIONATION OF GOLD(I) CHLORIDE COMPLEXES AT 25 TO 200-DEGREES-C, Geochimica et cosmochimica acta, 61(10), 1997, pp. 1971-1983
The disproportionation of aqueous Au(I) chloride complexes at elevated
temperature has been investigated experimentally using the solubility
method. At 300 degrees C, the dominant gold species in aqueous HCl so
lutions is AuCl2-. Upon cooling, this aurous complex partially decompo
ses according to the following reaction: 3AuCl(2)(-) = 2Au(s) + AuCl4-
+ 2Cl(-) Log K-Al values were obtained at 100 degrees C (4.42 +/- 0.2
2), 150 degrees C (2.86 +/- 0.12), and 200 degrees C (1.45 +/- 0.19).
The results are in excellent agreement with the earlier potentiometric
study of Nikolaeva et al. (1972) at 25-80 degrees C. The combined dat
a were used to obtain the following polynomial: log K-Al = -13.55 + 85
93/T - 700610/T-2 (T = Kelvin, valid from 25 to 200 degrees C). The ra
te of reaction (A1) at 25 degrees C was investigated by monitoring the
production of AuCl4- after quench using a UV spectrophotometer. The r
ates 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 AuCl2-.kg H2O-1.minute(-1). In general, faster rates
were obtained for samples with high initial AuCl2- concentrations. Add
ition of gold foils caused an abrupt increase in rate, indicating that
the reaction is catalyzed by the native metal. Gold crystals formed d
uring the disproportionation reaction at 25 degrees C show a variety o
f morphologies, including examples with anomalous fivefold symmetry. O
ur results indicate that the stability of AuCl2- relative to AuCl4- in
creases quickly with temperature. At 25 degrees C, AuCl4- is unlikely
to be of geochemical importance, with the possible exception of oxidiz
ed, acidic solutions that are also rich in chloride ion. In contrast,
AuCl2- may be the dominant form of dissolved gold in brines with near-
neutral pH (e.g., seawater), as well as hydrothermal fluids that are b
oth saline and oxidized. Cooling or dilution of solutions saturated wi
th AuCl2- could result in deposition of Au via a disproportionation re
action, as in our experiments. Copyright (C) 1997 Elsevier Science Ltd
.