Thermodynamic studies of the gold(III)/(I)/(0) redox system in ammonia-thiosulphate solutions at 25°C
Senanayake, G., Perera, W.N. and Nicol, M.J. (2003) Thermodynamic studies of the gold(III)/(I)/(0) redox system in ammonia-thiosulphate solutions at 25°C. In: Hydrometallurgy 2003: Proceedings of the 5th International Symposium, 24 - 27 August, Vancouver, Canada pp. 155-168.
The reported values of the standard reduction potentials of the Au(NH 3)2+/Au(0) and Au(S2O 3)23-/Au(0) couples are reviewed to emphasise the experimental difficulties associated with the measurement of reliable values for these quantities. The standard reduction potential for Ag(I)/Ag(0) in ammonia and thiosulphate solutions measured at ionic strength 0.1 and 25&C is used to predict the following relevant stability constants (β) for gold complexes based on the linear free energy correlations: Au(NH 3)2+ β2 = 1013, Au(S2O3)23- β2 = 1024 and Au(NH3)43+ β 4 = 1033. The equilibrium constant (≈ 10 -11) for the reaction Au(S2O3)23- + 2NH3 = Au(NH3)2+ + 2S2O32- based on these stability constants is in reasonable agreement with the measured value (≈10-9) based on potentiometric and spectroscopic data when excess ammonia was added to a solution of Na3Au(S2O3)2. This also indicates that the stability constant of Au(NH3)2+ is 9 to 11 orders of magnitude less than that of Au(S 2O3)23-. Relevant species distribution and Eh-pH diagrams based on the revised data are presented.
|Publication Type:||Conference Paper|
|Murdoch Affiliation:||Parker Cooperative Research Centre for Integrated Hydrometallurgy Solutions|
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