Thermodynamic studies of the gold(III)/(I)/(0) redox system in ammonia-thiosulphate solutions at 25°C

The reported values of the standard reduction potentials of the Au(NH3)(2)(+)/Au(0) and Au(S2O3)(2)(3-)/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 25degreesC is used to predict the following relevant stability constants (beta) for gold complexes based on the linear free energy correlations: Au(NH3)(2)(+) beta(2) = 10(13), Au(S2O3)(2)(3-) beta(2) = 10(24) and Au(NH3)(4)(3+) beta(4) = 10(33). The equilibrium constant (approximate to 10(-11)) for the reaction Au(S2O3)(2)(3-) + 2NH(3) = Au(NH3)(2)(+) + 2S(2)O(3)(2-) based on these stability constants is in reasonable agreement with the measured value (approximate to 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(S2O3)(2)(3-). Relevant species distribution and Eh-pH diagrams based on the revised data are presented.