THERMODYNAMIC STUDIES OF THE GOLD(Ⅲ)/(Ⅰ)/(0) REDOX SYSTEM IN AMMONIA-THIOSULPHATE SOLUTIONS AT 25℃

摘要

The reported values of the standard reduction potentials of the Au(NH_3)_2~+/Au(0) and Au(S_2O_3)_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 25℃ is used to predict the following relevant stability constants (β) for gold complexes based on the linear free energy correlations: Au(NH_3)_2~+ β_2 = 10~(13), Au(S_2O_3)_2~(3-) β_2 = 10~(24) and Au(NH_3)_4~(3+) β_4 = 10~(33). The equilibrium constant (≈ 10~(-11)) for the reaction Au(S_2O_3)_2~(3-) + 2NH_3 = Au(NH_3)_2~+ + 2S_2O_3~(2-) 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 Na_3Au(S_2O_3)_2. This also indicates that the stability constant of Au(NH_3)_2~+ is 9 to 11 orders of magnitude less than that of Au(S_2O_3)_2~(3-). Relevant species distribution and Eh-pH diagrams based on the revised data are presented.