Mechanism and stability study of gold electrodeposition from thiosulfate-sulfite solution

Electrodeposition of gold from a basic "fresh" and "used" thiosulfate-sulfite solution containing 0.02 M Na₃Au(S₂O₃)₂ has been investigated on Au and Pt electrodes using various electroanalytical techniques (CV, LSV, RDE), controlled potential electrodeposition, as well as constant current and pulse current deposition. It was proposed that irreversible one electron reduction to gold- run in "fresh" solution-at the Pt electrode starts from Au(S₂O₃)(SO₃)₂^5-, a mixed gold thiosulfate -sulfite complex with a high stability constant (log β = 30.8). A diffusion coefficient of 1.77 × 10^-6 cm²/s for Au (S₂O₃) (SO₃)₂^5- was calculated from the slope of the linear plot i_p- ν^1/2 obtained with a Pt electrode, which is lower than the value of diffusion coefficient (D = 4.6 × 10^-6 cm²/s) obtained for the Au (S₂O₃)₂^3- complex (log β = 26.0) present in the "used" solution. The voltammograms for gold deposition on the Au electrode were significantly different from those on Pt indicating that the electron transfer was influenced by the coupled chemical reaction. The natures of possible chemical reactions are discussed.