Characterization of copper and ammonia interactions with the passive layer that impedes gold dissolution with thiosulfate based electrolytes at extended time frames was performed with electrochemistry and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). Generalized two-dimensional correlation spectroscopy (2DCOS) was also employed to assist interpretation of the collected SHINERS spectra. Growth of the complex passive layer mixture on a polycrystalline gold electrode immersed in a 0.10 M Na2S2O3 electrolyte was found to initially proceed through formation of sulfides adsorbed to the gold electrode surface (SERS band at ca. 326 and 309 cm−1) followed by rapid transformation to cyclo-S8 (474, 218, and 152 cm−1) and polysulfides/polythionates (459 cm−1). After stable formation of the passive film, the addition of Cu2+ resulted in the partial removal of the passive layer. The removal of the components of the passive layer proceeded according to the following sequence; polysulfides were fastest, cyclo-S8 next, followed by . The end product was observed to be a combination of copper and gold sulfides. In an independent study, the addition of ammonia displayed a similar ability to remove the passive layer constituents, albeit at much slower rates. From these observations it can be concluded that copper and ammonia play a vital function in preventing passive layer formation, allowing increased mass transport of thiosulfate and the oxidant across the electrode-electrolyte interface, thereby allowing a higher gold extraction efficiency.
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