Ion-Specific Modulation of Ferri/Ferrocyanide-Driven Gold Dissolution Uncovered by Quantitative Quartz Crystal Microbalance Analysis.

Gold electrodes in ferri/ferro cyanide (FFCN) are widely treated as chemically stable, outer sphere benchmark systems, despite the growing reports of drift in electrochemical measurements. Here, we use quartz crystal microbalance to quantify FFCN-induced gold mass loss and isolate the roles of anion, cation, pH, buffer chemistry, and self-assembled monolayer. In the absence of FFCN, all solutions show positive mass shifts, indicating that the supporting electrolytes alone do not produce measurable gold loss over the experimental time scale. With FFCN present, gold etching becomes highly tunable: changing the supporting electrolyte, adjusting the pH, and changing the buffers significantly shifts the magnitude of gold dissolution. Additional oxygen- and solution-handling experiments show that oxygen depletion strongly reduces gold loss, while solution aging and ambient light exposure increases FFCN-drive gold dissolution. Neutral, long, well packed self-assembled monolayers (SAMs) attenuate etching, while charged or short-chain SAMs significantly accelerate gold loss. Overall, we map a systematic landscape where FFCN on gold behaves as a chemically active etchant, not an inert probe. Thus, electrochemical measurements must explicitly account for FFCN etching, which is influenced by salt, pH, buffer, and surface chemistry, and solution handling conditions.