By using buffer solutions with a constant concentration of phosphate species, electrochemical dissolution of gold in pH 1, 3, 5, 7, 9, 11 and 12.7 was investigated. The concentration of phosphate species was kept constant in order to minimize the possible effect of anion concentration on gold dissolution. Oxidation/reduction of the metal surface leads to transient dissolution peaks, which are labelled as A1 and C1 on the dissolution profiles on Figure 1. An additional dissolution peak A2 appears when the oxygen evolution reaction (OER) takes place. Quantitatively, there are significant differences. The amount of gold that is transiently dissolved in an alkaline medium is more than an order of magnitude higher than that in an acidic medium at lower potentials. Oppositely, steady-state gold dissolution in alkaline pH in the potential region of OER is hindered. There is an apparent shift in the mechanisms at pH 5. At higher potentials, i.e 1.9 V RHE, there is an apparent loss of peak A2 and a growth of C1. The loss of dissolution of gold due to a change in the mechanism of OER in more alkaline electrolytes is clear at pH 7. To explain the observed difference in gold dissolution, a difference in the thickness of compact oxide formed in acid and base is suggested. The formation of thicker oxide as a result of an increase in pH becomes noticeable with the observed increase in both peaks A1 and C1 that can be seen when the pH is increased from 7 to 12.7. These new data on Au dissolution in a broad potential and pH range should be used to minimize Au dissolution in various applications.
