Gold electrochemistry in the acidic choline chloride-oxalic acid deep eutectic solvent

The electrochemistry of the Au(III)-Au(I)-Au(0) system was studied in the choline chloride- oxalic acid dihydrate deep eutectic solvent at 60 degrees C. At glassy carbon electrodes, the 2 e (-) waves of the Au(III)/Au(I) and the 1 e (-) waves of the Au(I)/Au(0) are well separated in the cyclic voltammograms, whereas at platinum electrodes two anodic peaks but only one single 3 e (-) cathodic peak are observed. Electrochemical quartz crystal microbalance and zero-current chronopotentiometric experiments demonstrate the occurrence of a comproportionation reaction AuCl4- + 2Au(0) + 2Cl(-) reversible arrow 3AuCl (2) (-) when gold is electrodeposited from an Au(III) solutions. The standard potentials of the Au(III)/Au(I), Au(I)/Au(0) and Au(III)/Au(0) couples were estimated, and even though E A(u ( III ) / Au ( I )) (0)< E (0)(Au ( I ) / Au ( 0 )) translates into an equilibrium constant of the reaction being unfavourable to the comproportionation ( K (c,comp) < 1), it is shown that the reaction can be quantitatively displaced to the right under nonequilibrium conditions imposed by mass-transport.