Comparison of the gold reduction and stripping processes at platinum, rhodium, iridium, gold and glassy carbon micro- and macrodisk electrodes

The gold Au-III + 3e(-) --> Au-0 reduction and Au-0 --> Au-III + 3e(-) oxidation stripping processes in dilute aqua regia electrolyte (0.1 M HCl + 0.32 M HNO3) were examined at platinum, rhodium, iridium, gold and glassy carbon disk electrodes, After ascertaining that the preferred material was platinum, the effect of electrode size was evaluated by using nine different platinum disk electrodes having diameters ranging from 2 to 2000 mu m. The optimum analytical response was obtained with a 50 mu m diameter platinum disk electrode, With this electrode diameter, a sharp symmetrical gold stripping peak was obtained and the deposition process occurred predominantly under conditions of radial diffusion so that stirring of the solution was not required. In contrast, larger sized platinum electrodes produced a broader, asymmetric stripping response for the gold oxidation peak, whereas electrodes of smaller diameter provided poorer signal-to-noise ratios. The limit of detection and limit of quantification were calculated to be 4.4 x 10(-7) M (86 ppb) and 13.1 x 10(-7) M (258 ppb), respectively, at the 50 mu m diameter platinum disk electrode under conditions of linear sweep stripping voltammetry at a scan rate of 200 mV s(-1) and a 140 s deposition time. The optimum electrode gave a very well defined gold oxidation signal with negligible background current when applied to the determination of gold in a gold ore sample.