COMPARISON OF THE GOLD REDUCTION AND STRIPPING PROCESSES AT PLATINUM,RHODIUM, IRIDIUM, GOLD AND GLASSY-CARBON MICRO-DISK 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 preferr ed material was platinum, the effect of electrode size was evaluated b y using nine different platinum disk electrodes having diameters rangi ng from 2 to 2000 mu m. The optimum analytical response was obtained w ith a 50 mu m diameter platinum disk electrode, With this electrode di ameter, a sharp symmetrical gold stripping peak was obtained and the d eposition process occurred predominantly under conditions of radial di ffusion so that stirring of the solution was not required. In contrast , larger sized platinum electrodes produced a broader, asymmetric stri pping response for the gold oxidation peak, whereas electrodes of smal ler diameter provided poorer signal-to-noise ratios. The limit of dete ction 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 stri pping voltammetry at a scan rate of 200 mV s(-1) and a 140 s depositio n time. The optimum electrode gave a very well defined gold oxidation signal with negligible background current when applied to the determin ation of gold in a gold ore sample.