Current-potential oscillations on pyrite electrodes in alkaline sulfide solution

The anodic oxidation of pyrite in alkaline sulfide solution at room tempera ture leads to regular current-potential oscillations at potentials of about 0.5 V. The potentials oscillate with a frequency between nu = 8.4.10(-2) s (-1) and nu = 1.45 s(-1) in the range of 0.2 V to 0.7 V. The higher potenti al is accompanied by a decrease of the current from 30 mA to 15 mA. Stirrin g of the solution doesn't change the characteristics of the oscillations. T he oscillations stop by increasing the temperature at roughly 50 degrees C at the lower potential or by decreasing the temperature at roughly -5 degre es C at the higher potential. A potential of about 0.5 V on the pyrite electrode results in the formation of a surface layer of iron(III) sulfide. This layer is relatively passivat ing; the potential increases and the current decreases. Subsequently, the l ayer of iron(III) sulfide decomposes into iron(II) sulfide and disulfide io ns, and a new oscillation cycle begins. Iron(III) sulfide is stable at temp eratures below 0 degrees C and doesn't exist at temperatures above 50 degre es C. Therefore, no oscillations are observed at temperatures higher than 5 0 degrees C and lower than -5 degrees C.