OSCILLATORY DISSOLUTION OF AN IONIC SINGLE-CRYSTAL SURFACE OBSERVED WITH THE SCANNING ELECTROCHEMICAL MICROSCOPE

The scanning electrochemical microscope has been used to induce and monitor dissolution from the (100) face of copper sulfate pentahydrate single crystals in regions of the surface where the average dislocation spacing is much greater than the size of the ultramicroelectrode probe. The chronoamperometric characteristics for the reduction of Cu2+ at the probe electrode, positioned close to the crystal surface, indicate that the crystal is initially highly reactive toward dissolution. However, after a short period of time (ca. 0.1-0.3 s) the dissolution rate rapidly decreases until the solution adjacent to the crystal surface becomes sufficiently undersaturated to induce the dissolution process again. This is found to occur at a critical saturation ratio, c/c*, of 2 x 10(-7), where c is the concentration of Cu2+ and c* is the concentration in saturated solution. Thereafter, dissolution proceeds via a unique oscillatory process in which dissolution occurs in ''bursts'', each burst corresponding to the release of ca. 6 x 10(-9) mol cm(-2) of Cu2+ from the crystal. A simple theoretical model in which, in the absence of dislocations and steps, dissolution sites on the crystal surface are formed only when the interfacial saturation ratio is below the critical value is shown to account for the observed behavior and to successfully predict the period of the oscillations.