A NOVEL-APPROACH TO THE STUDY OF DISSOLUTION KINETICS USING THE SCANNING ELECTROCHEMICAL MICROSCOPE - THEORY AND APPLICATION TO COPPER-SULFATE PENTAHYDRATE DISSOLUTION IN AQUEOUS SULFURIC-ACID-SOLUTIONS

A new technique, employing the scanning electrochemical microscope (SECM), has been developed for the study of dissolution kinetics. The technique should allow spatially-resolved dissolution kinetics, of a number of materials with a wide range of solubilities, to be studied under conditions of well-defined local mass transport. The general concept is to employ the probe ultramicroelectrode (UME) of the SECM to induce and monitor the dissolution process of interest by depleting the concentration of one (or more) of the solution components of a target crystal surface via electrolysis. This is achieved using potential step chronoamperometry, in which the potential of the UME-placed in close proximity to the crystal-is stepped from an initial value where no electrode reaction occurs, and the solution is saturated, to a value where the electrolysis of the solution component occurs at a diffusion-controlled rate, and the solution in the gap between the UME and crystal surface becomes depleted, thus initiating the dissolution reaction. The consequent current flow at the UME provides information on the rate and mechanism of the dissolution process. A theoretical model for the SECM dissolution problem is developed numerically using the alternating direction implicit finite-difference method. Results defining relationships between current, time, distance, and kinetics are presented, and the range of kinetics open to study is identified. The applicability of the technique is illustrated with model studies on the dissolution of the (100) surface of copper sulfate pentahydrate, in aqueous sulfuric acid solutions, which demonstrate that rapid interfacial dissolution kinetics of soluble materials can readily be probed