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 (SE CM), has been developed for the study of dissolution kinetics. The tec hnique should allow spatially-resolved dissolution kinetics, of a numb er 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 conc entration of one (or more) of the solution components of a target crys tal surface via electrolysis. This is achieved using potential step ch ronoamperometry, in which the potential of the UME-placed in close pro ximity to the crystal-is stepped from an initial value where no electr ode reaction occurs, and the solution is saturated, to a value where t he electrolysis of the solution component occurs at a diffusion-contro lled rate, and the solution in the gap between the UME and crystal sur face becomes depleted, thus initiating the dissolution reaction. The c onsequent current flow at the UME provides information on the rate and mechanism of the dissolution process. A theoretical model for the SEC M dissolution problem is developed numerically using the alternating d irection implicit finite-difference method. Results defining relations hips between current, time, distance, and kinetics are presented, and the range of kinetics open to study is identified. The applicability o f the technique is illustrated with model studies on the dissolution o f the (100) surface of copper sulfate pentahydrate, in aqueous sulfuri c acid solutions, which demonstrate that rapid interfacial dissolution kinetics of soluble materials can readily be probed.