SCANNING ELECTROCHEMICAL MICROSCOPE-INDUCED DISSOLUTION - THEORY AND EXPERIMENT FOR SILVER-CHLORIDE DISSOLUTION KINETICS IN AQUEOUS-SOLUTION WITHOUT SUPPORTING ELECTROLYTE

The kinetics of silver chloride dissolution in aqueous solutions conta ining no supporting electrolyte have been investigated using the well- defined and high mass transport properties of the scanning electrochem ical microscope (SECM). In this application a probe ultramicroelectrod e (UME), positioned close to a silver chloride surface (pressed pellet or electrochemically grown film), is used to induce and monitor the d issolution process via the reduction of Ag+, from an initially saturat ed solution. Theory relating the current flow to rate laws in which di ssolution is governed by either a first- or second-order dependence on the interfacial undersaturation has been developed numerically, using the alternating direction implicit finite difference method to solve the mass transport equation appropriate to the system of interest. It is shown that the two rate laws may readily be distinguished from stea dy-state approach (current-distance) measurements. Moreover, it is pos sible to measure rate constants (particularly in the fast kinetic Limi t) with greater precision compared to the situation where an inert ele ctrolyte is present, as considered earlier [J. Phys. Chem. 1995, 99, 1 4824]. Experiments covering a range of mass transport rates, through t he use of probe UMEs with radii of 5, 12.5, and 25 mu m, demonstrate, for the first time, that the dissolution of silver chloride, in the fo rms of interest, in aqueous solution occurs via a second-order rate la w in interfacial undersaturation. The rate constant and corresponding undersaturations at the silver chloride/aqueous interface, during diss olution, have been identified.