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 containing no supporting electrolyte have been investigated using the well-defined and high mass transport properties of the scanning electrochemical microscope (SECM). In this application a probe ultramicroelectrode (UME), positioned close to a silver chloride surface (pressed pellet or electrochemically grown film), is used to induce and monitor the dissolution process via the reduction of Ag⁺, from an initially saturated solution. Theory relating the current flow to rate laws in which dissolution 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 steady-state approach (current?distance) measurements. Moreover, it is possible to measure rate constants (particularly in the fast kinetic limit) with greater precision compared to the situation where an inert electrolyte is present, as considered earlier [J. Phys. Chem. 1995, 99, 14824]. Experiments covering a range of mass transport rates, through the use of probe UMEs with radii of 5, 12.5, and 25 μm, demonstrate, for the first time, that the dissolution of silver chloride, in the forms of interest, in aqueous solution occurs via a second-order rate law in interfacial undersaturation. The rate constant and corresponding undersaturations at the silver chloride/aqueous interface, during dissolution, have been identified.