SCANNING ELECTROCHEMICAL MICROSCOPE INDUCED DISSOLUTION - RATE LAW AND REACTION-RATE IMAGING FOR DISSOLUTION OF THE (010) FACE OF POTASSIUM FERROCYANIDE TRIHYDRATE IN NONSTOICHIOMETRIC AQUEOUS-SOLUTIONS OF THE LATTICE IONS

The scanning electrochemical microscope (SECM) has been used to induce and monitor dissolution from the (010) surface of (monoclinic) potassium ferrocyanide trihydrate crystals in initially saturated aqueous solutions containing 3.5 mol dm(-3) potassium chloride. The reaction has been investigated by SECM chronoamperometry in which the potential of a disk ultramicroelectrode tip, positioned close to the crystal surface, was stepped from a value at which no reactions occurred to a value at which the oxidation of ferrocyanide to ferricyanide occurred at a diffusion-controlled rate, thereby initiating the crystal dissolution process. The resulting chronoamperometric characteristics, obtained with a range of tip sizes (electrode radii of 2.5, 5.0, and 12.5 mu m) and tip to crystal separations, demonstrate that the dissolution reaction initially occurs at a rapid rate and quickly attains a steady-state. Quantitative analysis of this data in terms of appropriate candidate dissolution rate laws indicates that the dissolution process is second, rather than first, order in the interfacial undersaturation at the crystal/solution interface. At longer times in the chronoamperometric measurements, the current decreases, suggesting that the crystal becomes less active to dissolution. The theoretical model demonstrates that this occurs when the concentration of ferricyanide, accumulating in the tip/crystal gap, reaches a value at which the solution becomes supersaturated with respect to potassium ferricyanide. This provides the thermodynamic force for the nucleation and growth of potassium ferricyanide on the potassium ferrocyanide trihydrate surface, thus blocking the dissolution reaction. Taking account of this phenomenon, the SECM has been successfully employed to image the dissolution activity of the (010) surface, particularly around pits which were electrochemically pre-etched in the surface. Additionally, by adding a small concentration (2 x 10(-3) mol dm(-3)) of potassium ferricyanide to the saturated solution, as a calibrant of the tip to crystal distance, it has been possible to use the SECM to image both the topography and dissolution activity of the potassium ferrocyanide trihydrate (010) surface.