Interfacial properties of precipitate-based ion-selective electrodes : Rotating disk impedance measurements of the Ag2S/Ag (aqueous) interface

Three-electrode rotating disk impedance measurements were made from 31.6 kHz to 0.0178 Hz on Ag₂S/Ag⁺ (aqueous) and Ag₂S/Ag systems. Membranes were prepared from materials precipitated in excess of silver or sulfide ions, and stoichiometric mixtures. Impedances were analyzed, as a function of rotation rate and bathing activities, to isolate bulk conductivities, internal diffusion, surface kinetic and dissolution/crystallization impedances. High-frequency bulk resistivities, R_∞, varied by four with precipitation and pressing conditions. Resistivities were the same for solution and ohmic configurations for each preparation. For ohmic contacts, R_∞ and R(DC) were identical. Solution contact cells in 10^-1 M and 10^-2 M bathing silver ion solution gave identical frequency-dependent impedances which were independent of rotation rate. Thus, solution diffusional impedances and solution dependent surface kinetics were eliminated, and a finite Warburg, interior-Ag⁺-defect, diffusion impedance was indicated. Summation of bulk membrane and contact resistances, and this Warburg impedance served as a 'background' correction in analyzing dilute bathing solution interfacial impedances for surface effects. Corrected impedances in 10^-3 –10^-5 M AgNO₃ showed solution diffusional behaviour combined with surface kinetic and dissolution impedances. An iterative linear least-squares method resolved these quantities. The surface resistance suggests a potential-dependent rate constant; dissolution time constants were solution-independent and smaller than those for solution diffusion. Thus, dissolution can be a rate-limiting step in establishment of steady-state potentials.