COUPLED DIFFUSION ADSORPTION MODEL FOR RESPONSE OF PRECIPITATE-BASED IODIDE-SELECTIVE ELECTRODES TO PRIMARY-ION ACTIVITY STEPS

Switched wall-jet streams of iodide solutions impinging on AgI-based ion-selective electrodes (ISEs) give potential responses after the first 20 ms. The responses to iodide activity steps of constant ratio a (final)/a(initial) are highly dependent on the starting activity. This is contrary to most present theories of ISE response, particularly those descrbing reversible surface ion exchange controlled by diffusion to a surface or first-order reaction kinetics. Experimentally, diffusion is involved over all concentration ranges but is coupled with some surface adsorption isotherm. At low activities, steps up or down are slower than diffusion because of the need to form or remove some of the adsorbed layer. At high activities, step responses approach pure diffusion control, since adsorption is nearly saturated. Dependence on the starting activity requires a nonlinear adsorption isotherm, which is consistent with adsorption data showing logarithmic dependence of surface ion concentration on bathing activity. Experimentally, rate-controlling dissolution/precipitation processes are not likely.