Impedance study of adsorption of iodide ions at Bi(0 0 1) electrode from the aqueous solutions with constant ionic strength

The adsorption of iodide ions from the aqueous solution with constant ionic strength on the Bi(0 0 1) single crystal plane electrode has been studied by impedance spectroscopy. The complex impedance plane and Bode plots have been fitted and analyzed, using the non-linear regression analysis method. The probable limiting kinetic steps of the process were obtained and discussed. At moderate ac frequency (from 1 Hz to 10 Hz) the adsorption process of the I− ions is limited mainly by a slow adsorption stage. In the region of intensive increase of the differential capacitance in the capacitance vs. electrode potential curves (E < −0.7 V vs. Ag∣AgCl∣sat. KCl) and in the low ac frequency region the adsorption process is conditioned by concentration of KI and is limited by the rate of adsorption, diffusion and partial charge transfer steps. It was found that the classical Frumkin–Melik-Gaikazyan equivalent circuit with the slow diffusion-like and adsorption steps can be applied for fitting the experimental complex impedance plane plots for iodide ions adsorption on Bi(0 0 1) from aqueous solutions with constant ionic strength because the adsorption of iodide ions on Bi(0 0 1) electrode is quick and reversible and the adsorption resistance is very small. The more complicated Ershler equivalent circuit is not applicable for characterization of the adsorption process at Bi(0 0 1) electrode from aqueous KF + KI solutions with constant ionic strength.