Electrochecmical Impedance Spectroscopy analysis of an adsorption process with a coupled preceding chemical step

The impedance equations for the case of an adsorption process including a preceding homogeneous chemical step have been derived and compared to the corresponding equations for the case of a pure adsorption process. They involve behaviours with the frequency that differ from those of the impedance equations for a pure adsorption process. However, two limiting cases have been deduced in which the frequency behaviour of the impedance cannot be distinguished from the case of an adsorption process without any chemical step: the high frequency and the low frequency limits that hold for low and high values of the rate constant of the chemical reaction, respectively.The analysis as a function of the frequency is discussed on the basis of the complex frequency-normalized admittance plots and also based on the frequency dependence of the real and imaginary frequency-normalized admittance components. The influence of the values of the equilibrium and the rate constants of the chemical step is discussed for different adsorption kinetic regimes. In this way the conditions for detecting the chemical step are delimited and suitable analysis are proposed to obtain the adsorption parameters and the characteristic constants of the chemical step.The analysis is applied as an example, to the impedance of adenine adsorption on Au(111) electrodes from acidic solutions at a representative electrode potential. It is proved that the low frequency limit applies and the equilibrium constant for the chemical step is obtained.