Irreversible follow-up reaction with linear adsorption of the product studied by the galvanostatic current reversal method

Abstract

The case of an irreversible follow-up reaction of the first order with adsorption obeying a linear isotherm was solved for the galvanostatic method with current reversal. Adsorption equilibrium and a different rate of the chemical reaction in the adsorbed phase from that in the solution are assumed. The transition time, z', after the reversion of the current is in this case independent of the current,/, and time, before the reversion provided that the value of tl is large enough. The course of the dependence of the transition time, r, on the ratio of the current intensities, u = ///', depends on two parameters, namely the ratio of the reaction rates in the adsorbed state and in the solution, I = kad/k, and the ratio of the quantities of the substance in the solution and in the adsorbed state in the instant of the current reversal, denoted as a. The theoretical dependence of the product AT' are for practical values of u shown graphically for various values of a and /. The possibility of evaluating all the parameters a, k, I, and distinguishing the given case from other cases of adsorption is discussed. The reduction of p-diacetylben zene in wateralcohol mixtures is used as an illustrative example. In the earlier work 1 devoted to the galvanostatic reduction of p-diacetylben zene it was assumed that the measured rate constants of the follow-up reaction of the electrode product are influenced by its adsorption on the electrode. An analysis of the results showed that the adsorption was weak and therefore the results of the work 2 assuming complete adsorption of the electrode reaction product are not applicable. Instead, a model based on a mobile adsorption equilibrium characterized by a linear (Henry's) isotherm is plausible. The possibility of determining such an adsorbed product by the galvanostatic method with current reversal was already investigated3. The present work deals with the case where a chemical conversion of the electrode reaction product to an inactive substance proceeds both in the solution and in the adsorbed state parallel to its adsorption described by a linear isotherm. This inactivation is of the first order and irreversible.