Spectroscopy takes electrochemistry beyond the interface: A compact analytical solution for the reversible first-order catalytic mechanism

Abstract The monitoring of the non-interfacial phenomena of mass transport and homogeneous chemical reactions by combining spectroscopic and electrochemical methods is examined. The spectroelectrochemical study of a charge transfer process is addressed, considering the possibilities that the electro-consumed species is regenerated in solution by means of a first-order chemical reaction or that the diffusivity of the oxidized and reduced species differ from each other significantly. Novel closed-form explicit analytical expressions for the concentration profiles and the absorbance response of all the species for any voltammetric technique are deduced, taking into account the two main working arrangements: the normal-beam and parallel-beam modes. The analytical solutions are particularized to double potential step chronoamperometry and to cyclic voltammetry. The corresponding chronoabsorptometric and voltabsorptometric responses in parallel-mode are proven to provide additional information with respect to electrochemical-only measurements, as well as insights into the solution regions next to the interface where the concentration changes and equilibrium disruption take place. With this theory the absorptometric response can be related to the physicochemical dynamics of the redox system and optimum configuration and conditions for the study of diffusion and reaction layers, the species diffusion coefficients and the chemical kinetic and equilibrium constants can be established.