Evaluation of the Kinetic Parameters of First-Order Reactions Following Charge Transfer by Applying a Long-Path-Length Spectroelectrochemical Technique

Abstract

A long-path-length spectroelectrochemical technique was developed for investigating the EC mechanism and evaluating the kinetic parameters under a semi-infinite condition. The chronoabsorbances were obtained by a digital simulation method. Simulated results showed that the shapes of A-t curves are markedly dependent upon the relative values of the molar absorptivities of spieces A, B and C. When ε_A=0, ε_B/ ε_C≥5 or ε_C=0, 0≤ε_A/ε_B<0.5, the system absorbance passes through a maximum (A^max_t) interestedly. While ε_B=0, 0.2≤ε_A/ε_C≤1, there appears a minimum (A^min_t). Either A^max_t or A^min_t is closely related to the chemical reactions following charge transfer. When there is A^max_t at moment t_max in the A-t curve, increasing the value of the first-order rate constant (k₁) causes A^max_t and t_max to decrease; while increasing the diffusion coefficient (D_B) for species B causes A^max_t to increase, but t_max to decrease. This opposite effect enables us evaluate k₁ and D_B both simultaneously and precisely. The elctrooxidation of p-aminophenol in an acid aqua solution was experimentally investigated by a long-path-length spectroelectrochemical technique. The pseudo first-order rate constant and the diffusion coefficient for the electrooxidized intermediate quinone imine were successfully evaluated by this method.