Voltammetric sensing of thiols. The electrocatalytic oxidation of 4-acetamidophenol in the presence of cysteine: a mechanistic rotating disk electrode study

A numerical method is illustrated for the modeling of an important electrocatalytic reaction at the rotating disk electrode (RDE). This reaction, labeled EC2XE, occurs via the following route: A - 2e(-) reversible arrow B, B + X --> B - X (k(2)), B-X - 2e(-) reversible arrow [B-X](2+). The numerical method is based on a finite difference formulation of coupled mass transport and kinetic equations and makes use of a Hale coordinate transformation. Iterative solutions are performed using a Gauss-Newton scheme to solve the nonlinear differential equations. Numerical solutions are calculated to generate working surfaces in the effective number of electrons transferred, N-eff. Cyclic voltammetry is reported in hydrostatic conditions with a 4-acetamidophenol (paracetamol)/cysteine system, which is shown to proceed via the EC2XE reaction pathway. The resulting data is modeled using the commercial package DIGISIM, and a result of k(2) = 1.25 +/- 0.25 x 10(7) mol-l cm(3) s(-1) is obtained. RDE studies of the same system using steady-state linear sweep hydrodynamic voltammetry is next performed to collect limiting current data at rotation speeds of 2, 4, 8, 16, and 25 Hz. Working surface interpolation of the resulting data results in a mean value of 1.6 +/- 0.35 x 10(7) mol(-1) cm(3) s(-1) for the rate constant k(2).