Oxidation of ascorbic acid by air oxygen catalyzed by copper(II) ions in batch and continuous flow stirred tank reactors: bistability, oscillations and stochastic resonance

Abstract

We report various nonlinear phenomena observed in the oxidation of ascorbic acid (H₂Asc) by molecular oxygen in the presence of copper(II) ions in a continuous flow stirred tank reactor (CSTR) as well as in a batch reactor (closed system). Transient irregular oscillations were observed in a batch reactor by varying the initial concentrations of H₂Asc and copper(II) ions. The behavior of the system in a CSTR was studied by varying the flow rate (k_f) and the inflow concentrations of H₂Asc and copper(II) ions. At low inflow concentrations of copper(II) ions the system exhibits bistability and small amplitude regular oscillations. In a two-dimensional parametric plot with coordinates k_f and [Cu²⁺]₀ the regions of bistability and oscillations form a cross-shaped diagram if the inflow concentration of H₂Asc is kept constant. For increasing values of k_f oscillations emerge ia a Hopf bifurcation and disappear due to a homoclinic orbit. Simultaneously applied small amplitude periodic and stochastic perturbations imposed onto the flow rate close to the bistable region revealed the phenomenon of stochastic resonance, which is confirmed by an analysis of probability distribution functions, power spectra, and singular value decomposition.