A New pH Oscillator:  The Chlorite−Sulfite−Sulfuric Acid System in a CSTR

Abstract

A new pH oscillator has been discovered involving the system of NaClO2, Na2SO3, and H2SO4 in a continuous-flow stirred tank reactor (CSTR). While ClO2 - serves as an oxidant in numerous systems exhibiting nonlinear dynamical behavior, this is the first reported chlorite-based pH oscillator. Large-amplitude oscillations in pH and potential of a platinum electrode were observed over a rather narrow concentration range. Complex dynamical behavior also was observed, including aperiodic oscillations, bistability between steady states, bistability between steady state and oscillatory state, bursting, a possible third steady state, and damped oscillations in batch. Autocatalytic oxidation of HSO3 - by ClO2 - is a major source of positive feedback in H+. A fast Cl+-transfer reaction between HOCl and SO3 2- is an important source of negative feedback. Oscillations were also obtained in the presence of Na2CO3, with the dehydration reaction of H2CO3 providing additional negative feedback. Models are proposed to account for observed behavior using computer simulations. Comparisons are made to the ClO2 -−I- and ClO2−I- oscillating systems, as well as to the general model for chlorite-based chemical oscillators proposed by Rabai and Orban. A possible mechanism for chemical coupling is proposed.

Mechanism Involving Hydrogen Sulfite Ions, Chlorite Ions, and Hypochlorous Acid as Key Intermediates of the Autocatalytic Chlorine Dioxide–Thiourea Dioxide Reaction

“…Indeed, independently reported values of this rate coefficient span the range 2.2 ϫ 10 8 -8.2 ϫ 10 9 m -2 s -1 (see below). [43][44][45] Although this may explain the difference quite well, at least one serious problem mentioned above questions the validity of the kinetic model reported by Jones et al [18] Therefore, we conclude that this may also account for the difference.…”

“…The kinetic model proposed here indicates that hypochlorous acid and hydrogen sulfite ions are key intermediates for the autocatalytic decrease of chlorine dioxide. The hydrogen sulfite oxidation by oxychlorine compounds is responsible for pH autocatalysis [45] in unbuffered solution and may further enhance the nonlinear nature of the title reaction. The elucidation of the mechanism of the chlorine dioxide-thiourea dioxide reaction may also contribute to a better understanding of the mechanistic background of the oscillatory and chaotic chlorite-thiourea system, [48,49] for which an autocatalytic loop involving the key intermediates hypochlorous acid, hydrogen sulfite ions, and chlorite ions might also play a critical role.…”

Compatible mechanism to characterize three independent but cross-coupled reactions of chlorite ion

“…From this, one can easily calculate k 29 to be 1.40 Â 10 8 M À1 s À1 that is in perfect harmony with the rate coefficient published in our previous work 12 but still differs by an order of magnitude found in independent works. 21,22 It can however, be easily explained by the very different experimental conditions used in those investigations.…”

27 Mechanisms of reactions in solution

“…A new system has been discovered which produces large pH oscillations in a continuous flow stirred tank reactor consisting of ClO 2 Ϫ , SO 3 2Ϫ and H 2 SO 4 and a chemical coupling mechanism has been proposed. 145 3 Substitution…”