István Szalai

Associate Professor
Eötvös University, Institute of Chemistry
Nonlinear Chemical Dynamics Group

Oscillatory reactions

Bromate Oscillators with 1,4-Cyclohexanedione

During the course of an oscillatory reaction the concentrations of some species, typically the intermediates, periodically changes. In batch reactors obviously only transient oscillations can be observed. However, in an open reactor where the reagents are continuously fed (CSTR), sustained oscillations and other dynamic phenomena (like multistability and chaos) appear. The bromate - 1,4-cyclohexanedione reaction is a so-called uncatalyzed bromate oscillator. In the oscillatory reactions the appropriate coupling between a positive (e.g autocatalytic reaction) and a negative feedback give rise to oscillations. In the bromate oscillators the bromate - bromous-acid reaction gives the autocatalysis, and the inhibitors are the bromide ions. "Uncatalyzed" means that contrary to the classical Belousov-Zhabotisky reaction in the bromate - 1,4-cyclohexanedione reaction oscillations appear in the absence of any metal ion catalyst (e.g. Ce(IV), Mn(II), ferroin). During the oxidation of the 1,4-cyclohexanedione, 2-bromo-1,4-cyclohexanedione, 1,4-hydroquinone and 1,4-benzoquinone are identified as intermediates and endproduct. 1,4-hydroquinone plays an important role in the autocatalytic process.

The bromate - 1,4-cyclohexanedione - ferroin reaction has become one of the most popular tool for studying nonlinear spatio-temporal phenomena. The practical advantage of this reaction, the long-lived and bubble-free batch oscillatory manner, makes it suitable for investigation of chemical waves. One of the most remarkable feature of this system is its unusual wave dynamics. Stacked wave fronts, traveling shock structures and merging of waves were observed in this reaction by Steinbock group. These phenomena were attributed to the anomalous dispersion of the medium.

The most important tools for study nonlinear dynamics in chemical system are the open reactors (CSTR). In a CSTR there is a continuous inflow fed of reactants and there is an outflow of the reacting mixture such that the total volume in the reactor remains constant. The chemical state of the contents of the CSTR depends on the time scale of the reaction and the residence time of the reactor. Multiple steady sates can be observed when an autocatalytic reaction is operated in an open reactor. In the bromate - 1,4-cyclohexanedione - ferroin system, two stationary states (SS1 and SS2) can be found in a CSTR. The stability domain of these steady states overlap, that is bistability. In the range of bistability the sate of the system depends on its history. The phase diagram presented here shows a typical "cross-shaped" topology.

Publications in this topic:

  • Szalai, I.; Kurin-Csörgei, K.; Epstein I. R.; Orbán, M.: "Dynamics and Mechanism of Bromate Oscillators with 1,4-Cyclohexanedione" J. Phys. Chem. A, 2003, 107, 10074.

  • Szalai, I., Körös, E.:  "The 1,4-Cyclohexanedione-Bromate-Acid Oscillatory System 3. Detailed Mechanism" J. Phys. Chem. A, 1998, 102, 6892.

Briggs - Rauscher Reaction with 1,4-Cyclohexanedion

The Briggs-Rauscher reaction is one of the most fascinating oscillatory reaction. This reaction is widely used as a classroom demonstration of the chemical oscillators due to the observable spectacular periodical color changes. The classic BR system consists of hydrogen-peroxide, iodate, Mn(II) catalyst, malonic acid as organic substrate and sulfuric acid. Here we present a novel variant of the BR reaction with 1,4-cyclohexanedione. The BR reaction with CHD shows long lasting oscillations under batch condition in wide range of the initial reagent-concentrations. Remarkably a high frequency, temperature compensated oscillations appear at the end of oscillatory period. The iodine - CHD reaction was found to be similar to the bromine - CHD reaction, the primary products (BrCHD and ICHD) in both cases are unstable in acidic solutions.

Publications in this topic:

  • Keresztúri, K.; Szalai, I.: ,,Briggs-Rauscher Reaction with 1,4-Cyclohexanedione Substrate'' Z. Phys. Chem., 2006, 220, 1071. (doi: 10.1524/zpch.2006.220.8.1071)