Uncatalyzed Bromate Oscillator Research Articles

Ultraviolet-Visible Kinetic Study of the Uncatalyzed Bromate Oscillator with Phenol by Multivariate Curve Resolution

A kinetic study of the uncatalyzed bromate oscillator (UBO) using phenol was carried out by chemometric analysis of the UV/Vis data. The reaction was studied with different starting concentrations of the reagents (sulfuric acid and potassium bromate) and by changing phenol for 2-bromophenol and 4-bromophenol. The number of chemical absorbing species involved in the UV/Vis data was established by singular value decomposition and by the residual fit parameter. Multivariate curve resolution with alternating least squares generated valuable information about the different chemical species along the process (spectral and concentration profiles). These results show the presence of four principal components: two intermediates and two products. The products can be assigned as o-quinone and 4,4′-diphenoquinone, and the principal components associated with the intermediates represent the linear combination of all the intermediates in the reaction mixture. These data were used to generate a model of the UBO-phenol oscillator that reproduces qualitatively the experimental results.

Are Uncatalyzed Bromate Oscillators Truly Gas-Free?

Gas evolution is a common byproduct of chemical oscillations but is seldom treated as anything but a complication. Its precise quantification is usually underestimated, and if not obvious enough, gaseous products are easily neglected completely. Reported herein is how evolution of gas from uncatalyzed bromate oscillators with pyrocatechol, pyrogallol, and 1,4-cyclohexanedione was measured by a high-precision batch-mode gasometric method, and the data obtained with unprecedented time resolution are presented. Even though within this completely closed arrangement of measurement no oscillations in rate of gas evolution were observed, it is verified that neither of the substrates can be considered totally free of gas production. Nevertheless, the amount of gas evolved from 1,4-cyclohexanedione was confirmed to be very small, and near negligible, if only oscillatory phase is of interest. Conversely, under identical conditions, the peak rates of gas evolution from the other substrates were reached much sooner, and even though the results in all three cases suggest some extent of suppression of gas production by oscillations, as much as 1 equiv of gas in total can be produced from the phenols, especially from pyrogallol, where the degradation was measured to be approximately twice as extensive. Greater caution is, therefore, recommended whenever the gas-free nature of these oscillators is considered.

The Effect of Perturbants on the Uncatalyzed Bromate Oscillator

Abstract The effect of cationic (CTAN), anionic (SDS) and nonionic surfactants (Brij-30 and Triton X-100) as well as of tert-butanol (t-BuOH) on the uncatalyzed bromate oscillator with phenol as substrate was investigated at 25 ± 0.1oC in non-stirred batch conditions. Addition of the surfactants influenced the oscillatory parameters: a slight increase in induction period of the first series of oscillations, a significant increase in induction period of the second series of oscillations as well as a gradual decrease in oscillation numbers of both series until complete disappearance at a certain surfactant concentration. The changes in the oscillatory parameters have been ascribed to the solubilization of phenol and of bromination products in the micelles, to the inhibition of bromination of aromatic substrate due to the bromine solubilization and to the catalytic effect of charged micelle surface. The changes in the oscillatory behaviour with increasing alcohol concentration have been discussed in terms of its ability to act as a radical scavenger and to affect the 3D-structure of water.

The Effect of Anionic, Cationic and Nonionic Surfactants on the Uncatalyzed Bromate Oscillator

The response of an uncatalyzed bromate oscillator with phenol as substrate to the increasing concentrations of cationic (CTAN), anionic (SDS) and nonionic surfactants (Brij-30 and Triton X-100) was monitored at (25±0.1) °C under stirred batch conditions. Addition of the surfactants influenced the oscillatory parameters: a slight increase of the induction period of the first series of oscillations, a significant increase of the induction period of the second series of oscillations and a gradual decrease of the oscillation numbers of both series until complete disappearance at a certain surfactant concentration. The changes in the oscillatory parameters have been ascribed to solubilization of phenol and of bromination products in the micelles, to inhibition of bromination of the aromatic substrate due to bromine solubilization, and to the catalytic effect of the charged micelle surface.

Nonlinear Dynamics in the Uncatalyzed Bromate–Pyrocatechol Reaction

Abstract Spontaneous chemical oscillations are uncovered in the uncatalyzed bromate–pyrocatechol reaction conducted in a stirred batch reactor. Phase diagrams show that the oscillatory behavior is more sensitive to the ratio of [bromate]/[pyrocatechol] than their actual concentrations. The long induction time observed in the system is insensitive to the initial presence of brominated pyrocatechol species. Quenching experiments illustrate that the studied chemical oscillator is both bromide and radical controlled. In addition, this new uncatalyzed bromate oscillator exhibits both light-induced and light-quenched oscillatory behaviors. Preliminary mechanistic characterization with UV/vis absorption method and mass spectrometry suggests the formation of bromo-1,2-benzoquinone through the reaction between pyrocatechol and bromine.

The Effect of tetrabutylammonium ion on the uncatalyzed bromate oscillator

The effect of tetrabutylammonium ion (Bu4N+) on the uncatalyzed bromate oscillator with phenol as substrate was monitored at 25 ± 0.1°C under stirred batch conditions. The changes in the oscillatory behaviour with increasing concentration of the Bu4N+ ions have been ascribed to their ability to act as ion-pairing reagent.

Pattern Formation in Uncatalyzed Bromate Oscillator with Pyrogallol

Stationary mosaic and trigger wave patterns may spontaneously develop in an unstirred thin film of solution containing bromate, pyrogallol and sulfuric acid (uncatalyzed bromate system). The propagation velocity of the waves was found to depend not only on the initial concentrations of H+ and BrO3- ions but also on the initial concentration of pyrogallol. For the explanation of the wave velocity dependence, a reaction scheme of the pyrogallol-BrO3- system has been proposed. The scheme is based on the well-known bromate chemistry combined with an organic radical control that takes into account the reactions of pyrogallol with BrO3-, HBrO2 and HOBr. Movement of the waves could be recorded also potentiometrically. The kinetics and mechanism of the reaction between pyrogallol and bromate in an H2SO4 medium were studied using a conventional polarographic method.

Sequential Oscillations in the Uncatalyzed Bromate Oscillator and Their Simple Qualitative Model

The sequential oscillations of the uncatalyzed Belousov−Zhabotinsky reaction with phenol have been reported. The behavior of this system was found to be remarkably sensitive to stirring in a batch reactor. Oscillations showing identical patterns are obtained in a simple model proposed for a qualitative description of the sequential oscillations observed in uncatalyzed bromate system.

Effects of Chloride and Bromide Ions on Sequential Oscillations in the Uncatalyzed Bromate Oscillator

The uncatalyzed system BrO3--phenol-H2SO4 exhibits an astonishing variety of dynamic behaviour, including sequential oscillations in a closed, stirred batch reactor. The effect of initial addition of chloride and bromide ions on a sequential oscillating system has been studied. Various results were obtained, depending on the initial chloride and bromide concentrations. Increasing the concentration of chloride ions (from 5 × 10-5 to 6 × 10-4 mol dm-3), the parameters of sequential oscillations changed. The number of first oscillations increased from 4 to 21 and their amplitude decreased. The duration of the non-oscillatory period and the number of second oscillations decreased from 96 to 26 min and from 8 to 1, respectively. In the concentration range 3.5 mmol dm-3 < [Br-]o < 8.0 mmol dm-3, the system displays a dual-frequency, dual-amplitude and aperiodic oscillations with a transition period between high- and low-frequency oscillations. The induced oscillations were also observed when the system was perturbed by chloride or bromide ions. The oxidation of pyrocatechol with bromate is an autocatalytic reaction; the rate constants corresponding to the uncatalyzed and catalyzed reactions were evaluated and used in numerical simulations.

Pattern formation in uncatalyzed bromate oscillator with catechol

Spatial patterns of the uncatalyzed bromate – catechol – sulfuric acid system in a thin fluid layer were observed. A mixture of dots, strips and circles was formed by interaction of chemical reaction, diffusion and hydrodynamic instabilities.

A New Double Substrate Uncatalyzed Bromate Oscillator

A double substrate uncatalyzed bromate oscillator is reported. Oscillations occur for several minutes, the color of the solution alternating between dark brown and reddish yellow. The oscillatory wave form (Pt electrode) has a distinct pattern, with a progressive increase in the amplitude and time interval. The presence of both substrates namely 8-hydroxyquinoline and malonic acid is essential.

Simultaneous Calorimetric and Potentiometric Investigations on Some Uncatalyzed Bromate Oscillators (UBOs)

Simultaneous calorimetric and potentiometric/thermometric measurements were performed on three uncatalyzed bromate oscillators (UBOs) showing distinct temporal structures. The following UBOs were chosen: a) the gallic acid (3,4,5-trihydroxy benzoic acid)—bromate; b) the 1-hydroxy-4-[(1-hydroxy-2-methylamino)ethyl] benzene (HME)—bromate; and c) the 1,4-cyclohexanedione (CHD)—bromate system. Their oscillating reactions were monitored by an isoperibolic batch calorimeter of the Calvet-type at 20°C and with a quasi-adiabatic Dewar setup at room temperature, and in addition by platinum/calomel potentiometry and thermometry. Shape of the calorimetric curves, number and duration of the oscillations and their frequencies varied considerably between these three UBOs. The underlying chemical reaction schemes are discussed in connection with the energetic background.

Potential response in the uncatalyzed bromate oscillatory system

The potential response of the platinum indicator electrode in the uncatalyzed bromate oscillator (UBO) with three different substrates, namely gallic acid (GA), pyrogallol (PG) and veratraldehyde (VA) has been analyzed. The large amplitude oscillations obtained with a Pt electrode are due to the sequential response of the indicator electrode to the varying concentration ratios of the redox couples present in the reaction mixture.

New Indicators for Visualizing Pattern Formation in Uncatalyzed Bromate Oscillatory Systems

Many uncatalyzed bromate oscillators (UBO's) undergo oscillations in electrochemical potential without any observable color change in a closed, stirred (batch) system. We have investigated the effects of adding a wide variety of redox indicators, including diphenylamine derivatives, azo dyestuffs, triphenylmethane derivatives, diimine chelate complexes, and methylene blue to UBO's. We find that in many cases the indicators make the batch oscillations strikingly visible and also allow observation of spatial pattern formation (concentric rings and spirals) in a thin, unstirred layer of solution. In some systems, the indicators act merely to make visible changes in redox potential, while in others they play an active role in the chemistry, and added indicator can lengthen the duration of oscillations or regenerate oscillations or patterns once this behavior has ceased.

Temperature-triggered chemical oscillators. A peculiar temperature effect in perturbed uncatalyzed bromate-driven reactions

Some uncatalyzed bromate oscillators with p-substituted phenol derivatives as the organic substrates when perturbed with chloride ions show a remarkable behavior, namely, a sudden transition, at a well-defined threshold temperature, from the non-oscillatory to the oscillatory state, and back. The high sensitivity of the reacting systems towards chloride ion perturbation and the conditions for the occurrence of the phenomenon are described.

Bistability in an uncatalyzed bromate oscillator in a continuously fed stirred tank reactor

Uncatalyzed gallic acid oscillating system has been investigated in a continuously fed stirred tank reactor (CSTR). In the [Bromate]0–[Bromide]0 concentration space, a region has been located where a bistability is observed between an oscillatory branch and a flow branch. To our knowledge this is the first evidence of bistability in an uncatalyzed bromate oscillator. Some observations have been explained in terms of the skeleton mechanism proposed in the past.

Sequential oscillations in uncatalyzed bromate oscillator in a closed reactor

Two types of oscillatory regimes separated by a time pause occur during the oxidation of aniline with bromate in sulfuric acid (a Koros-Orban System) in a closed, stirred batch reactor. It is most likely that sequential oscillations are due to aniline and its oxidation/bromination product.

Spatial Structures in an Uncatalyzed Bromate Oscillator and Periodic Precipitation in Gels

Stationary and moving structures in the uncatalyzed bromate-aniline-sulfuric acid oscillator and the formation of Liesegang rings in gelatin and silica gel were investigated. Structural analogies were found in the systems which are of a different nature. They exhibit properties typical of Turing type structures.

Several types of oscillations in the acidic bromate-chromotropic acid reaction

The uncatalyzed, acidic bromate-chromatropic acid system exhibits even under batch-reactor conditions a broader range of dynamic behavior than other uncatalyzed bromate oscillators (UBO) having anilinic or phenolic substrates. Oscillations with the following characteristics are observed: (i) oscillations with small amplitudes in redox and bromide ion selective electrode potentials (≃10-50 mV) on a generally low level of bromide ion concentration ( 10 −6 M) with larger amplitudes (≃50-100 mV), type 2; (iii) irregular oscillations

Perturbations in bromate oscillators by gamma irradiation

The oscillatory characteristics of the catalyzed and uncatalyzed bromate oscillators are altered by gamma irradiation. These alterations could arise due to additional reactions to be included in the reaction scheme involving the species like H., OH., and HO2. generated as a result of gamma irradiation.