Chemical Oscillations With Sodium Perborate as Oxidant.

The peroxo compounds H2O2 and K2S2O8 represent one of the major constituents in many oscillatory chemical systems. In this contribution we demonstrate that beside H2O2 and S2 O 8   2 - the sodium perborate (NaBO3·H2O) can act as alternative oxidizing agent in oscillatory reactions. So far the H2O2 has been successfully substituted with NaBO3 in two oscillators: in the BO   3 - -S2 O 3   2 - -Cu(II) flow system potential and pH oscillations, in the strongly alkaline Cu(II)-catalyzed BO   3 - -SCN- batch reaction, which are rather different in their chemistry and dynamics, potential oscillations were observed. In spite of the significant differences in the oxidizing nature of H2O2 and NaBO3 we assume that the oscillatory cycle in the BO   3 - -substrate and in the H2O2-substrate systems is similar in many aspects, therefore the numbers of this new subgroup of the oscillators may be considered to be borate-mediated H2O2 oscillators. Mechanisms are suggested and simulations are shown to describe the oscillatory behaviors observed in the perborate chemistry based oscillators by using the assumption that the oxidation reactions of the intermediates (HO)3B(OOH)- and (HO)2B(OOH)   2 - anions, which are dominant species in alkaline and neutral pH solutions of perborate, are much faster than that of H2O2.

Periodic changes in the oxidation states of the center ion in the cobalt-histidine complex induced by the BrO3- - SO32- pH-oscillator.

The coupling of acid-base type pH-dependent equilibria to pH-oscillators expanded significantly the number and type of species which can participate in oscillatory chemical processes. Here, we report a new version of oscillatory phenomena that can appear in coupled oscillators. Oscillations in the oxidation states of the center ion bound in a chelate complex were generated in a combined system, when the participants of the oscillator as dynamical unit and the components of the complex formation interacted with each other through redox reaction. It was demonstrated, when the BrO3- - SO32- pH-oscillator and the Co2+ - histidine complex were mixed in a continuous stirred tank reactor, periodic changes in the pH were accompanied with periodic transitions in the oxidation number of the cobalt ion between +2 and +3. The oscillatory build up and removal of the Co(III)-complex were followed by recording the light absorption at the wavelength characteristic for this species with simultaneous monitoring the pH-oscillations. The dual role of the SO32- ion in the explanation of this observation was pointed out. Its partial and consecutive total oxidations by BrO3- give rise to and maintain sustained pH-oscillations in the combined system and its presence induces the rapid conversion of the Co2+ to a highly inert Co(III)-histidine chelate when the system jumps to and remains in the high pH-state. The oscillatory cycle is completed when the Co(III)-complex is washed out from the reactor and the reagents are replenished by the flow during the time the system spends in the acidic range of pH. The idea, to couple a core oscillator to an equilibrium through a redox reaction that takes place between the constituents of the oscillator and the target species of the linked subsystem, may be generally used to bring about forced oscillations in many other combined chemical systems.

Spatiotemporal dynamics of minimal bromate oscillators in an open one-side-fed reactor.

Minimal bromate oscillators represent the simplest version of the oscillatory reactions based on the chemistry of the oxybromine species. Here, we present numerical and experimental evidence of the existence of reaction-diffusion waves in the ferroin catalyzed minimal bromate oscillator. The wave dynamics depend not only on the characteristic chemical timescales but also on those of the diffusive matter exchange which occurs between the reaction-diffusion medium and its environment. We show that the extended reactivity of the ferroin catalyst towards the oxybromine species plays an essential role in the observed phenomena. For the cerium catalyzed minimal bromate oscillator the simulations support only the formation of spatial bistability.

pH-oscillations in the bromate-sulfite reaction in semibatch and in gel-fed batch reactors.

The simplest bromate oxidation based pH-oscillator, the two component BrO3(-)-SO3(2-) flow system was transformed to operate under semibatch and closed arrangements. The experimental preconditions of the pH-oscillations in semibatch configuration were predicted by model calculations. Using this information as guideline large amplitude (ΔpH∼3), long lasting (11-24 h) pH-oscillations accompanied with only a 20% increase of the volume in the reactor were measured when a mixture of Na2SO3 and H2SO4 was pumped into the solution of BrO3(-) with a very low rate. Batch-like pH-oscillations, similar in amplitude and period time appeared when the sulfite supply was substituted by its dissolution from a gel layer prepared previously in the reactor in presence of high concentration of Na2SO3. The dissolution vs time curve and the pH-oscillations in the semibatch and closed systems were successfully simulated. Due to the simplicity in composition and in experimental technique, the semibatch and batch-like BrO3(-)-SO3(2-) pH-oscillators may become superior to their CSTR (continuous flow stirred tank reactor) version in some present and future applications.

pH-regulated chemical oscillators.

The hydrogen ion is arguably the most ubiquitous and important species in chemistry. It also plays a key role in nearly every biological process. In this Account, we discuss systems whose behavior is governed by oscillations in the concentration of hydrogen ion. The first chemical oscillators driven by changes in pH were developed a quarter century ago. Since then, about two dozen new pH oscillators, systems in which the periodic variation in pH is not just an indicator but an essential prerequisite of the oscillatory behavior, have been discovered. Mechanistic understanding of their behavior has grown, and new ideas for their practical application have been proposed and, in some cases, tested. Here we present a catalog of the known pH oscillators, divide them into mechanistically based categories based on whether they involve a single oxidant and reductant or an oxidant and a pair of reductants, and describe general mechanisms for these two major classes of systems. We also describe in detail the chemistry of one example from each class, hydrogen peroxide-sulfide and ferricyanide-iodate-sulfite. Finally, we consider actual and potential applications. These include using pH oscillators to induce oscillation in species that would otherwise be nonoscillatory, creating novel spatial patterns, generating periodic transitions between vesicle and micelle states, stimulating switching between folded and random coil states of DNA, building molecular motors, and designing pulsating drug delivery systems. We point out the importance for future applications of finding a batch pH oscillator, one that oscillates in a closed system for an extended period of time, and comment on the progress that has been made toward that goal.

Periodic changes in the distribution of species observed in the Ni(2+)-histidine equilibrium coupled to the BrO3(-)-SO3(2-) pH oscillator.

The dynamical behavior of the system comprising of the pH-dependent complex formation between histidine and Ni(II) ions coupled to the BrO3(-)-SO3(2-) pH oscillator was studied. The pH oscillator was demonstrated to be capable of forcing the pH-sensitive nickel ion-histidine equilibrium to alternate periodically between the unreacted and the fully complexed states. The periodic interconversions gave rise to an oscillatory distribution of the species that participate in the equilibrium and resulted in oscillations in the free [Ni(2+)], [NiHis(+)], and [Ni(His)2]. The preconditions of the successful coupling of metal ion-amino acid complexes to a primary pH oscillator are briefly discussed. Model calculations were performed to simulate the dynamics observed in the BrO3(-)-SO3(2-) - Ni(2+)-His CSTR system.

Generation of spatiotemporal calcium patterns by coupling a pH-oscillator to a complexation equilibrium.

Sustained spatiotemporal pH and calcium patterns are produced in a non-equilibrium inorganic reaction-diffusion system by coupling two modules, the bromate-sulfite-ferrocyanide pH-oscillator and the pH-sensitive complexation of Ca(2+) by ethylenediaminetetraacetate. The development of chemical waves is mainly determined by the oscillatory module, however, the formation of the localised stationary patterns results in the synergistic interaction between the modules.

[Recent results in research on oscillatory chemical reactions]. / Ujabb eredmények a periodikus kémiai reakciók kutatásában.

The mechanisms of the complicated periodical phenomenas in the nature (e.g. hearth beat, sleep cycle, circadian rhythms, etc) could be understood with using the laws of nonlinear chemical systems. In this article the newest result in the research of the subfield of nonlinear chemical dynamics aimed at constructing oscillatory chemical reactions, which are novel either in composition or in configuration, are presented. In the introductory part the concept of chemical periodicity is defined, then the forms as it can appear in time and space and the methods of their study are discussed. Detailed description of the experimental work that has resulted in two significant discoveries is provided. A method was developed to design pH-oscillators which are capable of operating under close conditions. The batch pH-oscillators are more convenient to use in some proposed applications than the equivalent CSTR variant. A redox oscillator that is new in composition was found. The permanganate oxidation of some amino acids was shown to take place according to oscillatory kinetics in a narrow range of the experimental parameters. The KMnO4 - glycine - Na2HPO4 system represents the first example in the family of manganese based oscillators where amino acids is involved. In the conclusion formal analogies between the simple chemical and some more complicated biological oscillatory phenomena are mentioned and the possibility of modeling periodic processes with the use of information gained from the studies of chemical oscillations is pointed out.

Oscillations in the permanganate oxidation of glycine in a stirred flow reactor.

Oscillatory behavior is reported in the permanganate oxidation of glycine in the presence of Na2HPO4 in a stirred flow reactor. In near-neutral solutions, long-period sustained oscillations were recorded in the potential of a Pt electrode and in the light absorbance measured at λ = 418 and 545 nm, characteristic wavelengths for following the evolution of the intermediate [Mn(IV)] and reagent [MnO4(-) ] during the course of the reaction. No evidence of bistability was found. The chemical and physical backgrounds of the oscillatory phenomenon are discussed. In the oscillatory cycle, the positive feedback is attributed to the autocatalytic formation of a soluble Mn(IV) species, whereas the negative feedback arises from its removal from the solution in the form of solid MnO2. A simple model is suggested that qualitatively simulates the experimental observations in batch runs and the dynamics that appears in the flow system.

Generation of pH-oscillations in closed chemical systems: method and applications.

All pH-oscillators reported to date function only under open (flow reactor) conditions. We describe an approach to generating pH-oscillations in a closed system by starting from an open system pH-oscillator, finding semibatch conditions under which it oscillates with an inflow of a single reactant to an otherwise closed reactor containing the remaining components, and replacing this inflow with a layer of silica gel impregnated with the key reactant. We present data showing the successful application of this technique to the BrO(3)(-)-Mn(2+)-SO(3)(2-), IO(3)(-)-Fe(CN)(6)(4-)-SO(3)(2-), and BrO(3)(-)-Fe(CN)(6)(4-)-SO(3)(2-) systems. In all three cases, sulfite ion is the species that is replenished via dissolution from the gel layer.

Oscillatory concentration pulses of some divalent metal ions induced by a redox oscillator.

Oscillations in the concentration of divalent ions Cd(2+), Ca(2+), Zn(2+), Co(2+) and Ni(2+) are induced by adding these species to the BrO(3)(-)-SO(3)(2-) chemical oscillator in a flow reactor. Producing periodic pulses in the concentrations of these non-redox ions extends our earlier approach to generating forced periodic behavior. Instead of driving pH-dependent equilibrium reactions of the target ion by a pH oscillator backward and forward, we now couple a redox core oscillating reaction to two consecutive reactions taking place between the components of the oscillator and the target element. In the systems examined here, the oscillatory reductant SO(3)(2-) binds the free metal ion in a MSO(3) precipitate, reducing its level to a minimal value when [SO(3)(2-)] is high, followed by release of the metal ion when the sulfite is oxidized by BrO(3)(-).

Oscillations in the concentration of fluoride ions induced by a pH oscillator.

Sustained oscillations in the concentration of free fluoride ions can be generated when the BrO3--SO32--Mn(II) oscillator is coupled either to Al3+-F- complex formation or to the Ca2+-F- precipitation process in a flow reactor. By careful analysis of the relevant equilibria and optimization of the reactant concentrations, one can obtain [F-] oscillations of several orders of magnitude as measured with an ion-selective electrode. The BrO3--SO32--Mn(II)-Al(NO3)3-NaF system also exhibits bistability, that is, simultaneously stable steady states of high and low [F-].

Periodic pulses of calcium ions in a chemical system.

By coupling the bromate-sulfite-ferrocyanide oscillating chemical reaction with the complexation of calcium ion by EDTA, we construct a system that generates periodic pulses of free Ca(2+) with an amplitude of 2 orders of magnitude and a period of ca. 20 min. These pulses may be observed either with a calcium ion-selective electrode or with Arsenazo(III) as an indicator. We describe the systematic design procedure and the properties of this first abiotic calcium-based chemical oscillator.

New experimental data and mechanistic studies on the bromate-dual substrate-dual catalyst batch oscillator.

The bromate-hypophosphite-acetone-Mn(II)-Ru(bpy)(3)(2+) batch oscillator was recently suggested for studying two-dimensional pattern formation. The system meets all major requirements that are needed for generation of good quality traveling waves in a thin solution layer. The serious drawback of using the system for studying temporal and spatial dynamical phenomena is its unknown chemical mechanism. In order to develop a mechanism that explains the observed long-lasting batch oscillations the bromate-hypophosphite-acetone-Mn(II)-Ru(bpy)(3)(2+) oscillator was revisited. We studied the dynamics both in the total system and in some composite reactions, and kinetic measurements were carried out in three subsystems. From the new experimental results we concluded that the two oscillatory sequences observed in the full system are originated from two oscillatory subsystems, the Mn(II)-catalyzed bromate-hypophosphite-acetone and the Ru(bpy)(3)(2+)-catalyzed bromate-bromoacetone reactions. Here we propose a mechanism which is capable of simulating the dynamical features that appeared in the complex system.

Systematic design of chemical oscillators using complexation and precipitation equilibria.

Concentration oscillations are ubiquitous in living systems, where they involve a wide range of chemical species. In contrast, early in vitro chemical oscillators were all derived from two accidentally discovered reactions based on oxyhalogen chemistry. Over the past 25 years, the use of a systematic design algorithm, in which a slow feedback reaction periodically drives a bistable system in a flow reactor between its two steady states, has increased the list of oscillating chemical reactions to dozens of systems. But these oscillating reactions are still confined to a handful of elements that possess multiple stable oxidation states: halogens, sulphur and some transition metals. Here we show that linking a 'core' oscillator to a complexation or precipitation equilibrium can induce concentration oscillations in a species participating in the equilibrium. We use this method to design systems that produce periodic pulses of calcium, aluminium or fluoride ions. The ability to generate oscillations in elements possessing only a single stable oxidation state (for example, Na+, F-, Ca2+) may lead to reactions that are useful for coupling to or probing living systems, or that help us to understand new mechanisms by which periodic behaviour may arise.