Oscillating reactions experiments

The 1970s saw an explosion of theoretical and experimental studies devoted to oscillating reactions. This domain continues to expand as more and more complex phenomena are observed in the experiments or predicted theoretically. The initial impetus for the smdy of oscillations owes much to the concomitance of several factors. The discovery of temporal and spatiotemporal organization in the Belousov-Zhabotinsky reaction [22], which has remained the most important example of a chemical reaction giving rise to oscillations and waves. 

The kinetics experiments are subdivided into classical kinetics experiments, Table XX photochemistry, Table XXI catalysis, oscillating reactions and miscellaneous topics in kinetics, Table XXII. 

Table XXII. Experiments on Catalysis, Oscillating Reactions and Miscellaneous Topics in Kinetics...

In a similar spirit, Lengyel et al. (1990) have proposed and analyzed a particularly elegant model of another oscillating reaction, the chlorine dioxide-iodine-malonic acid (CIO2 -I2 -MA) reaction. Their experiments show that the following three reactions and empirical rate laws capture the behavior of the system ... 

A mathematical model may be constructed representing a chemical reaction. Solutions of the mathematical model must be compatible with the observed behavior of this chemical reaction. Furthermore if some other solutions would indicate possible behaviors so far unobserved, of the reaction, experiments maybe designed to experimentally observe them, thus to reinforce the validity of the mathematical model. Dynamical systems such as reactions are modelled by differential equations. The chemical equilibrium states are the stable singular solutions of the mathematical model consisting of a set of differential equations. Depending on the format of these equations solutions vary in a number of possible ways. In addition to these stable singular solutions periodic solutions also appear. Although there are various kinds of oscillatory behavior observed in reactions, these periodic solutions correspond to only some of these oscillations. 

Experimenting with the combined catalytic action of Ce(III, IV) and ferroin on the oscillating reaction system bromate/malonic acid/H2S04 for different ratios... 

Farage and Janjic (1982-1 and 2) observed oscillations in the concentrations of bromide and redox potentials during the uncatalyzed oxidation of 1,4-cyclohexane-dione by bromate in sulfuric, nitric (1982-1), perchloric and orthophosphoric (1982-2) acid solutions. The system does not require a catalyst such as the redox couple Ce(IV)/Ce(III) or Mn(III)/Mn(II) of the B-Z reaction. Experimenting with this system Farage and Janjic (1982-3) observed that temperature, stirring and oxygen affect the frequency or amplitude of oscillations. 

Figure 4. Hcnematic oj me moaijiea JKK experiment used to measure the force produced by the bulk PMAA gel as it changes volume in the oscillating reaction.

It is worth asking whether perturbation methods might yield as much or more information about oscillating reactions, where it might be possible to probe not only constant or monotonically varying concentrations but also amplitude and phase relationships. Schneider (1985) reviewed a variety of model calculations and experiments on periodically perturbed chemical oscillators. The results, which show such features as entrainment, resonance, and chaos, are of considerable interest in the context of nonlinear dynamics, but shed little light on the question of mechanism. 

While these experiments are interesting, it remains to be seen if using an oscillating reaction to initiate polymerization can be more useful than current approaches. 

I had a similar experience in 1954 when I spoke, probably for the first time, about the possibility of oscillating reactions. At that time, I had published a short paper with Radu Balescu on the possibility that far from equilibrium we could have chemical oscillations, in contrast with what happens near equilibrium. This work was connected with involvment in the so-called "universal evolution criterion", derived with Paul Glansdorff. My lecture of 1954 had no more success than the one of 1946. The chemists were very skeptical about the possibility of chemical oscillations and in addition, said an outstanding chemist, even if it would be possible, what should be the interest The interest of chemical kinetics was at that time the discovery of well-defined mechanisms, and specially of potential energy surfaces, which one could then connect with quantum mechanical calculations. The appearance of chemical oscillations or other exotic phenomena seemed to him to be of no interest in the direction in which chemical kinetics was traditionally engaged. All this has changed, but to some extent the situation of chemistry in respect to physics remains under the shadow of this distrust of time. 

Addition of iodide and the pseudohalide azide (NJ) had a similar effect as addition of chloride, whereas addition of nitrate, sulfate or perchlorate ions had no effect. The oscillation period after the initial state was found to be identical with that of the unperturbed system. When the amount of chloride added is less than the amount of cerium(IV) present initially, no observable inhibition is expected. This system is more sensitive to addition of chloride after the oscillating reaction has started than to initial addition of chloride ions smaller amounts of chloride ions are required to induce inhibition or to suppress the oscillations completely. Because of this inhibitory effect, the glass vessels used to prepare the solutions for a Belousov-Zhabotinsky experiment should be clean and free of chloride ions. Moreover, in the preparation of the ferroin indicator solution 1,10-phenanthroline must be used in its free base form and not in the form of the hydrochloride salt. If one wants to monitor the periodic changes in the chemical potential of the solution, one must use a reference electrode that does not leak chloride ions. Conventional calomel electrodes or silver/silver chloride electrodes are not suitable, but double-junction version of these electrodes are adequate. 

Abstract. The urea-urease system is a pH dependent enzymatic reaction that was proposed as a convenient model to study pH oscillations in vitro here, in order to determine the best conditions for oscillations, a two-variable model is used in which acid and substrate, urea, are supplied at rates kh and ks from an external medium to an enzyme-containing compartment. Oscillations were observed between pH 4 and 8. Thus the reaction appears a good candidate for the observation of oscillations in experiments, providing the necessary condition that kh > ks is met. In order to match these conditions, we devised an experimental system where we can ensure the fast transport of acid to the encapsulated urease, compared to that of urea. In particular, by means of the droplet transfer method, we encapsulate the enzyme, together with a suitable pH indicator, in a l-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) lipid membrane, where differential diffusion of H+ and urea is ensured by the different permeability (Pm) of membranes to the two species. Here we present preliminary tests for the stability of the enzymatic reaction in the presence of lipids and also the successful encapsulation of the enzyme into lipid vesicles. 

Visually, of course, this occurs because the ratio [Ce4+]/tCe3+] is coupled to the steady-state concentration. This in turn can be made yet more visible for demonstration purposes by the addition of Fe(phen)3+/Fe(phen)2+. The feedback loop is controlled by [Br-]ss. At the same time [Br-] too is oscillating in inverse relation to HBr02, by virtue of a competition between those reactions that form Br- and those that conserve it. Some of these effects are shown in Fig. 8-1, which depicts various oscillations in [Ce4+]/fCe3+] and in [Br-]. This figure shows the results for experiments under two sets of conditions. It illustrates how the amplitude and the frequency of the oscillations depend on the concentrations. 

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