Cerous bromate

Geiseler and Follner [10] discovered bistability in cerous-bromate system and found that under certain set of constraints, the system in sulphuric acid medium can exist in either one or two stable states. Bar-Eli and Noyes [11] have explained their results on the basis of the following mechanism. 

All the component reactions investigated are found to be exothermic. Initial temperature rise of bromide + bromate reaction was found to be the highest (0.55°C/min) while that of cerous + bromate + malonic acid was to be found to be quite low (0.085°C/min). Thus in the first stage, when the reaction was mixed, the latter reaction involving autocatalysis predominates and the temperature rise is very slow. On the other hand, when Br + BrOj reaction involving inhibition reaction becomes dominant, there is a sharp rise in temperature. The thermochemical behaviour is thus in conformity with the FKN mechanism (Br -control mechanism). 

An acidic bromate solution can oxidize various organic compounds and the reaction is catalyzed by species like cerous and manganous ions that can generate 1-equivalent oxidants with quite positive reduction potential. Belousov (1959) first observed oscillations in Celv]/[Cem] during Ce (III) catalysed oxidation of citric acid by bromate ion. Zhabotinskii made extensive studies of both temporal and spatial oscillations and also demonstrated that instead of Ce (III), weak 1- equivalent reductants like Mn(II) and Fe (II) can also be used. The reaction is called Belousov-Zhabotinskii reaction. This reaction, most studied and best understood, can be represented as... 

Cerous iodates and the iodates of the other rare earths form crystalline salts sparingly soluble in water, but readily soluble in cone, nitric acid, and in this respect differ from the ceric, zirconium, and thorium iodates, which are almost insoluble in nitric acid when an excess of a soluble iodate is present. It may also be noted that cerium alone of all the rare earth elements is oxidized to a higher valence by potassium bromate in nitric acid soln. The iodates of the rare earths are precipitated by adding an alkali iodate to the rare earth salts, and the fact that the rare earth iodates are soluble in nitric acid, and the solubility increases as the electro-positive character of the element increases, while thorium iodate is insoluble in nitric acid, allows the method to be used for the separation of these elements. Trihydrated erbium iodate, Er(I03)3.3H20, and trihydrated yttrium iodate, Yt(I03)3.3H20,... 

The experimental system is the same as that used in earlier work and details are given elsewhere [5,9]. The reactor is a baffled CSTR of volume 26.4 ml held at 25°C. The reactants are fed with a peristaltic pump. The mixed feed concentration of malonlc acid, sodium bromate, sulfuric acid, and cerous ion are 0.3, 0.14, 0.2, and 0.001 M respectively. The outputs from platinum and bromide ion electrodes are connected to a microcomputer. 

In the BZ reaction, malonic acid is oxidized in an acidic medium by bromate ions, with or without a catalyst (usually cerous or ferrous ions). It has been known since the 1950s that this reaction can exhibit limit-cycle oscillations, as discussed in Section 8,3. By the 1970s, it became natural to inquire whether the BZ reaction could also become chaotic under appropriate conditions. Chemical chaos was first reported by Schmitz, Graziani, and Hudson (1977), but their results left room for skepticism—some chemists suspected that the observed complex dynamics might be due instead to uncontrolled fluctuations in experimental control parameters. What was needed was some demonstration that the dynamics obeyed the newly emerging laws of chaos. 

Belousov (1959) observed sustained oscillations in the ratio of concentrations of the ceric and cerous ions, Ce(IV)/Ce(III), during the cerium catalyzed oxidation of citric acid by bromate in aqueous sulfuric acid. 

Acidic bromate oxidation catalyzed by cerous and manganous ion Chaos... 

Even the simplest case of one-to-one entrainment at a common frequency offers a surprisingly rich array of possibilities. Crowley and Epstein (1989) studied the behavior of two coupled BZ oscillators in the experimental configuration shown in Figure 12.2. They used acetylacetone instead of malonic acid to prevent the formation of carbon dioxide bubbles. The compositions of the feedstreams (sodium bromate, cerous nitrate, sulfuric acid, acetylacetone) were the same for the two CSTRs, except that the acetylacetone concentration was 0.015 M for one reactor and 0.016 M for the other. This difference gave rise to a difference in uncoupled frequencies of about 15%, with tq = 99.6 s for one reactor and 112 s for the other. In Figure 12.7, we see what happens as the coupling parameter p, the ratio of the mass fiow between reactors to the mass flow through each CSTR, is increased. 

The new bromate oscillators really date from the experimental fulfillment of BAR-ELI s [23] prediction that a system consisting of bromate, bromide and cerous (or manganous) ions would show sustained oscillations in a CSTR. This "minimal bromate oscillator" was soon found by ORBAN et al. [24] and later independently by GEISELER [25]. Figure 1 shows the excellent agreement between the calculations based on the NFT mechanism [23,26] and the actual experimental conditions for oscillation. 

The spatial structures recorded from sealed tubes (10 mm diameter) were obtained in the Belousov Zhabotinskii reaction medium under similar conditions with initial concentrations potassium bromate 0.09M, sulphuric acid 0.4M, malonic acid 0.3M and cerous nitrate 1.2mM, Sufficient ferroin dye was included for a visual colour contrast of pastel blue waves in pale orange background to be enhanced by black and white photography (1, 2). 

The oscillating flow reaction of bromate, bromide, and cerous ions, referred to as a minimal oscillator Cl,2] and well understood in terms of elementary reaction steps [3], was investigated computationally in an isothermal CSTR where periodic perturbations were imposed on either the total flow rate or one of the reactant inflow concentrations. Both the perturbation amplitude and frequency were varied over wide ranges. The investigations were primarily conducted with sinusoidal perturbations however, square pulse and saw-tooth periodic functions were also applied. 

While work of this type has focused primarily on chlorite systems, several new bromate oscillators, simpler in their chemistry and mechanism than the BZ system, have also been discovered. The first of these, the "minimal" bromate oscillator (Orbln, De Kepper and Epstein, [22] ) consists of bromate, bromide and either cerous or manganous ions flowed into the CSTR. It is essentially the BZ reaction with malonic acid replaced by an input of bromide (which in the batch reaction is generated by a... 

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