Bromate reduction

A voltaic cell consists of two half-cells. One of the half-cells contains a platinum electrode surrounded by chromium(III) and dichromate ions. The other half-cell contains a platinum electrode surrounded by bromate ions and liquid bromine. Assume that the cell reaction, which produces a positive voltage, involves both chromium(III) and bromate ions. The cell is at 25°C. Information for the bromate reduction half reaction is as follows ... 

Recently, the reduction by activated carbon filters of bromate (BrOj ), an ozone disinfection molecule produced by reactions between ozone and bromide initially present in water, has received increasing attention. Bromate reduction (BrOj /Br") was effective in a virgin activated carbon grain filter or in a BAG filter. The presence of natural organic matter (NOM) or inorganic ions (nitrate. 

Zhabotinskii, et al. (1982), following the concentration of Ce(IV) in a CSTR with open up inflows of Br03, Ce(III) and Br by spectrophotometry, with and without added bromomalonic acid, found that production of Br- is not proportional to bromomalonic acid concentrations. A radical mechanism for bromate reduction was also proposed. 

Kovalenko and Tikonova (1980) used inert electrodes and discovered a correlation between the oscillations in the redox potential and the concentration changes in the B-Z reactants. The graphite and vitreous carbon electrodes were used to measure the concentration ratio of the redox forms of the catalyst metal ions while the concentration of bromate reduction products were measured by the inert platinum electrodes. 

Downing, L. S. and Nerenberg, R. 2007a. Kinetics of microbial bromate reduction in a hydrogen-oxidizing, denitrifying biofilm reactor. Biotechnology and... 

Bromate reduction is biphasic and complex. In the initial stage, is produced from the reaction of vm and BrOa" with a rate constant in the range 3—61 mol i s i. Subsequent oxidation of Vm by BrOs" was identified as the slower process. The rates of reaction with 189 and 3—61 mol i s"i for IOs and BrOa" respectively,... 

With the single exception of the chlorite-bromate-reductant systems [14], for which a mechanism has been developed by joining the NFT model [26] with THOMPSON S [42] mechanism for the BrOj -ClOo reaction, no mechanism has been published for any chlorite oscillator. In Table 2, we give a recently developed mechanism [43] for the minimal chlorite-iodide oscillator. Of special significance is the fact that it contains no radical species, but rather the binuclear intermediate CIO,. Calculations with this mechanism give excellent agreement with a wide variety of experimental results. One example is given in Fig. 5. 

Another method for preparing alkah bromates is by electrolysis of bromine in alkah solutions. Anodes coated with Pb02 are used and a small amount of dichromate is added to prevent reduction of BrO at the cathode (76). 

The modes of thermal decomposition of the halates and their complex oxidation-reduction chemistry reflect the interplay of both thermodynamic and kinetic factors. On the one hand, thermodynamically feasible reactions may be sluggish, whilst, on the other, traces of catalyst may radically alter the course of the reaction. In general, for a given cation, thermal stability decreases in the sequence iodate > chlorate > bromate, but the mode and ease of decomposition can be substantially modified. For example, alkali metal chlorates decompose by disproportionation when fused ... 

The oxidizing power of the halate ions in aqueous solution, as measured by their standard reduction potentials (p. 854), decreases in the sequence bromate > chlorate > iodate but the rates of reaction follow the sequence iodate > bromate > chlorate. In addition, both the thermodynamic oxidizing power and the rate of reaction depend markedly on the hydrogen-ion concentration of the solution, being substantially greater in acid than in alkaline conditions (p, 855). 

Discussion. These anions are both determined as silver bromide, AgBr, by precipitation with silver nitrate solution in the presence of dilute nitric acid. With the bromate, initial reduction to the bromide is achieved by the procedures described for the chlorate (Section 11.56) and the iodate (Section 11.63). Silver bromide is less soluble in water than is the chloride. The solubility of the former is 0.11 mg L 1 at 21 °C as compared with 1.54 mg L 1 for the latter hence the procedure for the determination of bromide is practically the same as that for chloride. Protection from light is even more essential with the bromide than with the chloride because of its greater sensitivity (see Section 11.57). 

The polarographic method is applicable to the determination of inorganic anions such as bromate, iodate, dichromate, vanadate, etc. Hydrogen ions are involved in many of these reduction processes, and the supporting electrolyte must therefore be adequately buffered. 

The U(IV) reduction of bromate follows two paths in acid perchlorate or nitrate media , the rate law being... 

Bromate treatment in the dough gives an increase in elasticity and a reduction in extensibility. These are of course the desired characteristics for making bread. 

Biological reduction. Biological reduction of chlorate and bromate was still in development at the time of decision-making [2, 3]. It is known that this anaerobic process works well on a bench-scale. Studies on a full-scale basis are presently being conducted. 

Recycle and cathodic reduction. The most elegant solution for the Diaphragm Electrolysis Plant (DEP) appears to be recycling of the hypochlorite solution and reduction of the chlorate and bromate on the cathode of the electrolysis cell - the hypochlorite solution is added to the feed brine of the cells and the chlorate and bromate are converted to chloride and bromide at the cathode. 

The most important chemical reactions that take place in the electrolysis process and which affect the cathodic reduction of chlorate and bromate are provided in the following subsections. 

In the cell, chlorate and bromate are converted to chloride and bromide at the cathode by reduction with nascent hydrogen, although the chlorate reacts only partially. The chlorate remaining in the cell-liquor can be reduced with thiosulphate or another chemical reducing agent ... 

Recycling the hypochlorite to the feed brine has provided an excellent possibility of eliminating completely the chlorate and bromate emissions of the chlorine destruction unit of a diaphragm electrolysis plant. The main advantage of the hypochlorite recycling and cathodic reduction procedure is the reduction of bromate to bromide. 

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... 

E—The bromate ion, Br03, is gaining electrons, so it is being reduced. Reduction always occurs at the cathode. 

In this instance an excess of potassium bromate is employed. Therefore, any bromide formed [Eq. (a)] is oxidized to bromine, and the excess bromate and the bromine are assayed bromometrically. The reduction of bromate to bromine may be designated as in [Eq. (ft)]. 

The reduction of potassium iodate to iodide is usually not feasible in a direct titrimetric method (unlike the reduction of potassium bromate to bromide) and hence, has no viable application in the official procedures ... 

Potassium bromate, KBrOs, is less soluble than the bromide. Thus, most potassium bromate may be removed by filtration. Remaining bromate can be converted to bromide by reduction with iron. After filtering iron from the solution, potassium bromide is obtained by evaporation and crystallizaton. 

Indeed, other electrocatalytic processes were studied, including the pioneering work on chlorate reduction by the six-electron reduction product of 12-molybdophosphate in water-dioxane solutions [169]. This process, extended to bromate, has become a classical test of the electrocatalytic abilities of several POMs [see Table 13]. 

This mechanism can be illustrated by the reaction of ferrous ions with hydrogen peroxide (42), the reduction of organic peroxides by cuprous ions (63), as well as by the reduction of perchlorate ions by Ti(III) (35), V(II) (58), Eu(II) (71), The oxidation of chromous ions by bromate and nitrate ions may also be classified in this category. In the latter cases, an oxygen transfer from the ligand to the metal ion has been demonstrated (8), As analogous cases one may cite the oxidation of Cr(H20)6+2 by azide ions (15) (where it has been demonstrated that the Cr—N bond is partially retained after oxidation), and the oxidation of Cr(H20)6+2 by 0-iodo-benzoic acid (6, 8), where an iodine transfer was shown to take place. 

In another part of my review, I referred to the reduction of different oxyanions, including perchlorate, bromate, and nitrate by metal ions like chromous or vana-dous. I suggested that an oxygen transfer may occur in these eactions. At present, we have evidence, at least in two cases, that oxygen transfer actually takes... 

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