Electrodes bromide-sensitive

Fig. 1.1. Typical experimental records of oscillatory behaviour in the Belousov-Zhabotinskii reaction (a) platinum electrode which responds primarily to the Ce3+/Ce4+ couple (b) bromide-sensitive electrode measuring In [Br ].

Figure 1. Oscillatory behavior following an initial induction period in the potentials of a platinum redox electrode and a bromide sensitive electrode in a BZ reaction mixture containing an aqueous solution of bromate, malonic acid, a ceric salt, and sulfuric acid. Reproduced from Field et al. (6). Copyright 1972 American Chemical Society,...

Figure 8.21 Example of the control algorithm where the next amplitude map is obtained from successive maxima of a bromide-sensitive electrode in the BZ reaction. Inset shows original map (left) and shifted map (right) in the neighborhood of the unstable steady state A. When the flow rate is shifted by 0.2%, the point A is shifted down to a point that evolves to the steady state A. (Adapted from Petrov et al., 1993.)... 

Figure8.22 Phase portraits of stabilized (a) period-1 and (b) period-2 orbits embedded in a chaotic attractor in the BZ reaction. Scattered points show chaotic trajectory (delay time r = 1.3 s) before stabilization. (c) Time series showing potential of bromide-sensitive electrode. Control via change in input flow rate of cerium and bromate solutions was switched on from 27,800 s to 29,500 s to stabilize period-1 and from 30,000 s to 32,100 s to stabilize period-2. (Adapted from Petrov et al., 1993.)...

Figure 10.7 Experimental phase portraits showing crisis. Axes represent potential of bromide-sensitive electrode at times t and t + 32.9 s. r = (a) 54.925 s, (b) 109.85 s. Filled circles represent initially (r = 0) stable fixed points, filled squares are initially unstable fixed points. As r is increased, the lower attractor collides with an unstable fixed point and undergoes sizable expansion (crisis). (Adapted from Chevalier et al., 1991.)...

The periodic change of colour from red to blue corresponds to the formation of Ce(III) and Ce(VI) during the oscillations. Oscillations also appear if a bromide sensitive electrode is used to monitor and/or record [bromide]. Small temperature oscillations may be also observed. 

Care had to be taken in the preparation of suitable homogeneous coated tubular bromide-selective electrodes from silver bromide. Maximum contact area is obtained by using a tubular electrode that is well-coated. The maximum sensitivity was obtained when the electrode was coated, tested, left in ca. 500 mg/dm3 bromide solution, recoated, etc., until maximum response was obtained, and then conditioned overnight in 20 mg/dm3 bromide solution. It was also necessary to carry out an actual test run of about ten minutes. 

Perbromic acid and perbromates are most readily assayed by determination of their oxidizing power after reduction with hydrogen bromide, as described earlier in this article. Traces of fluoride in the acid or salts may be determined potentio-metrically, using a fluoride-sensitive electrode (Orion Research, Inc.) and an expanded-scale pH meter. Acid or alkaline solutions should be neutralized or buffered with acetic acid and sodium acetate before the determination. The electrode response should be calibrated against similar solutions of known fluoride content. 

In the case of oscillatory reaction under discussion, reactions are ionic in nature and oscillating species are ions. The oscillating species Br and Ce +/Ce + are detected by bromide and platinum sensitive electrodes in conjunction with standard calomel electrode. The essential challenging task of developing a reaction mechanism is to postulate how the concentration of Ce + and Br builds-up in the course of time and how it is periodically inhibited. In the light of Brusselator model discovered by... 

The important techniques for measuring the oscillations in the chemical reaction are the potentiometry. The advantage of this technique is that by using a bromide ion-sensitive electrode, the composition of bromide ions in the reaction system can be monitored easily. However, platinum electrode is susceptible to changes in the oxidation state of the metal-ion catalyst. The measured electrode potential can be used to monitor oscillations in [Br ] and [Mox]/[Mjed] with the help of a suitable reference electrode. 

An interesting example of oscillation in a chemical system has been described in the bromate-cerium(iv)-malonic acid reactions in sulphuric acid media. In a stirred acidic solution containing initially KBrOa, malonic acid, and cerium(iv) sulphate, the concentrations of the cerium(rv) and of bromide ion produced may be monitored potentiometrically using a tungsten and a bromide-ion-sensitive electrode, respectively, and have been shown to undergo marked repeated oscillations. A mechanism has been postulated in which both the overall chemical reactions and those responsible for the oscillations have been identified. In the conditions of malonic acid (0.1—0.5 mol 1 ), BrOj ( 0.06 mol 1 ), and Ce ( 10" mol 1 ) in 0.8M-sulphuric acid, there are three overall reactions,... 

This electrode is unique in that the bilirubin oxidase is active at neutral pH, whereas the laccase cited above is not, even though the redox potential of laccase is somewhat higher. Additionally, the bilirubin oxidase is much less sensitive to high concentrations of other anions such as chloride and bromide, which deactivate laccase. It was shown that mutations of the coordination sphere of bilirubin oxidase led to an increased redox potential of the enzyme, which increased current density and reduced current decay to 5%/day over 6 days at 300 rpm. The latter improvement was attributed to improved electrostatic attraction between the enzyme and the redox polymer. An electrode made with high-purity bilirubin oxidase and this redox polymer has recently been shown to outperform a planar platinum electrode in terms of activation potential and current density of oxygen reduction. ... 

Dye oxidation (e.g., tetrazolium reductase activity with 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide, MTT 2-[4-iodophenyl]-3-[4-nitrophenyl]-5-[2,4-disulfophenyl]-2H tetrazolium monosodium salt, WST-1 3- (4,5 -carboxymethoxyphenyl) -2-(4-sulfophenyl)-2 H-tetra-zolium, MTS 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide inner salt, XTT 2,2 -di-p-nitrophenyl-5,5 -diphenyl-3,3 -(3,3 -dimethoxy-4,4 -diphe-nylenej-ditetrazolium chloride, NET), Alamar blue assays, ATP concentration (e.g., luciferase assay), oxygen consumption (e.g., oxygen electrodes, phosphorescent oxygen-sensitive dyes), mitochondrial protein and nucleic acid synthesis mitochondrial mass (e.g., mitotracker dyes) mitochondrial membrane potential (e.g., tetramethylrho-damine methyl ester, TMRM tetramethylrhodamine ethyl ester, TMRE)... 

The potential of the silver-silver chloride electrode is sensitive to traces of bromide in the solution used to deposit AgCl. The presence of 0.01 mole percent (mol %) of bromide in a KC1 electrolyte is sufficient to alter the potential of electrodes immersed in the solution by 0.1-0.2 mV.28 The potentials are not greatly affected by traces of iodide or cyanide. Light of ordinary intensities does not have a marked effect on the potential of the electrodes, but exposure to direct sunlight should be avoided. 

Sheppard S. E., Vanselow W. and Happ G. P. (1940), Photovoltaic cells with silver-silver bromide electrodes. 111. Optical sensitizing by dyes , J. Phys. Chem. 44,411-421. 

Electrodes suitable for the potentiometric determination of surfactants are either specially designed liquid or solid membrane electrodes or ion-selective electrodes that in addition to being selective to a particular ion, also quantitatively respond to surfactants. For example, a nitrate ion-selective electrode responds to anionic surfactants, a calcium ion-selective electrode is sensitive to quaternary ammonium salts, and a barium ion-selective electrode can be used for assaying polyethoxylates [43], In some cases it is possible for one to perform potentiometric determination of a counter-ion, e.g. one can titrate alkylpyridinium chloride or bromide salts with silver nitrate solution using silver wire as an indicator electrode [38]. 

Other experiments have reported the high sensitivity of glassy carbon electrodes modified by MWNT-CHT for cathodic stripping voltammetric measurements of bromide. 

Ion-selective electrodes (ISEs) are potentiometric sensors that include a selective membrane to minimize matrix interferences. The most common ISE is the pH electrode, which contains a thin glass membrane that responds to the H concentration in a solution. Other parameters that can be measured include fluoride, bromide, nitrate, and cadmium, and gases in solution such as ammonia, carbon dioxide, nitrogen oxide, and oxygen. ISEs do have their limitations including lack of selectivity and sensitivity and problems connected with conditioning of electrodes. Detection limits for nitrate-N, for example, are typically 0.098mgl for commercial field devices and have chloride as a major interferent. 

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