Dushman reaction

Redox Reaction lodide/lodate - Dushman Reaction... 

The Dushman reaction was chosen for its changes in UV-visible properties to demonstrate the capability of such in-line monitoring in a micro reactor [22],... 

Dushman Reaction Investigated in Micro Reactors Organic synthesis 91 [OS 91] Redox reaction of iodate and iodide... 

Figure 4.98 With decreasing of residence time a lower Dushman reaction product is observable, indicated by in-line spectrometry [22].

P 35] A reaction system with two competitive parallel reactions was used for mixing characterization [36], The Dushman reaction involves the mixing of iodate, iodide and sodium acetate in one solution and a strong acid such as sulfuric acid or hydrochloric acid in another solution. If mixing is fast, the neutralization of the acid and the base dominates as the faster reaction. The redox reaction of iodide and iodate then is a slow process nearly no iodine is formed as the redox product... 

Process (VIA) is the net reaction in excess arsenite ([H3As03]o/[IOJ]0 > 3) it is equivalent to (VIB) + 3 (VIC). Process (VIB), the Dushman reaction, is normally rate determining. Therefore, the net process (VIA) is autocatalytic in [I-], which causes a dramatic color change to arise at the stoichiometric point due to the sudden appearance of I2. Bognar and Sarosi exploited this fact to devise a chronometric technique for the determination of traces of iodide43. Process (VIC), the Roebuck reaction44, has played an important role in the development of chemical kinetics and teaching of physical chemistry, as it was the first multi-step reaction for which it was shown that the quotient of the independently determined forward and reverse rate laws equals the equilibrium quotient obtained from the law of mass action. 

Table 5. Some mechanistic steps in the Dushman reaction...

In these acidic media, with both iodate and iodide present, the Dushman reaction (VIB) will compete significantly with (VII A), thereby inhibiting (VIIB). The net reaction is given by the relation (VII) = 1/2[(VII A) + (VIIB)]. 

A complete mechanism for the chlorite-iodide system entails many steps. In addition to those consistent with the rate laws for processes (VII) (A) and (B), the elementary steps of the Dushman reaction need to be included. A set of relevant steps for these processes in collected in Table 7. 

Reactions of the three halide ions with iodate are reported as obeying different kinetic laws (Table 28). For the chloride and bromide reactions, the evidence is not extensive, and even the great attention given to the iodide reaction has not produced complete agreement about the reaction orders. Earlier papers refer to this last as the Dushman reaction, on account of a kinetic study which established the reaction as close to fifth order overall (rate = k[H ] [IOJ][I ] ). Kubina examined the same reaction in the presence of arsenite (which did not affect the rates) and claimed that the rate expression was different, viz. 

Scheme 6.6 Dushman reaction used for evaluation of mixing rate...

The efficiency of mixing is usually determined by a standard chemical method, the so-called Dushman reaction.Mixing of a strong acid (FICl) and a solution of I , lOs , and CFi3C02Na (sodium acetate) leads to a combination of an ultrafast reaction (neutralization of the acid) and a fast reaction (I2 formation by an acid promoted redox reaction between iodide ions and iodate ions) as shown in Scheme 7.1. These two reactions having H as a common reaction component compete in a parallel way. If the mixing is very fast, the ultrafast process masks the slower second... 

In Landolt -type reactions, iodate ion is reduced to iodide through a sequence of steps involving a reductant species such as bisulfite ion ) or arsenous acid (H2ASO3). The reaction proceeds through two overall stoichiometric processes. The Dushman reaction involves the reaction of iodate and iodide ions... 

The exchange of oxygen between iodate ions and water has been investigated by a rapid chemical-quenching technique.Over the pH range 2.1—12.5, exchange occurs by and OH -ion-catalysed paths, and the results provide further evidence for associative substitution mechanisms with l. New direct measurements of the rate of the Dushman reaction, at [I"]< 10 mol F have been obtained and confirm the importance of the polymerization of HIO3 in... 

Furuichi, R. and Liebhafsky, H.A., Rate of the Dushman reaction in iodic acid at low iodide concentration complexity of ionic acid. Bull. Chem. Soc. Jpn., 48, 745-750, 1975. 

Schmitz, G., Kinetics of Dushman reaction of low I concentrations, Phys. Chem. Chem. Phys. 2, 4041-4044, 2000. 

Villermaux-Dushman Reaction This method, also referred to as iodide-iodate reaction, is based on a system of two parallel competing reactions [64] ... 

On the basis of the extensive experimental results obtained for the Villermaux-Dushman reaction, Commenge and Falk [22] elaborated a protocol for estimating the characteristic mixing time in microstructured devices. They used the lEM... 

Ratio of the quantity of acid moles consumed by Villermaux-Dushman reaction, value of Yat complete segregation ... 

The present chapter aims to be complementary to the studies and reviews already published to present theoretical basis elements for the understanding of mixing principles in laminar flows, mainly developed in micromixers. Among different characterization techniques of mixing efficiency, this chapter more specifically focuses on the chemical test method, called the Villermaux-Dushman reaction, that we have developed over many years and which is named in memory of Professor Jacques Villermaux. It will be shown how to obtain the mixing time and how to relate it to operating parameters such as the Reynolds number of the flow and the specific power dissipation per unit mass of fluid. A non-exhaustive comparison of several micromixers will be presented. 

There are numerous experimental studies on micromixer characterization and, among the different methods, the well-known Villermaux-Dushman reaction is one of the most used. A few papers have proposed the comparison of the mixers performances thanks to these chemical reactions. As explained previously, mixing quality is a relative concept with regard to chemical reaction and chemical test methods... 

Several studies have been considered, particularly those with no experimental errors in the use of the Villermaux-Dushman reaction protocol and with providing important information such as mixer geometry (charmel sizes, internal volume), flow rate and pressure drop. The following studies have been analyzed ... 

Of course, the mixing quality is a very important parameter for microstructured liquid-liquid reactors. There are some approaches to characterize the mixing quality of such devices. Up to now, the most important method for characterizing the mixing quaUty of micromixers is the Villermaux-Dushman reaction [5]. This is a two-reaction system with a fast and a very fast reaction, which has been adapted for use with microstructured mixers. The better the mixing, the less product of the slower reaction is formed. This product (I3 ) is easily detected by UV spectroscopy. 

There is comparatively little detailed information on the kinetics of the backre-action (3), i. e. on the formation of I2 in solutions containing I and 103 (the so-called Dushman reaction) in the pH range under consideration. This question is of interest with regard to the evaluation of I2 supply from the disproportionation products in the event that the I2 fraction in the solution is diminished by volatilization. As far as is known, reaction (1) proceeds very rapidly also from the right to the left side, whereas the rate of the backreaction (3) seems to be too slow (Palmer and Lyons, 1988) to be of signiflcance as a determinant of iodine volatility, compared to the effect of radiolytic oxidation of I". Only in the case of low radiation doses would the Dushman reaction be the controlling parameter for iodine volatilization rates from aqueous solutions. 

Unlike the containment, the annuli do not represent a closed system. This means that equilibrium considerations cannot be applied here, since the buildup of a partition equilibrium is continuously disturbed by the flowing atmosphere which consists of steam and permanent gases and contains traces of I2. Consequently, the processes which take place inside the annuli are very complex, and a simple calculation of the extent and the rate of iodine revolatilization from the liquid phases is not possible. Assuming an instantaneous establishment of the equilibrium state in each volume element of the flowing atmosphere would result in an overconservative approach, leading to the result that iodine would be only temporarily retained in the annuli. Therefore, the thermodynamic approach has to be complemented by kinetic considerations which include the rate of formation of I2 from 1 and lOa" by the Dushman reaction, as well as the kinetics of the diffusion-controlled transport of I2 from the liquid to the gas phase. In addition, revolatilized I2 can be temporarily or even permanently trapped by the paint on the walls of the annuli and the auxiliary building, which would additionally diminish or at least delay iodine release from the plant. 

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