Iodate reaction

The arsenous acid-iodate reaction is a combination of the Dushman and Roebuck reactions [145]. These reactions compete for iodine and iodide as intermediate products. A complete mathematical description has to include 14 species in the electrolyte, seven partial differential equations, six algebraic equations for acid-base equilibriums and one linear equation for the local electroneutrality. 

The disproportionation of hypoiodite to form iodate (reaction (4.10)) is believed to be slow in slightly alkaline solutions such as seawater [6,7]. 

The solution becomes acidic due to the formation of sulfuric acid. This solution is treated with an equivalent amount of fresh iodate mother hquor. lodide-iodate reaction in acid medium yields iodine ... 

Before considering the chlorite reaction, we mention one further aspect of the arsenite-iodate reaction, its spatial inhomogeneity. Epik and Shub first reported chemi-... 

Table 6. Some mechanistic steps in the arsenite-iodate reaction...

Like the arsenite-iodate reaction, chlorite-iodide is a clock reaction50,51 however, it is more complex. This reaction shows a dramatic rise in the intensity of the brown color of iodine, followed by an even more abrupt fade-out. This behavior resembles that of the ferrous-nitrate clock, where the color is due to formation of the FeNOz+ complex52. ... 

The mechanism proposed for the iodate reaction is closely related to that above, viz. 

The iodate reaction is well known in the form of lecture demonstrations (the Landolt Reaction). The phenomenon excited intense early interest. By 1904, it was already possible to cite twenty-five references °. In an extended series of papers, Eggert et a/. have developed the concept that the reaction involves the three consecutive processes... 

The rate of oxidation of thiosulphate ion by chlorate and bromate is not small by comparison with other reactions of these halates. The chlorate reaction appears to have received only very fragmentary kinetic examination. The latter has been studied , but only reported as having a rate equal to fc[ ][BrOJ ] ]. The iodate reaction has been reported to have the stoichiometry... 

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

Guichardon, P. and Falk, L. (2000) Characterisation of micromixing efficiency by the iodide-iodate reaction system. Part I Experimental procedure. Chem. Eng. Sci., 55 (19), 4233-4243. 

The most natural, and most common, method to look at and present one s data is the way in which those data are taken. In a typical experiment, we measure some function of concentration (e.g., electrode potential or absorbance) as a function of time at one set of constraints. A plot of signal vs. time is known as a time series. Time series can be exceedingly dull, for example, in the case of a steady state, or they can be quite difficult to interpret, as in the case of a system that may or may not be chaotic. Nevertheless, they can yield valuable information, and they are certainly the first thing one should look at before proceeding further. Figure 2.12 shows a time series that establishes the occurrence of bistability in the arsenite-iodate reaction. 

Figure 2.12 Time series showing the iodide concentration in the arsenite-iodate reaction in a CSTR. The system is initially in an oxidized steady state (the potential shows a slight downward drift because of an experimental artifact). At the time s indicated by the arrows, a measured amount of acid is injected. With a small injection, the system returns to the steady state, demonstrating the stability of that state. With a larger injection, there is a transition to a second, reduced, steady state. (Adapted from De Kepper et al., 1981a.)...

The design algorithm outlined above was first applied to a real chemical system in 1980. De Kepper et al. (1981b) chose two autocatalytic reactions, the chlorite-iodide and arsenite-iodate reactions, to work with. Chlorite reacts with iodide in two stages. The first, and dynamically more important, step produces iodine autocatalytically (Kern and Kim, 1965) according to... 

The team first established that the arsenite iodate reaction showed bistability in a CSTR (De Kepper et aL, 1981a). They then introduced chlorite as a feedback species and obtained oscillations as shown in Figure 4.10. Note, particularly in the trace of the iodide-selective electrode, the alternation between periods of slow concentration change and rapid jumps between pseudo-steady-state levels, similar to the behavior shown schematically in Figure 4.8c. 

In order to do this, we need to be clever and a little bit lucky. The most thorough analytical treatment of wave propagation in any system to date is the study by Showalter and coworkers of the arsenous acid-iodate reaction (Hanna et al., 1982). The key here is that the essential part of the kinetics can be simplified so that it is described by a single variable. If we treat one-dimensional front propagation, the problem can be solved exactly. 

A model that reproduces the homogeneous dynamics of a chemical reaction should, when combined with the appropriate diffusion coefficients, also correctly predict front velocities and front profiles as functions of concentrations. The ideal case is a system like the arsenous acid-iodate reaction described in section 6.2, where we have exact expressions for the velocity and concentration profile of the wave. However, one can use experiments on wave behavior to measure rate constants and test mechanisms even in cases where the complexity of the kinetics permits only numerical integration of the rate equations. 

Before stating the model precisely, it should be pointed out although elementary, it is similar to the Arsenious acide-Iodate reaction and thus can provide useful results. 

Reaction System The iodide-iodate reaction (Villermaux-Dushman reaction) is based on a system of two competing parallel reactions as described above (Equation 2.60). The neutralization reaction ri is very fast and can be considered as instantaneous. The redox reaction T2 is fast. By adding an add to a mixture of iodide and iodate in a H2B03 /H3B03 buffer, iodine will be formed only if a local proton excess exists. Therefore, the amount of iodine formed can be used to characterize the mixing time in the reactor. Best results are obtained if the mixing time is on the same order as the reaction time (t = tr2). 

---