Landolt reaction

1 Drivers Ibr Performing Landolt Reactions in Micro Reactors 

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Figure A3.14.2. Characteristic features of a clock reaction, illustrated for the Landolt reaction, showing (a) variation of product concentration witii induction period followed by sharp reaction event (b) variation of overall reaction rate witli course of reaction.

The Landolt reaction (iodate + reductant) is prototypical of an autocatalytic clock reaction. During the induction period, the absence of the feedback species (Irere iodide ion, assumed to have virtually zero initial concentration and fomred from the reactant iodate only via very slow initiation steps) causes the reaction mixture to become kinetically frozen . There is reaction, but the intemiediate species evolve on concentration scales many orders of magnitude less than those of the reactant. The induction period depends on the initial concentrations of the major reactants in a maimer predicted by integrating the overall rate cubic autocatalytic rate law, given in section A3.14.1.1. 

Beneficial Micro Reactor Properties for Landolt Reactions... 

Landolt Reactions Investigated in Micro Reactors Oiganic synthesis 93 [OS 93] Modified Landolt reaction... 

Bassam Z. Shakhashiri, "Color Variations of the Landolt Reaction," Chemical Demonstrations, A Handbook for Teachers of Chemistry, Vol. 4 (The University of Wisconsin Press, Madison, 1992), pp. 26-28. Three pairs of colorless solutions are mixed. After 10 seconds the three mixtures turn red, yellow, and blue, respectively. Each clock reaction is the reduction of iodate by bisulfite. The observed colors are related to the amount of triiodide produced, which depends on the relative amounts of iodate and bisulfite reacted, and whether or not starch is present. 

We now turn to the selection of reactions exhibiting autocatalysis. Chemical reactions with abrupt, non-linear changes of concentrations have been known for some time. They are the spectacular clock reactions, first described by Landolt in the nineteenth century41. The key to understanding the sudden and predictable (you could set your watch by their occurrence, hence the name clock ) color changes was provided over 60 years ago by Eggert and Schamow42. They analyzed the Landolt reaction (IV) (in the presence of excess iodate)... 

Other work dealing with this reaction includes a study of the exchange reaction in fused alkali nitrate solutions and an examination of the effect of sulphide ion on catalysis by vanadium(V) and molybdenum(VI) . Another group of experimental investigations of the iodate-iodide reaction has been primarily concerned with the Landolt reaction (p. 388). These are satisfactorily interpreted in terms of Dushman s original rate expression. 

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... 

LandolTs reaction can be demonstrated in the following manner Dissolve 1 8 grm. of iodic acid in a litre of water also prepare a litre of an aq. soln. of 0 9... 

Detailed studies on the Landolt reaction and on its use for lecture demonstrations were carried out between 1917 and 1922 by J. Eggert and A. Skrabal. The reaction mechanism presented below was confirmed. Skrabal writes as follows. ... 

All these requirements can be realised with the Landolt reaction. 

E.C.Edblom, L.Gyorgyi, M.Orban and I.R.Epstein, A Mechanism for Dynamical Behavior in the Landolt Reaction with Ferrocyanide, Journal of the American Chemical Society, 109,4876-4880(1987). 

Kinetic catalytic methods for determination of species can be classified in a manner similar to that of the kinetic noncatalytic methods described elsewhere in this encyclopedia (Table 1). Methods commonly used to measure induction periods are commented on in dealing with Landolt reactions below. 

Homogeneous catalytic reactions can be classified into two broad categories (1) ordinary catalytic reactions and (2) Landolt reactions. 

Consequently, kx>ki and P will only be detected after L (the Landolt reagent) has disappeared altogether in the second reaction. If reaction [VII] is accelerated by a catalyst, then the time elapsed until the product (P) appears will be a measure of the catalyst concentration, which is directly related to the induction period, t, by an empirical equation of the form [Cjo = kjti or [Cjo = depending on the particular system. Some slow reactions can be converted into Landolt reactions by using a retardant as the Landolt reagent. 

Edblom, E. C. Orban, M. Epstein, I. R. 1986. A New lodate Oscillator The Landolt Reaction with Ferrocyanide in a CSTR, J. Am. Chem. Soc. 108, 2826-2830. Edelson, D. Allara, D. L. 1980. A Computational Analysis of the Alkane Pyrolysis Mechanism Sensitivity Analysis of Individual Reaction Steps, Int. J. Chem. Kinet. 12, 605-621. 

The expression fast reactions is obviously ambiguous. We can clarify its usage here by referring to the familiar iodine clock reaction (Landolt reaction). If the reducing agent is denoted by R and its oxidized form by Ox, the principal reaction steps of the Landolt reaction may be written as... 

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