Landolt clock reaction

The system used in the Landolt clock reaction, IOj/SOl , when treated with [Fe(CN)6] , shows oscillations in a CSTR. A modification of the mechanism proposed previously resolves a number of problems. A slightly different system, I03/HS03/S20i produces a small number of high-amplitude pH oscillations (up to 2 units of pH) over a narrow range of conditions in a closed system. ... 

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. 

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)... 

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... 

The dependence of reaction rates on pH and on the relative and absolute concentrations of reacting species, coupled with the possibility of autocatalysis and induction periods, has led to the discovery of some spectacular kinetic effects such as H. Landolt s chemical clock (1885) an acidified solution of Na2S03 is reacted with an excess of iodic acid solution in the presence of starch indicator — the induction period before the appearance of the deep-blue starch-iodine colour can be increased systematically from seconds to minutes by appropriate dilution of the solutions before mixing. With an excess of sulfite, free iodine may appear and then disappear as a single pulse due to the following sequence of reactions ... 

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