Briggs-Rauscher reaction

Oscillogram recorded in July 1972 by Briggs and Rauscher

The Briggs-Rauscher reaction (also known as the oscillating iodine clock ) is one of the small number of known reactions that oscillate in homogeneous solution . The reaction is particularly suitable because of the color changes to illustrate the mechanism of oscillating reactions. The color changes run from colorless through yellowish to deep blue, which returns to the colorless initial state in order to run through the cycle several times.

history

The first homogeneous oscillating chemical reaction was found by WC Bray in 1921. It is a reaction of hydrogen peroxide (H ₂ O ₂ ) and iodate (IO ₃ ^- ) in acidic solution. Because of the experimental difficulties, the reaction received little attention and was not suitable for demonstrating the effect. In 1958, BP Belousov discovered the Belousov-Zhabotinsky reaction in the Soviet Union , which was also received with skepticism, as oscillating reactions were unknown up to this point in time. AM Zhabotinsky published his research on this reaction in 1964 and in May 1972 in the Journal of Chemical Education. These publications became aware of the reaction to two scholars at Galileo High School in San Francisco. They then developed the oscillating Briggs-Rauscher reaction by replacing the bromate of the Belousov-Zhabotinsky reaction with iodate and starch as indicators. The so-called oscillating iodine clock showed the reactions taking place.

mechanism

Play media file

The detailed reaction mechanism is complex. Two key processes can be described as important steps:

• a non-radical process A: the slow reaction of free iodine and malonic acid in the presence of iodate to produce an intermediate iodide ion.

• a radical process B: a fast autocatalytic process of the manganate and free radicals, which convert the water and iodate into free iodine and oxygen. This process consumes iodine up to the rate-limiting step. Process B can only occur at low concentrations of iodide, which otherwise triggers a feedback process.

Initially, the iodide concentration is low and process B generates free iodine which slowly accumulates. Meanwhile, process A slowly generates the intermediate iodide ion from the free iodine at an increasing rate. At a certain point, the reaction suppresses process B and thus stops the production of free iodine (and thus iodide) until the concentration has decreased so far that process B runs again. The process continues until all reactants are used up.

The reaction can be roughly described by the following equation:

${\displaystyle \mathrm {IO_{3}^{-}+\ 2\ H_{2}O_{2}\ +CH_{2}(COOH)_{2}+\ H^{+}\to \ ICH(COOH)_{2}+2\ O_{2}+3\ H_{2}O} }$

The change in color of the process corresponds to the kinetics of the two processes. The slowly increasing yellowish color is due to the production of free iodine by process B. When process B stops, the increasing concentration of iodide ions leads to a color change to blue, since both iodine and iodide ions are necessary for the formation of the starch complex . Because process A is still running, the color change goes back to the colorless solution. The eventual resumption of process B is not visible, but can be detected electrochemically.

Negative feedback with a delay mediated by process A is the general mechanism to generate oscillating reactions in physical systems, but is rarely observed in non-biological homogeneous chemical reactions.

Web links

• A detailed history of oscillating reactions ( Memento from September 16, 2008 in the Internet Archive ) (PDF; 177 kB).

Videos

• Reaction unmoved with small spatial changes
• Unmoved variant with major spatial changes ( Memento from June 26, 2008 in the Internet Archive )
• 19 cycles of the Briggs-Rauscher reaction

Temperature effects

• Series of 4 videos to demonstrate the influence of temperature: 10 ° C 22 ° C 40 ° C 60 ° C

preparation

• Demonstration from about.com
• Demo by John A. Pojman (use of 3% H ₂ O ₂ )

Individual evidence

1. ^ WC Bray: A Periodic Reaction in Homogeneous Solution and Its Relation to Catalysis . In: J. Am. Chem. Soc. . 43, No. 6, 1921, pp. 1262-1267. doi : 10.1021 / ja01439a007 .
2. ↑ BP Belousov: Sbornik Referatov po Radiatsionni Meditsine In: Medgiz. Moscow 1958, p. 145. (Translated into English: A Periodic Reaction and Its Mechanism In: RJ Field, M. Burger (Ed.): Oscillations and Traveling Waves in Chemical Systems. J. Wiley & Sons, New York 1985).
3. ↑ AM Zhabotinskii: Периодические окислительные реакции в жидкой фазе (Periodic oxidation reactions in liquid phase) . In: Doklady Akademii Nauk SSSR . 157, No. 2, 1964, pp. 392-393.
4. ↑ RJ Field: A reaction periodic in time and space. A lecture demonstration. In: Journal of Chemical Education. 49, 1972, p. 308, doi : 10.1021 / ed049p308 .
5. ↑ Hans Degn: Oscillating chemical reactions in homogeneous phase. In: Journal of Chemical Education. 49, 1972, p. 302, doi : 10.1021 / ed049p302 .
6. ↑ ^{a b} Thomas S. Briggs, Warren C. Rauscher: An oscillating iodine clock. In: Journal of Chemical Education. 50, 1973, p. 496, doi : 10.1021 / ed050p496 .
7. ^ RM Noyes, SD Furrow: The oscillatory Briggs-Rauscher reaction. 3. A skeleton mechanism for oscillations . In: J. Am. Chem. Soc. . 104, No. 1, 1982, pp. 45-48. doi : 10.1021 / ja00365a011 .
8. ^ ^{A b} S. D. Furrow: Chemical Oscillators Based on Iodate Ion and Hydrogen Peroxide. In: RJ Field, M. Burger (Ed.): Oscillations and Traveling Waves in Chemical Systems. J. Wiley & Sons, New York 1985.