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===Fluorescent Demonstration===
===Fluorescent Demonstration===
By replacing the starch with a fluorescent dye, Weinberg (2007) produced a demonstration visible in total darkness under UV illumination<ref>{{cite journal|last=Weinberg|first=Richard B.|title=An Iodine Fluorescence Quenching Clock Reaction|journal=J. Chem. Educ.|date=2007|volume=84|pages=797|url=http://jchemed.chem.wisc.edu/journal/Issues/207/May/abs797.html}}</ref>.
By replacing the starch with a fluorescent dye, Weinberg (2007) produced a demonstration visible in total darkness under UV illumination<ref>{{cite journal|last=Weinberg|first=Richard B.|title=An Iodine Fluorescence Quenching Clock Reaction|journal=J. Chem. Educ.|date=2007|volume=84|pages=797|url=http://jchemed.chem.wisc.edu/journal/Issues/2007/May/abs797.html}}</ref>.


===Use as a biological assay===
===Use as a biological assay===

Revision as of 00:17, 8 July 2007

Oscillogram made in July 1972 by Briggs and Rauscher.

The Briggs-Rauscher oscillating reaction is one of a small number of known oscillating chemical reactions. It is especially well suited for demonstration purposes because of its visually striking color changes: the freshly prepared colorless solution slowly turns an amber color, suddenly changing to a very dark blue. This slowly fades to clear and the process repeats, about ten times in the most popular formulation, before ending as a dark blue liquid smelling strongly of iodine.

History

The first known homogeneous oscillating chemical reaction, reported by W. C. Bray in 1921,[1] was between hydrogen peroxide (H2O2) and iodate (IO3-) in acidic solution. Due to experimental difficulty, it attracted little attention and was unsuitable as a demonstration. In 1958 B. P. Belousov in the Soviet Union discovered the Belousov-Zhabotinsky reaction (BZ reaction),[2] which is suitable as a demonstration, but it too met with skepticism (largely because such oscillatory behavior was unheard of up to that time) until A. M. Zhabotinsky, also in the USSR, learned of it and in 1964 published his research[3]. In May of 1972 a pair of articles in the Journal of Chemical Education[4][5]brought it to the attention of two science instructors at Galileo High School in San Francisco. They discovered the Briggs-Rauscher oscillating reaction[6] by replacing bromate (BrO3-) in the BZ reaction by iodate and adding hydrogen peroxide. By then adding starch indicator they produced the very popular demonstration reaction seen today.

Description

Initial conditions

The initial aqueous solution contains hydrogen peroxide, an iodate, divalent manganese (Mn++) as catalyst, a strong chemically unreactive acid (sulfuric acid (H2SO4) or perchloric acid (HClO4) are good), and an organic compound with an active ("enolic") hydrogen atom attached to carbon which will slowly reduce free iodine (I2) to iodide (I-). (Malonic acid (CH2(COOH)2) is excellent for that purpose.) Starch is optionally added as an indicator to show the abrupt increase in iodide ion concentration as a sudden change from amber (free iodine) to dark blue (the "iodine-starch complex", which requires both iodine and iodide.)[7]

NOTES: The reaction is "poisoned" by chloride (Cl-) ion, which must therefore be avoided. The reaction will oscillate under a fairly wide range of initial concentrations. For recipes suitable for demonstration purposes, see Shakhashiri[8] or #Preparations.

Behavior in time

The reaction shows recurring periodic changes, both gradual and sudden, which are visible to the eye: slow changes in the intensity of color, interrupted by abrupt changes in hue. This demonstrates that a complex combination of slow and fast reactions are taking place simultaneously. For example, following the iodide ion concentration with a silver/silver iodide electrode[6] shows sudden dramatic swings of about 4 orders of magnitude separated by slower variations. This is shown by the oscillogram above.

Stirring the solution throughout the reaction is helpful for sharp color changes, otherwise spatial variations may develop(see #Videos). Bubbles of free oxygen are evolved throughout, and in most cases, the final state is rich in free iodine.

Variants

Changing the initial concentrations

As noted above, the reaction will oscillate in a fairly wide range of initial concentrations of the reactants.[9] For oscillometric demonstrations, more cycles are obtained in dilute solutions, which produce weaker color changes. See for example the graph, which shows more than 40 cycles in 8 minutes.

Changing the organic substrate

Malonic acid has been replaced by other suitable organic molecules,[10] such as acetone (CH3COCH3) or 2,4 Pentanedione (CH3COCH2COCH3) ("acetylacetone"). More exotic substrates have been used.[11][12] The resulting oscillographic records often show distinctive features, for example as reported by Szalai.(http://www.chem.elte.hu/departments/anal/szalai/chd.html)

Continuous flow reactors

The reaction may be made to oscillate indefinitely by using a continuous flow stirred tank reactor (CSTR), in which the starting reagents are continuously introduced and excess fluid is drawn.[13][14]

Two dimensional plot

By omitting the starch and monitoring the concentration of I2 photometrically, (i.e. measuring the absorption of a suitable light beam through the solution) while simultaneously monitoring the concentration of iodide ion with an Ag/AgI electrode[6], a distorted spiral XY-plot will result. In a continuous-flow reactor, this becomes a closed loop (limit-cycle).

Fluorescent Demonstration

By replacing the starch with a fluorescent dye, Weinberg (2007) produced a demonstration visible in total darkness under UV illumination[15].

Use as a biological assay

The reaction has been proposed as an assay procedure for antioxidants in foodstuffs.[16] The sample to be tested is added at the onset of oscillations, stopping the action for a period proportional to its antioxidant activity. Compared to existing assay methods, this procedure is quick and easy and operates at the pH of the human stomach.[17]

Chemical mechanism

The detailed mechanism of this reaction is quite complex.[18][9]. Nevertheless, a good general explanation can be given.

The essential features of the system depend on two key processes (These processes each involve many reactions working together):

  • A ("non-radical process"): The slow consumption of free iodine by the malonic acid substrate in the presence of iodate. This process involves the intermediate production of iodide ion.
  • B ("radical process"): A fast autocatalytic process involving manganese and free radical intermediates, which converts hydrogen peroxide and iodate to free iodine and oxygen. This process also can consume iodide up to a limiting rate.

But process B can operate only at low concentrations of iodide, creating a feedback loop as follows:

Initially, iodide is low and process B generates free iodine, which gradually accumulates. Meanwhile process A slowly generates the intermediate iodide ion out of the free iodine at an increasing rate proportional to its (i.e. I2) concentration. At a certain point, this overwhelms process B, stopping the production of more free iodine, which is still being consumed by process A. Thus, eventually the concentration of free iodine (and thus iodide) falls low enough for process B to start up again and the cycle repeats as long as the original reactants hold out.

The overall result of both processes is (again, approximately)[9]:

IO3- + 2H2O2 + CH2(COOH)2 + H+ → ICH(COOH)2 + 2O2 + 3H2O

The color changes seen during the reaction correspond to the actions of the two processes: the slowly increasing amber color is due to the production of free iodine by process B. When process B stops, the resulting increase in iodide ion enables the sudden blue starch color. But since process A is still acting, this slowly fades back to clear. The eventual resumption of process B is invisible, but can be revealed by the use of a suitable electrode[6].

A negative feedback loop which includes a delay (mediated here by process A) is a general mechanism for producing oscillations in many physical systems, but is very rare in nonbiological homogeneous chemical systems (The BZ oscillating reaction has a somewhat similar feedback loop).

References

  1. ^ W. C. Bray (1921). "A Periodic Reaction in Homogeneous Solution and Its Relation to Catalysis". J. Am. Chem. Soc. 43 (6): 1262–1267. doi:10.1021/ja01439a007.
  2. ^ Belousov, B. P. (1958), "A Periodic Reaction and Its Mechanism", Sbornik Referatov po Radiatsionni Meditsine, Medgiz, Moscow, p. 145. (transl. in Field and Burger, Op.Cit., below)
  3. ^ Zhabotinskii, A. M. (1964), "Periodic Liquid-Phase Oxidation Reactions", Dokl. Akad. Nauk SSR, 157, 392.
  4. ^ Field, R. J. (1972), "A Reaction Periodic in Time and Space", J. Chem. Educ., 49, 308.
  5. ^ Degn, Hans (1972), "Oscillating Chemical Reactions in Homogeneous Phase", J. Chem. Educ., 49, 302-307.
  6. ^ a b c d Thomas S. Briggs and Warren C. Rauscher (1973), "An Oscillating Iodine Clock", J. Chem. Educ. 50, 496.
  7. ^ J. A. Thoma and D. French (1960). "The Starch-Iodine-Iodide Interaction. Part I. Spectrophotometric Investigations". J. Am. Chem. Soc. 82 (16): 4144–4147. doi:10.1021/ja01501a004.
  8. ^ Shakhashiri, B. Z. (1992) Chemical Demonstrations: A Handbook for Teachers of Chemistry Vol. IV pp 3-13, University of Wisconsin Press, Madison, WI.
  9. ^ a b c Furrow, S. D. in Field, R. J. and M. Burger(1985), Oscillations and Traveling Waves in Chemical Systems, J. Wiley & Sons, New York.
  10. ^ S. D. Furrow (1995). "Comparison of Several Substrates in the Briggs-Rauscher Oscillating System". J. Phys. Chem. 99 (28): 11131–11140. doi:10.1021/j100028a013.
  11. ^ Furrow, Stanley D. (2002). "NewSubstrates for the Oscillating Briggs-Rauscher Reaction". J. Phys. Chem. A. 106: 5841–5850. doi:10.1021/jp0138779. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  12. ^ Szalai, Istvan (August 2006). "Briggs-Rauscher Reaction with 1,4-Cyclohexanedione Substrate". Z. Phys. Chem. 220 (8): 1071–1082. doi:10.1524/zpch.2006.220.8.1071.
  13. ^ A. Pacault, P. Hanusse, P. De Kepper, C. Vidal and J. Boissonade (1976). "Phenomena in homogeneous chemical systems far from equilibrium". Acc. Chem. Res. 9 (12): 438–445. doi:10.1021/ar50108a003.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  14. ^ Merino, J. M.(1992), "A simple,continuous-flow stirred-tank reactor for the demonstration and investigation of oscillating reactions", J. Chem Educ. 69, p. 75.
  15. ^ Weinberg, Richard B. (2007). "An Iodine Fluorescence Quenching Clock Reaction". J. Chem. Educ. 84: 797.
  16. ^ R. Cervellati, K. Höner, Stanley D. Furrow, C. Neddens and S. Costa (2001). "The Briggs-Rauscher Reaction as a Test to Measure the Activity of Antioxidants". Helvetica Chimica Acta. 84 (12): 3533–3547. doi:10.1002/1522-2675(20011219)84:12%3C3533::AID-HLCA3533%3E3.0.CO;2-Y. {{cite journal}}: Unknown parameter |doilabel= ignored (help)CS1 maint: multiple names: authors list (link)
  17. ^ R. Cervellati, C. Renzulli, M. C. Guerra and E. Speroni (2002). "Evaluation of Antioxidant Activity of Some Natural Polyphenolic Compounds Using the Briggs-Rauscher Reaction Method". J. Agric. Food Chem. 50 (26): 7504–7509. doi:10.1021/jf020578n.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  18. ^ R. M. Noyes and S. D. Furrow (1982). "The oscillatory Briggs-Rauscher reaction. 3. A skeleton mechanism for oscillations". J. Am. Chem. Soc. 104 (1): 45–48. doi:10.1021/ja00365a011.

Videos

Preparations

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