Oscillating response

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In chemistry, an oscillating reaction is understood to be a form of the course of complex chemical reactions in which the concentration of intermediate products and catalysts does not assume a steady state, but rather shows periodic fluctuations. This is a special case of dissipative structures . Under certain conditions, non- monotonous changes can also occur - the system then behaves chaotically . A distinction can be made between oscillating reactions in heterogeneous and homogeneous media, the heterogeneous reactions being explicitly linked to phase interfaces. Oscillating reactions are common. However, one is interested in them not only for theoretical reasons (the best-known example is the Belousov-Zhabotinsky reaction ) or technical reasons (reaction management in the chemical industry ), they are of immense importance for life. They act as a clock for periodic processes ( sinus nodes of the excitation system of the heart ) or synchronize the nerve activity in the brain . They also play an important role in the electrochemical dissolution of metals into acid and the oxidation of carbon monoxide, hydrogen sulfide and hydrocarbons.

history

Color change of a Belousov-Zhabotinsky reaction

While there was initially only a broad interest in oscillating reactions in systems in homogeneous media, it was heterogeneous systems in which such behavior was observed first - and very early. The first description of such a reaction, the polarization reversal in the iron / silver electrode pair in nitric acid silver nitrate solution, was published in 1828 by Gustav Theodor Fechner . In 1829 Friedlieb Ferdinand Runge described the beating mercury heart in the form we know today. In 1833, John FW Herschel , known as the astronomer and inventor of the cyanotype , discovered periodic reactions in the dissolution of iron in nitric acid for certain concentrations of the acid. Oscillating reactions are often to be found in electrochemical processes , where they were also reported early and numerous, so u. a. 1842 by Christian Friedrich Schönbein or 1844 by James Prescott Joule .

Interestingly, at the beginning of the twentieth century, the chemist Alfred J. Lotka was theoretically concerned with periodic reactions. In this work, which at the time also went unnoticed, he presents an autocatalytic reaction scheme that finds equilibrium in an oscillating manner. Lotka later made important contributions to population dynamics ( Lotka-Volterra rules , pig cycle), the mathematical modeling of which contains extensive analogies to that of oscillating reactions.

JS Morgan discovered in 1916 in the reaction of formic acid (methanoic acid) with conc. Sulfuric acid to carbon monoxide and water under certain conditions an oscillating gas evolution. If the escaping carbon monoxide is ignited, then one can observe the flame getting bigger and smaller. This oscillation is attributed to a supersaturation of the solution and subsequent removal of the supersaturation.

The first oscillating reaction in a homogeneous medium, the Bray-Liebhafsky reaction , was described in 1921 by William C. Bray . He examined the catalytic decomposition of hydrogen peroxide in the presence of iodate and noticed a periodically fluctuating evolution of oxygen. However, the publication received little attention; It was also claimed that the periodic behavior was based on impurities that would create heterophase interfaces. At the time, the latter was considered a prerequisite for such oscillations to occur.

It was only when Boris Belousov was very hesitant about a publication - again almost unnoticed in a non-specialist paper, since the relevant specialist journals refused to accept his article or Belousov could not accept their extensive revision proposals - leading to research into the homogeneous bromate / cerium (IV) system -Salt / malonic acid , beginning with the work of Anatoli Schabotinski in 1964, showed that such homogeneous reactions do exist. In 1972 Richard J. Field , Endre Körös and Richard M. Noyes published a mechanism ( FKN mechanism ) for modeling the Belousov-Zhabotinsky reaction : with their list of 18 partial reactions with 21 participating species, they demonstrated the high complexity of this system.

In 1973 Briggs and Rauscher discovered an impressive oscillating reaction ( Briggs-Rauscher reaction ) when they combined the Bray-Liebhafsky reaction and Belousov-Zhabotinsky reaction. This oscillating ioduhr showed a rhythmic color change between colorless, golden brown and deep blue.

requirements

The occurrence of an oscillating reaction is tied to certain conditions:

  • The system is far from thermodynamic equilibrium (high exergonic reaction, ΔG <0).
  • The system has at least one reaction step that includes positive / negative feedback (e.g. through autocatalysis or autoinhibition ) and thereby creates a non-linear relationship.
  • The system must be open to the exchange of substances and energy with the environment.

Non-linearity of the response

The non-linearity can be caused, for example, by autocatalytic sub-steps, rhythmic passivation of the electrodes (in electrochemical processes), or temperature changes. An effect occurs that is referred to as feedback in control engineering. The results of certain sub-steps (e.g. change in temperature / concentration / electrode state) have an effect on the rate constants or concentrations and thereby accelerate or delay the reaction process.

Modeling

Since a large number of individual coupled steps are always necessary, the mechanisms of the oscillating reaction are not exactly known to this day. However, there are various reaction models that show oscillation under certain conditions. The simplest model of a reaction A → B comes from Alfred J. Lotka :

(1) (2) (3)

The substeps (1) and (2) are autocatalytic reactions. They cause the feedback.

Another model for the reaction sequence of the oscillating reaction is the Brusselsator developed by Ilya Prigogine and co- workers .

A realistic minimal oscillating chemical reaction system contains only mono- and bimolecular chemical reactions (the Brusselsator contains a trimolecular reaction) and is dissipative (the Lotka system is not dissipative). A minimal system from a mathematical point of view contains three reactants and five irreversible reactions and a minimal system from a chemical point of view contains only three reactants and four irreversible reactions (see discussion in).

Bistability

In addition to oscillation, bistabilities sometimes also occur in such a system. There are two stable reaction states (one with a high reaction rate / another with a low reaction rate or intermediate concentration) that the system can optionally adopt. If the system is disturbed from the outside, one state may be preferred.

Electrochemical oscillations

The pulsating mercury heart is basically an electrochemical oscillation, as it combines two metals (mercury and iron) in one local element .

A galvanic cell normally generates a direct voltage . The following experiment generates a pulsating voltage under certain conditions: A lead dioxide electrode, as found in a lead accumulator, is combined with a stainless steel electrode that is coated with platinum and palladium . Diluted sulfuric acid is used as the electrolyte . In addition, the stainless steel electrode is surrounded by a formaldehyde solution (methanol solution ) and acts as a fuel anode . When this galvanic cell is loaded, the following overall reaction occurs:

At the cathode, lead (IV) in the lead dioxide is reduced to lead (II) in the lead sulfate with the absorption of electrons . At the same time, formaldehyde is oxidized to water and carbon dioxide at the anode , releasing electrons. The pulsating voltage comes about because the anode is coated with intermediate products of the oxidation. This reduces the voltage generated and the potential at the anode becomes more positive. The intermediate products are then further oxidized, the electrode surface is exposed again and the voltage rises again until the electrode surface is again covered by intermediate products. Depending on the externally applied load resistance, the result is a sawtooth voltage between 0.3 and 0.6 Hz , which can be made visible with an oscilloscope . A connected light bulb flickers to the rhythm of the pulsating tension.

Another example is a galvanic element made from iron and copper sheet with an electrolyte made from a potassium bromate solution in dilute sulfuric acid . Here, iron is oxidized to iron (II) ions, the bromate ions being reduced to bromide ions . The measured voltage pulsates, which can be explained by the fact that the iron electrode is briefly covered by an extremely thin oxide layer and thus passivated. This leads to a sudden increase in potential. In the next step, the oxide layer is dissolved again by hydrogen ions, the iron is activated again and the potential drops. This cycle is run through several times.

Occurrence in biological systems

Many biochemical reactions have the prerequisites ( e.g. through competitive inhibition of enzymes) to oscillate under given conditions. This was observed e.g. B. in reactions of glycolysis or cellular respiration . The timing of the heartbeat is also based on oscillating reactions (see sinus node ). The biological rhythms examined in chronobiology are oscillations with considerably longer periods beginning in the minute range (see also circadian rhythms , ultradian rhythms ). These do not only depend on external factors (sunlight, temperature); rather, the timing is generated in certain cell areas (e.g. in the suprachiasmatic nucleus ). The exact biochemical background is still in the dark. Oscillating systems in living beings can also be visualized as complex two- and three-dimensional structures, such as those formed by intracellular Ca 2+ waves.

See also

Web links

Individual evidence

  1. ^ MG Th. Fechner: About reversals of polarity in the simple chain. In: Schweiggers Journal for Chemistry and Physics. 53, 1828, pp. 129-151.
  2. ↑ Hartwig Möllencamp, Bolko Flintjer & Walter Jansen: 200 years of “Pulsating Mercury Heart ”: On the history and theory of a fascinating electrochemical experiment. In: Chemkon. Volume 1, No. 3, 1994, DOI: 10.1002 / ckon.19940010303 , pp. 117-125.
  3. ↑ JFW Herschel: Note sur la manière d'agir de l'Acide nitrique sur le Fer. In: Annales de chimie et de physique. 54, 1833, pp. 87-94.
  4. ^ AJ Lotka: Contribution to the theory of periodic reactions. In: J. Phys. Chem. 14, 1910, pp. 271-274.
  5. ^ H. Brandl: Oscillating chemical reactions and structure formation processes. Aulis, Cologne 1987, ISBN 3-7614-0993-1, p. 70.
  6. ^ WC Bray: A Periodic Reaction in Homogeneous Solution and Its Relation to Catalysis. In: J. Am. Chem. Soc. 43, 1921, pp. 1262-1267.
  7. ^ BP Belousov: A Periodic Response and its Mechanism (in Russian). In: Sbornik referatov po radiatcionnoj meditsine za 1958 god. 147, 1959, p. 145.
  8. ↑ AM Zhabotinsky: The periodic course of the oxidation of malonic acid in solution (in Russian). In: Biofizika. 9, 1964, p. 306.
  9. ^ RJ Field, E. Körös, RM Noyes: Oscillations in Chemical Systems II. Thorough Analysis of Temporal Oscillation in the Bromate-Cerium-Malonic Acid System. In: J. Am. Chem. Soc. 94, 1972, pp. 8649-8664.
  10. ↑ Field / Schneider: Oscillating chemical reactions and nonlinear dynamics, Chemistry in our time, 22nd year, 1988, No. 1, p. 17.
  11. ^ H. Brandl: Oscillating chemical reactions and structure formation processes. Aulis, Cologne 1987, ISBN 3-7614-0993-1, p. 45.
  12. ^ T. Wilhelm, S. Schuster, R. Heinrich: Kinetic and thermodynamic analyzes of the reversible version of the smallest chemical reaction system with Hopf bifurcation, Nonlinear World, 1997, 4, pp. 295–321.
  13. ^ T. Wilhelm, R. Heinrich: Smallest chemical reaction system with Hopf bifurcation, J. Math. Chem., 1995, 17, pp. 1-14.
  14. ^ T. Wilhelm: The smallest chemical reaction system with bistability, BMC Syst. Biol., 2009, 3, 90.
  15. ↑ Schwarzer / Vogel / Hamann: Electrochemical direct generation of pulsating voltages, Chemistry in our time, 8th year 1974, No. 6, p. 173.
  16. ↑ M. Oetken, M.Ducci: An impossible battery - the alternating current battery, Praxis der Naturwissenschaften Chemie 1/49. Year 2000, p. 16.
  17. ↑ S. Honma, K. Honma: The biological clock: Ca2 + links the pendulum to the hands. In: Trends in Neurosciences 26, pp. 650-653, 2003.
  18. ↑ J. Lechleiter, S. Girard, E. Peralta, D. Clapham: Spiral calcium wave propagation and annihilation in Xenopus laevis oocytes. In: Science 252, 123-6, 1991.