Clock, chemical

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 ... 

Wagner, G. A. (1999), Age Determination of Young Rocks and Artifacts, Physical and Chemical Clocks in Quaternary Geology and Archaeology, Springer, New York. 

The field of oscillating reactions, or periodic reactions, or chemical clocks, came out of this background indeed quite a number of chemical systems have been described, which show this oscillating, periodic, regular behavior (Field, 1972 Briggs and Rauscher, 1973 Shakhashiri, 1985 Noyes, 1989 Pojman etal, 1994 Jimenez-Prieto etal., 1998). 

The induction period, followed by a sharp increase in rate is, however, the most characteristic feature of autocatalysis in closed vessels. One manifestation of this behaviour is the clock reaction . An experimental system which is a typical chemical clock and which also exhibits cubic autocataiysis is the iodate-arsenite reaction. In the presence of excess iodate, the system which is initially colourless eventually undergoes a sudden colour change to brown (or blue in the presence of starch). The potential of an iodide-sensitive electrode shows a barely perceptible change during most of the induction period, but then rises rapidly, reaching a peak at the point of colour change. 

What is described above is an idea of the so-called chemical clock, that is a reaction with periodic (oscillating) change of reactant concentrations its period could be estimated as 5t > nc/p. In the condensed matter theory a leap in densities is interpreted as phase transitions of the first order. From this point of view, the oscillations correspond to a sequence in time of phase transitions where the two phases (i.e., big clusters of A s containing inside rare and small clusters of B s and vice versa) differ greatly in their structures. 

In the case of a chemical clock, the asymptotic (f -A oo) solution depends on time, there are not only singular points but also singular trajectories. An example is the stable limit cycle - Fig. 2.4, i.e., a closed trajectory to which all phase trajectories existing in its vicinity strive. 

Of interest is the study of conditions under which such a limit cycle emerges in a system. The chemical clock serves as an example of the so-called temporary structures study which was stimulated by a fundamental problem of order emerging from chaos. In the last decade it became a central part of a new discipline called synergetics [1, 21, 22],... 

A spectacular kinetic effect, known as the chemical clock , has been observed when an acidified sulfite solution is reacted with iodate in the presence of the starch indicator... 

As a certain concentration of CHBr (C02H)2 is needed for reaction 9 to occur long induction period for oscillations is expected, a phenomenon, which is also observed experimentally. During this induction period, the concentration of Br" is small and mechanism II dominates due to the slow conversion of Ce4+ into Ce3+ and the accumulation of brommalonic acid (reaction 8). Step 9 (8.71) results in the change of the blue color of solution to red resetting the chemical clock for the next oscillation. In fact, the oxidized form of the catalyst can also react directly with malonic acid, so there may be less than one bromide ion per cerium (III) ion produced. 

Nonclassical Carbocations Real or Are Our Chemical Clocks Too Slow ... 

The natural amino acids asparagine and glutamine provide chemical clocks for the disintegration of protein-based plastics they have the... 

Examples of protein-based plastics with transition temperatures below 0°C are given in Figure 2.22 and Figure 1.10 they remain strong plastics in water until the chemical clocks, that is, the timed conversion of carboxamides to carboxylates, raise the transition temperature from below to above the temperature of the water. At this point, the plastics, having fulfilled their useful life, gracefully soften and disappear. 

Programmed Biodegratlation of Matrices Due to Presence of Chemical Clocks... 

In Vitro Swelling of Plastic and Elastic States due to Chemical Clocks... 

Bromate and iodate salts are prepared on a much smaller scale than chlorates. Under appropriate conditions, these ions undergo oscillating chemical reactions known as chemical clocks. The best known clock reaction is observed when an acidified solution of sodium sulfite (Na2S03) is mixed with an excess of iodate in the presence of starch indicator. After a suitable induction period allowing for sodium sulfite reduction of iodate to iodide [Eq. (44)], the blue, starch-iodine color periodically appears and disappears as the iodide is oxidized to iodine [Eq. (45)], and the iodine is reduced back to iodide [Eq. (46)]. 

The chemical oscillator is nowadays a typical oscillator. A respectable number of them are now known, and this is continually on the increase. It constitutes a dissipative oscillator (i.e. limit cycle), whose motion must be maintained by a constant supply of fresh reactants. Its amplitude and period are extremely sensitive to the conditions. An amusing application of this is the setting up of a real chemical clock (26). The observed oscillations usually have a very marked relaxational character. They frequently comprise a single arch. There is, however, also a series that results from the periodic repetition of a basic motif comprising... 

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