Mechanism of the BZ reaction

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What the BZ reaction (BZR) is?

From the viewpoint of physicist this is a nonlinear chemical system which maintains oscillations and propagating pulses (chemical waves).
Typical scales are:

Period of oscillations - 1 min
Wavelength - 1 mm
Velocity - 1 mm/min

See an example of the dynamics (mpeg movie) on my home page.
Click here to read some historical notes about the BZR

How to prepare the BZR?

The simplest way is to mix the following solutions:

Malonic Acid - 0.2 M/L
Sodium Bromate - 0.3 M/L
Sulfuric Acid - 0.3 M/L
Ferroin - 0.005 M/L

If add a droplet (5 ml) of the solution to a container (Petri dish, 6 cm in diameter) so that the thickness of the layer is 0.5-1 mm, there appears colorful spatio-temporal patterns. In thicker layers there is an interference of hydrodynamic flows with the reaction.

Why the BZR maintains oscillations? (zero-dimensional case)

The BZR involves more than 40 elementary reactions which result in changes of several dozens of intermediate substances. Fortunately, the basic behavior is understandable if take into account just several elementary processes. Consider the following scheme (tiny modified of the scheme proposed by Professor A.M.Zhabotinsky):

The main substances here are HBrO2 = Bromous Acid; Br-= Bromide ion; ferroin and its oxidized form - ferriin.

STEP-1:

(upper part of the scheme) Bromous Acid is involved into an autocatalytic reaction, its amount increases exponentially with time, like in an explosion process. The other result of this process is the production of ferriin (oxidized form of ferroin). For an observer the latter process is seen as a change in color from red (ferroin's color) to blue (ferriin's color).

STEP-2:

when the amount of ferriin becomes large, due to its interaction with organic components, ferriin starts to change back slowly to ferroin (physically this is just one electron transportation into a molecule), AND Bromide ions are produced during this reaction (lower part of the scheme).

STEP-3:

Bromide is an effective inhibitor of the autocatalytic process. Because of Bromide, the production of Bromous Acid is stopped and its amount is reduced.

STEP-4:

The active processes in the system is started at STEP-2 slow change of ferriin to ferroin (blue changes to red). With time the medium turns red, Bromide ions are binded, and the system is able to repeat the STEP-1.

The presented scheme is just a sketch of real processes occurring in the system. Nonetheless, it gives basic understanding of the dynamics and allows to locate the main substances involved into the reaction: Bromous Acid, Bromide ions and ferroin.

How the main components of the BZR change with time?

The evolution of main components is sketched as:

X-axis stands for time and Y-axis - concentrations. Digits in the right-hand part corresponds to the BZR steps described above. The main observation here, is that there are two different time scales, i.e. there are fast processes (time of order of Tfr), and slow processes (of order Tref, subscript "ref" means refractoriness).