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Clock reactions

Clock reactions represent a unique class of chemical reactions – Landolt’s clock reaction being the most known of them. In this particular reaction, two colorless solutions are mixed and nothing happens at the first moment. However, after a few seconds the solution suddenly changes its color to dark blue. This is a so called clock mechanism: we cannot observe any visual changes immediately after reactants are mixed, but color changing takes place suddenly after some time – this time is known as induction period.

Clock reaction mechanisms can be explained by considering a hypothetic system which consists of four reactants: A, B, C and D. In this system, reactant A reacts with reactant B to produce a product P:

The formed product P than reacts with reactant C and is therefore consumed – yielding product R:

If the concentrations of A and B are sufficiently high (regarding to the concentration of C), the overall effect of the Reaction 1 and Reaction 2 is consumption of reactant C. As soon as the reactant C is completely removed from the reaction system, the product of the Reaction 1 (P) cannot further reacts. Out of this reason, concentration of product P raises rapidly. At this critical point, the concentration of P molecules is high enough to reacts with the fourth reactant D. In this reaction a colored product is formed and this is the moment when we observe the sudden change in solution color:

However, it is necessary that Reaction 2 is much faster than Reaction 3, thus allowing the product P to reacts selectively with C, but not with D.

In the most known Landolt's clock reaction, potassium iodate and sulfite reacts with each other according to the clock mechanism to yield iodine, which than reacts with starch producing a dark blue colored complex. Various combinations of reactants that follow the clock mechanism are described in this section.

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