# Very slow chemical clock reaction

I'd like to preface this with a declaration of my understanding that this is not necessarily possible.

I was wondering if anyone knew of a chemical clock reaction where the period before the first change (the induction period?) is very long (between 3 hours and a day) and the change is à la Landolt or Old Nassau reactions (i.e they cross a concentration threshold to create sudden color change).

Bonus Question: Is there any remote possibility that a reaction like this can happen in pigments or dyes once they are absorbed by a cloth?

• I definitely remember about reaction that took about 2 hours and only visible changes started to appear at the end of 2-nd hour, about 10 minutes to end of time period. Don't remember the details, but reaction was autocatalytic, so at the end it runs like crazy and very slow at the beginning. The idea is quite simple, so reactions of this type should be common. Problem: usually reactions used are exotermic, meaning that reaction mixture for autocatalytic process can easily end on your ceiling. Unfortunately, I cannot provide you with specific example at the moment. – permeakra Aug 25 '12 at 15:43

I assume you are aware of the common clock-reactions, such as the iodine clock, etc. By varying concentration of reagents and pH of the solution, you could run several repetitions to generate a trend-line, and from that extrapolate the estimated pH and reagent concentration to provide you with the time delay you are looking for. The iodine clock is a two-step reaction as outlined below:

In the first, slow reaction, iodine is produced:

$$\ce{H2O2 + 2I- + 2H+ -> I2 + 2H2O}$$

In the second, fast reaction, iodine is reconverted to 2 iodide ions by the thiosulfate:

$$\ce{2S2O3^2- + I2 -> S4O6^2- + 2I-}$$

By decreasing pH in reaction 1 and increasing the thiosulfate level in reaction 2, you should be able to lengthen the ‘clock time’. Let me know how things turn out!

• I am wondering if really long timed clock reactions would be affected by ambient oxygen dissolving into the solution and oxidizing some species. – uLoop Aug 7 '16 at 13:59