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Clock reactions

Töilet Plünger - 27-2-2014 at 21:35

Is it possible to reverse the iodine clock reaction so that the solution starts off as black and suddenly turns clear? I'm guessing it's possible because the Briggs-Rauscher reaction turns clear in its last step of the cycle.

I would think for a demo, beaker A should contain hydrogen peroxide, sodim triiodide, and starch, and beaker B should have sulfuric acid and sodium thiosulfate.

I also want to figure out how to design a clock reaction. The iodine clock looks cool, but I'd like to be able to change the reaction's steps so that different reagents can be used and different colors can be made. Complexation reactions are interesting, but I'm not sure what kind of competing reactions would be needed.

[EDIT: expanded subject]

[Edited on 2014228 by Töilet Plünger]

thesmug - 27-2-2014 at 22:20

Quote: Originally posted by Töilet Plünger

Is it possible to reverse the iodine clock reaction so that the solution starts off as black and suddenly turns clear? I'm guessing it's possible because the Briggs-Rauscher reaction turns clear in its last step of the cycle.

I would think for a demo, beaker A should contain hydrogen peroxide, sodim triiodide, and starch, and beaker B should have sulfuric acid and sodium thiosulfate.

As far as I know that's not possible.

blogfast25 - 28-2-2014 at 01:40

I think it might be possible to engineer something like that but it's obviously not easy or it would be all over UToob. The original iodine clock is about competing reactions, one much faster than the other.

You'd need something like this:

I2 + A === > 2 I- + C ... R1

A + B === > D ... R2

With R2 much faster than R1. When B is depleted the remainder of A then eliminates the I2. Something like that...

Töilet Plünger - 28-2-2014 at 06:54

I just realized thiosulfates aren't stable in acidic solution. So a different reducer will have to be found.

And what happens when you run the reaction without starch? Other than color, does it make a significant difference?

MrHomeScientist - 28-2-2014 at 09:02

It would probably be clear -> amber. The starch should only affect the color, since all it does is complex with triiodide to make that very dark blue. I imagine it would be like the intermediate stage in the oscillator, as some shade of amber.

blogfast25 - 28-2-2014 at 12:42

The complex is apparently a clathrate:

http://en.wikipedia.org/wiki/Clathrate_compound

One reducing agent that does reduce iodine to iodide in acid medium is Sn2+, like SnCl2 (aq). It's been used in iodometric titrations at low pH.

[Edited on 28-2-2014 by blogfast25]

Töilet Plünger - 28-2-2014 at 15:13

I have tin(II) salts; I'll see if I can try this myself. Would ascorbic or citric acid be a good reducer as well?

blogfast25 - 1-3-2014 at 04:11

Quote: Originally posted by Töilet Plünger

Would ascorbic or citric acid be a good reducer as well?

I'm not sure but I doubt it.