# Chemists! Why doesn't my iodine clock reaction turn blue?
**URL:** https://boards.straightdope.com/t/chemists-why-doesnt-my-iodine-clock-reaction-turn-blue/647627
**Category:** Factual Questions
**Created:** [January 18, 2013, 4:58pm UTC](https://boards.straightdope.com/t/chemists-why-doesnt-my-iodine-clock-reaction-turn-blue/647627 "2013-01-18T16:58:16Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@Billy-o](https://boards.straightdope.com/u/Billy-o)
#### Post date: [January 18, 2013, 4:58pm UTC](https://boards.straightdope.com/t/chemists-why-doesnt-my-iodine-clock-reaction-turn-blue/647627/1 "2013-01-18T16:58:16Z")
I perform some public chemistry demos, and would like to include the iodine clock reaction, because [it’s pretty cool looking](http://www.youtube.com/watch?v=KWJpKNQfXWo). I don’t, though, because I haven’t been able to get reliable results.
My reaction _does_ turn color at the appropriate time, but it turns yellow instead of dark blue. But not always! Every now and then, I try the experiment again and _do_ get the dark blue color. I haven’t been able to determine what’s different when the experiment goes properly – I haven’t changed the method.
Usually, getting a yellow color indicates that starch is not present, but I definitely have it in there. My initial thought is that one of the reagents has gone bad, but I always make them fresh from dry chemicals. And since the reaction does occur at the proper time (just the wrong color), I suspect none of the reactants is at fault.
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### Author:  [@Busy\_Scissors](https://boards.straightdope.com/u/Busy_Scissors)
#### Post date: [January 18, 2013, 5:21pm UTC](https://boards.straightdope.com/t/chemists-why-doesnt-my-iodine-clock-reaction-turn-blue/647627/2 "2013-01-18T17:21:54Z")
I’ve not tried this reaction (although I am a chemist - I’ve met the guy in that video a couple of times, great ambassador for the subject), but it sounds from what Prof Poliakoff is saying that concentration will be key here - are you being rigorous with the concentrations of the relevant actors? Nae black/blue colour suggests excess bisulfite preventing the accumulation of iodine.
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### Author:  [@Billy-o](https://boards.straightdope.com/u/Billy-o)
#### Post date: [January 18, 2013, 5:49pm UTC](https://boards.straightdope.com/t/chemists-why-doesnt-my-iodine-clock-reaction-turn-blue/647627/3 "2013-01-18T17:49:46Z")
Thanks, **Busy Scissors**. I think the concentrations are OK, because the color change does happen when expected – but it’s a clear-to-yellow change, not the clear-to-blue one expected.
The yellow color is the result of the I[sub]3[/sub][sup]-[/sup] ion, which indicates the completion of the reaction. When the demo is working properly, that yellow I[sub]3[/sub][sup]-[/sup] immediately turns blue because of the starch. So you’d think the problem is I forgot the starch, but I didn’t. Hmmm.
And I agree – Prof. Poliakoff is a fantastic ambassador of science. Would love to meet him some day.