I perform some public chemistry demos, and would like to include the iodine clock reaction, because it’s pretty cool looking. 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.
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.
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.