Iodine Clock Reaction

I’ve preformed this reaction in two separate labs, once in high school and once in college PChem. It didn’t really work in either case. The text books insist that a blue color should appear, but ours turned yellow.
I’ve been told, by whom I can’t remember, that this reaction is linked with one of the ivy league schools, Princeton, I believe. Anyway, the story I’ve heard is that you mix the reactants together and they turn yellow instantly. Then, you sing the school’s fight song. When you reach the end, the mixture turns blue. This is supposed to be significant as blue and yellow are the school’s colors.
Has anyone every done this and had it work? Or are there others, like me, that are highly disappointed with the lack of color change on Physical Chemistry Lab?

Well, when we did this experiment for my P-chem lab, we definitely noticed the change to a pale blue when the reaction was complete. I’m not certain why you wouldn’t notice the change. I suppose that one possibility would be that you didn’t wait long enough.

The reason I say this is that the rate of the reaction depends on the concentration of the reactants. Consequently, if your initial concentrations weren’t high enough, it would take a pretty long amount of time for the color to change to blue. I don’t have all the exact details of our runs with me, but our shortest time for the change was 90 seconds. Our longest was about about seven minutes, which I’m sure is longer than the the school’s fight song. (Coincidentally, I’ve never heard that story before, so I can’t verify its truth.)

For those who aren’t familiar with this reaction, it is often used to demonstrate the effect of concentration of reactants on the initial reaction rate. One of the big reasons for this is the sudden color change that occurs upon the completion of the reaction. The solution is either colorless or pale yellow up until the end, when it suddenly changes very rapidly to blue. This occurs because one of the reactants gets entirely used up, so a competing process (that was originally unfavorable) begins to occur. By measuring the time until the color change, the initial rate can be determined.

On a less technical note, this reaction can also be used as a neat demo to show how you can initially start with clear solutions, mix them together and get a color change.

There are a few iodine clock reactions – do you know the name of the one you tried? I’ve never actually seen or tried a clock reaction, for some reason, but I know a bit about them.

I’m going to use the Landolt reaction here. It goes from colorless to blue-black, so it’s not the one you mean, but it’s fairly straightforward.

The yellow color is I[sub]2[/sub], which is formed by decomposition of iodate in acid to give water and aqueous iodine. The blue/blue-black color is produced by a complex of starch with pentaiodide or triiodide ion (I[sub]5[/sub][sup]-[/sup] or I[sub]3[/sub][sup]-[/sup]), depending on the reaction. The iodide-starch complex is formed by a reaction of iodine with iodide ion.

To form the blue-black starch complex, both iodine and iodide must be present. In the Landolt reaction, iodine doesn’t accumulate until one of the reactants, HSO[sub]3[/sub][sup]-[/sup], is consumed, because HSO[sub]3[/sub][sup]-[/sup] rapidly converts I[sub]2[/sub] to I[sup]-[/sup]. When the HSO[sub]3[/sub][sup]-[/sup] is consumed, I[sub]2[/sub] accumulates and reacts (quickly) with iodide to give the starch complex.

The yellow-to-blue-black reaction would do the opposite – it would prevent I[sup]-[/sup] from accumulating until one of the reactants was consumed. The I[sub]2[/sub] produced by decomposition of KIO[sub]3[/sub] would accumulate and give the yellow color.

So, what’s happening is that the part of the reaction that’s supposed to be producing iodide isn’t. Try increasing the amount of KIO[sub]3[/sub]. (If you want to cheat, add some KI and it’ll go blue-black.)

I’d need to know what reaction you were trying to suggest anything further…