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We're studying the inhibiting effects of alcohols on the iodine clock reaction & have heard that iodine can complex with alcohols as well as starch. We're using starch as an indicator and have noticed some different color changes for endpoints particularly when using high concentrations of alcohols. Can you offer any advice on this system or direct us to useful support materials?
Question Date: 2002-02-28
Answer 1:

It sounds like you're doing some interesting work on the iodine clock reaction. As I understand it, there are several variations of the iodine clock reaction. However, I think they basically all come down to a slow reaction that forms triiodide ions in the presence of an acid, and a fast reaction where triiodide ions are converted back to iodine ions in the presence of thiosulfate (or bisulfate, I think works too). When all the thiosulfate is used up, then the iodine, and react with the start to form a complex giving the solution its color. Now, I don't know that much about iodine-alcohol complexes, but I'll try to note a few things that I would think about.

Iodine can form complexes with at least some alcohols. One example would be with polyvinyl alcohol - it forms a bluish / purplish complex with iodine. Depending on what acid and what alcohols you're using, I can imagine several potential situations. If the concentration of hydrogen ions (acid) decreases, then the formation of the triiodide ions would probably be somewhat inhibited. However, this wouldn't really change the colors that are obtained, but rather the rate at which the color changes. If you're seeing radically different colors, then you may want to look at possible reactions between your acid and alcohol, and how those products may or may not react to form iodine complexes.

Unfortunately, I don't think I can help you much more beyond this. In addition to looking up information on the iodine clock reaction, you may also want to look at the Briggs-Rauscher oscillating reaction, which is similar, but involves more chemicals and different color changes.

Answer 2:

That's a fun experiment, isn't it? I don't have any useful advice about this specific experiment, because I hadn't heard about the alcohol effects, but I'd suggest a search on www.google.com for iodine starch alcohol. I'm too busy, or I'd do the search myself. If the experiment is the one I know about, then you're watching saliva degrade starch? So the alcohol probably denatures (kills) the enzyme in the saliva? You could test that hypothesis by incubating the saliva for a while with vs without alcohol and checking the iodine-starch reaction for the 2 different saliva treatments. Good luck with your research.

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