[sol]

I'm looking for a chemical reaction (maybe even a physical reaction) that will show a color change over a period of time. The purpose is an elapsed time indicator using color.

Any help would be appreciated. Thanks.

[Albert]

Try oscillatory reactions, like Belousov-Zhabotinsky's one.

[pantone159]

I'm looking for a chemical reaction (maybe even a physical reaction) that will show a color change over a period of time.

There is a reaction using iodine, H2O2, ascorbic acid, and starch, that may fit the bill.  I am not at home this moment, so I don't have the reference and details at hand, but I'll post them in a couple of days when I get back.  It can be done with over-the-counter chemicals.

The scheme is like this:
Prepare solution A by adding, in order, tincture of iodine, ascorbic acid, and starch.
Prepare solution B with 3% H2O2, the amount is in excess of the ascorbic acid.

Solutions A and B are colorless.  Mix them, stir, and wait around 30 seconds.  Initially, the mixture is colorless.

The H2O2 oxidises the I- to I2, but the ascorbic acid reduces it back to I- right away, so I2 (and I3-) stay at negligible concentrations, and the mixture remains colorless.  Eventually, however, the ascorbic acid is exhausted.  Then, I2 (and then I3-) accumulate, and the mixture suddenly becomes dark blue, almost black.

This is quite fun.  It was specifically developed as a 'clock' reaction that could be performed with consumer chemicals.

[Albert]

Just in case someone is looking for practical information about B-Z's reaction, this is a quite useful page: http://ed.augie.edu/~gjhonsbr/BZ_lab.html

[pantone159]

Just in case someone is looking for practical information about B-Z's reaction

Thanks, I am!  I really want to try this someday.

Unfortunately, in Texas, I am not allowed to posess malonic acid without a license.  (Idiotic law.)  That will be a complication.

[pantone159]

Ok, the reference is:

The Vitamin C Clock Reaction, Stephen W. Wright, J Chem Ed, Vol 79 No 1, Jan 2002, p 41-43.

The reagents to use are:
1 - 0.1 M ascorbic acid (vitamin C) solution.  This does not keep, so it must be fresh.  If you have to, you can supposedly use a vitamin c tablet, but vitamin c powder is easy for me to find, in the health food section of the grocery store.
2 - 2% Tincture of iodine.  (This is a mixture of iodine and iodide ion.)  Other versions of tincture of iodine supposedly exist, and you'd have to modify the recipe to use them.  The 2% (of iodine) is the most common for me.
3 - 3% hydrogen peroxide
4 - Starch solution.  I used concentrated laundry starch, from which I made a 25% (by weight) solution.
5 - Distilled water.  Supposedly, tap water is usually acceptable, and if not, vinegar is ok.

Procedure:
Prepare Solution A with 5 ml of 0.1 M ascorbic acid, 5 ml of tincture of iodine, plus X ml of water.  (The clock period depends on X, see below.)
Prepare Solution B with 15 ml of 3% H2O2, 3 ml starch solution, plus X ml of water.
Pour solutions A and B into a separate container, swirl to mix, and wait.  After some time, depending on X, the mixture changes from clear (perhaps slightly cloudy from the starch) to dark blue, almost black.
If X = 30 ml, the time is 20-25 sec, if X = 60 ml the time is 40-50 sec, if X = 120 ml the time is 80-95 sec.

For a variation, which also can be done with consumer chemicals, I replaced the starch solution with some blue food color.  I used McCormick brand coloring, which is probably FD&C Blue 1.  (I cannot tell for sure.)  I used 1/2 drop, prepared by diluting 1 drop of the food color with a little water, and using half of this mixture.
With the food color, the color changes from blue to approximately forest green, at around the same time.  (Perhaps slightly later.)  This change is less sudden than the starch indicator, and the green color gradually changes over several minutes from forest green, to olive green, eventually to brown.  I'm not sure what is happening with the indicator here.  (The paper does not describe this, just the starch.)

Disposal instructions are to add ascorbic acid to the mixture until the color change is reversed (colorless with starch, back to blue with the food color), then pour down the drain.

[pantone159]

For a variation, which also can be done with consumer chemicals, I replaced the starch solution with some blue food color.  I used McCormick brand coloring, which is probably FD&C Blue 1.

I don't think any reaction is happening with the iodine and the FD&C Blue 1.  As the clock alarm strikes, brown iodine is being formed.  (With the starch, this forms the blue-black complex.)  The brown + blue color makes green.  With other colors of food color, this change is a lot less dramatic.