Why doesn’t it glow for days and days?

It’s Halloween, and like most parents here in the US I had my son out for trick-or-treating. One of the things he got was a couple of glow sticks, which he insisted on taking to bed with him. That’s when he found the one he got a few days ago, and with some alarm he told us it wasn’t glowing. We told him that they stop glowing after a day, and that’s when he asked his question.

Honestly, I don’t know. Beyond it being a chemical reaction of some sort. So, let’s find out!

The construction of a glow stick is simple enough:  there’s a tube containing one chemical inside a larger tube that contains a different chemical.  The inner tube is fragile, the outer flexible, and when you break the inner tube to mix the chemicals the glow happens. But why?

Inside the larger tube is actually two chemicals. One is a fluorophore, which is a chemical that emits light when excited. The other is diphenyl oxilate, a chemical that released enough energy to excite the fluorophore when it oxidizes. And the chemical inside the brittle tube is hydrogen peroxide. So when you break the inner tube, the hydrogen peroxide oxidizes the diphenyl oxilate, causing the fluorophore to become excited and glow.

The color of the glow depends on the exact fluorophore used, and both the brightness and duration depend on the ratio of hydrogen peroxide to diphenyl oxilate- the brighter the glow, the shorter the duration. And that thing about making it last longer by putting it in the freezer does work!  Heat speeds the reaction up, and cold slows it down.

So why doesn’t it last for days and days?  Because there’s a limited amount of diphenyl oxilate to oxidize, and when it’s finished the fluorophore stops getting excited.

Happy Halloween’

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