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Don’t blame the chlorine, folks!

August 1, 2013

This is a slightly belated entry for the #chemsummer blog carnival. Between thesis-ing and writing for the Journal Sentinel, I’m a little behind on a lot of these things! Hope you’ll enjoy it anyway.

When I think summer, I think backyard barbecues, running through sprinklers, and jumping into swimming pools. Or diving into swimming pools, actually – one of the most terrifying moments of my elementary-school life was when my friend Katie (a competitive diver) convinced me to jump off the 3-meter board at a local pool.

Katie had blonde hair. And she spent so much time in the water that by mid-summer, her hair took on a semi-permanent green tinge.

Me, I have brown hair, so I never suffered this terrible fate. But now, 20 years later, my chemist brain asks — why green?

Growing up, we always thought it was the chlorine. And it was one of those things that we just *knew* and didn’t need to question.

But the answer, it turns out, isn’t chlorine. It’s copper.

Copper gets into swimming pools in a couple of ways. Copper salts, like copper sulfate, are used as algicides. But copper can also get into the water as copper plumbing in the pool corrodes.

And while copper metal is a pretty reddish-brown color, copper compounds can be red or bright green or blue or even black. This color change happens because the energy levels of the copper atoms shift as the copper oxidizes (loses electrons) and binds to other chemical apendages (ligands).

Taking images of cross-sections of single strands of hair shows that the copper is mostly confined to the hair’s outer layer and doesn’t penetrate too far inside.

From what I could find, however, scientists aren’t exactly sure how copper binds to hair, though it sounds like it might be because carboxylic acid sidechains in the keratin in hair grab onto the copper ions.

And this provides a clue to why some of the oft-recommended home remedies might work for getting rid of that greenish tinge: two of the classic recommendations, lemon juice and tomato juice, both contain large amounts of citric acid, which can also grab onto, or chelate, metal ions.

If these acidic juices don’t work, other chelating agents can do the trick as well. EDTA is supposedely a common ingredient in swimmers’ shampoo, and case-reports suggest penicillamine shampoos also work. But normal shampoo probably won’t — detergents don’t do much to grab that metal and get it out of there.

But perhaps the best solution is simply to not get the copper into your hair in the first place. Swim cap, anyone?

Further reading:

Here’s a research paper on how different hair pre-treatments affect the amount of copper hair picks up, and includes scanning electron microscope images of where the copper sits in the hair fiber

Here’s a nice description of crystal field theory, which explains what controls the color of transition metal compounds


From → Cool Stuff

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