sbreheny
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Registered: 30-1-2014
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Interesting Iodine effect
Hi all,
I just thought I'd share an interesting thing which happened to me. I have been storing prilled elemental iodine in a high quality glass chemical
bottle with a heat and chemical resistant lid, for about 6 months. Slowly over time, the Iodine has either stained or reacted with the lid to produce
a very even and sharply demarcated black discoloration of the plastic. I think the demarcation is due to the sealing ring of plastic which goes around
the mouth of the bottle. It is so uniform that I originally thought that it was part of the original lid but then I compared it to another lid and saw
the stark difference. In the image, the cap on the right is the one exposed to the Iodine.
Sean
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woelen
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This is a known and common effect of iodine (and also of bromine). Every plastic, in which it is stored becomes stained. Iodine gives a dark brown
stain, which in combination with the blue of your caps looks black. Bromine gives orange/brown stains. Even teflon becomes stained. The stain is very
hard to remove. Only if the cap is allowed to be soaked in a solution of Na2SO3 or Na2S2O3 for months, then it is reduced a lot, but completely
removing it is (nearly) impossible.
I think that this staining is not due to reaction with the plastic, but due to porousness of plastics. Many plastics in some way can absorb other
chemicals. With most plastics you don't see this, because these chemicals are colorless, but with the strongly colored halogens you can easily see
this. For the same reason, plastic bottles with conc. HCl or 65% HNO3 always have some humidity on them. Some HCl is absorbed by the plastic and is
released on the outside, which picks up moisture from air which causes condensation of liquid at the outside (especially in winter in cold and damp
storage).
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Morgan
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I once put some iodine crystals in a 2 liter bottle and pressurized it with air for the heck of it and let it sit for a year. The clear PET plastic
stained brown but remained transparent and the I2 formed lots of little iodine crystals all over the walls of the bottle, like these found on Flickr.
If you can find the firsthand account of the discovery of iodine by Courtois, it's a good few sentences. I read it in a book about the elements.
https://www.flickr.com/photos/paulslab/6065225366/
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