why lugol solution becomes transparent

[mete]

I have a Sigma Aldrich lugol solution (in a brown glass bottle). I had a small amount from that in an opaque plastic (with dropper cap). I was not using it for some time and I realized it turned into a transparent liquid. I thought maybe there is a precipitation and I did shake the plastic bottle a bit but it did not help. Why would this happen, because of light or another contamination ?

[iPeace]

Iodine in solution is the familiar brown or amber color. The iodide ion is colorless. Iodine is an oxidizer, meaning it will take an electron from anything willing to offer one, an become iodide. Lugol's solution is iodine & KI in water. None of those is a willing electron donor, so my bet would be the plastic bottle or the rubber cap. I doubt that the iodine precipitated out of solution, but you could try heating it in a water bath to see if the color returns.

[mete]

Thanks for the info, good to know. I got rid of that one so I cannot try heating, I started using a glass dropper now. If it never happens again, I guess it was due to plastic.