Mabus - 23-4-2018 at 06:44
Recently at my workplace I got two different bottles of bromthymol blue, each around 8-10 years old. The first bottle is a simple white plastic bottle
without plastic or PTFE seal, similar to those used for pills. The second bottle is amber glass with a black plastic cap that didn't seal properly and
the bottle was sealed with duct tape. Before these two bottles, the only source I had was a small older bottle, where the compound has a lilac/mauve color.
However, upon opening these two bottles, I noticed that their color is different. The bromthymol blue from the first bottle has a pink-beige color, while the bromthymol blue from the second bottle is white, with a faint pinkish hue.
Is pure bromthymol blue colorless? Could the difference in color be due to atmospheric oxidation?
EDIT: Oh and I forgot to mention, I've tested all three samples at different pH (acidic, neutral, basic), they behave like bromthymol blue would.
[Edited on 23-4-2018 by Mabus]
Amos - 23-4-2018 at 07:29
Bromothymol blue contains phenols, which, similar to anilines and above almost all other chemicals in general, are well known for darkening with age
because of oxidation. The impurities produced are extremely strongly colored but often constitute a very small percentage of mass.
[Edited on 4-23-2018 by Amos]
Mabus - 23-4-2018 at 07:48
I see. I noticed other old indicators being darker in color, I guess this could be something typical for such compounds.