Re: What caused BTB to turn green in the refrigerator?

This is a great question and experimental observation!! Bromothymol blue is commonly used as a pH (acid-base) indicator (6.2-7.6) due to its change in color across a known pH range re f 1. To view the structure of bromothymol blue click here.

The temperature variation is indirectly having an effect on the color of the solution. Carbon dioxide is a by-product of respiration and thus a large fraction of atmospheric gases (initially, though there is thought to be a sink that depletes it) (ref 2). Carbon dioxide is absorbed into a solution from the atmosphere, and thus the beakers containing bromothymol blue. This abosorbtion of CO2 decreases the pH of the solution (if it is just water the pH is already highly variable).

This alteration of the pH changes the ionization state of the conjugated molecule and produces a different colour. These colours are different between the microwave and fridge largely due to a higher CO2 concentration in the denser (cooler) air within the fridge. Also, time of exposure to the atmosphere is a big factor. To test this, you could store a fresh solution under Argon gas at room temperature and in the fridge and a lack or slow colour change will most likely occur. I hope this answers your question. If not then feel free to email me directly at frank@osucau.okstate.edu and I will forward more information to you.

happy researching!!

/frank