[halo364]

Hi all!

In a recent Orgo lab, we synthesized a sample of fluorescein and dissolved .01g of it in 10mL of .10M NaOH. Then, while keeping the solution under a UV light source, we added 2.0M HCl drop wise until the color of the solution changed from a neon greenish-yellow to more of a dull gold (the change took place at around 22 drops, and the pH dropped rapidly from about 8 to about 2).  Now we're being asked to explain this color change, and I'm stuck. I know that fluorescein absorbs light at 494 nm and emits it at 521 nm, and multiple sources have confirmed that pH does have an effect on the color of the solution, but none have even attempted to explain why.

If anyone could start to point me in the right direction here, it would be very much appreciated. My current best guess it that the acid somehow interferes with fluorescein's ability to absorb and give off light, but I'm not sure why or how this happens (or whether it's even right).

Any hints (or answers!) would be greatly appreciated. Thanks a lot!

[Corribus]

Look at the structure of fluorescein.  Anything jump out at you that might happen to it at very low pH values?

[camptzak]

Heres a link to the image. It looks like there is a electrophilic point at the carbon opposite the ether linkage.

[Babcock_Hall]

If anyone could start to point me in the right direction here, it would be very much appreciated. My current best guess it that the acid somehow interferes with fluorescein's ability to absorb and give off light, but I'm not sure why or how this happens (or whether it's even right).

Clearly your first hypothesis is wrong, because fluorescein has color under both acidic and basic conditions.  If you have ever used phenolphthalein or pH paper, then you have already made use of the fact that some compounds change their wavelengths of maximum absorbance with pH.