I regularly use the dye bromothymol blue (BTB) for demonstrations of things like ocean acidification and the way that carbon dioxide (in the form of carbonic acid) acidifies the blood. It's nifty stuff!

I saw a site suggesting a preparation to make a sensitive 'voice-activated' color change when an audience member speaks into the container, saying, "TURN YELLOW!" Has anybody else tried this? Has BTB been particularly temperamental in your hands?

Hi FerdieLance,
I looked up the reference on the web. The experiment used phenol red and suggested also trying bromothymol blue. It looked like it would take a fair amount of talking before the colour change would happen.
For a voice activated change, I used a "clock reaction" like in "Clear water, black water" or "Instant Ink", which instantly changes from colourless to black as you say the magic words. It gives great reactions.

Richard