If you mean to test for the presence or absence of CO2 & O2 in photosynth & respiration, I use the bromothymol blue-phenol red combination.
Stock bromo blue
Dissolve 0.5g in 0.35ml 2M NaOH & make up to 1L
Stock phenol red
Dissolve 1g in 30ml 0.1M NaOH & make up to 1L
Indicator
Add 4ml bromo blue stock solution to 20ml phenol red stock solution & make up to 2L
This should give you a pale reddish solution. I've found having it quite diluted helps the pond weed & snails survival rates better.
The presence of CO2 turns it orange/yellow, the presence of O2 turns it pink/purple.
Because the pH of the water you use may be different, you might need to tweak the solution with acid or base to end up with the reddish colour.
Sometimes when I make it up it ends up a bit purple so I add a bit of HCl.
If you mean just the bromothymol blue to test for expired CO2 that the kids breathe with the effect of exercise prac then I use this recipe...
Stock bromo blue
Dissolve 1g in 16ml 0.1M NaOH & make up to 1L
Dilute 25ml / L for class use (& add a few drops of NaOH if the diluted solutions ends up a bit green)
Which prac do you want a copy of? I can send you either depending on what you're after
Cheers,