Science Buddies: "Ask an Expert"

I am doing an experiment that i found at science buddies called "Effects of Exercise: Changes in carbon dioxide output". As suggested i purchased the ph test kit (tetra-ph) from a fish store... In doing the experiment the solution went from blue to a pale green... about a ph of 6.4. It did not go to pure yellow (ph<6). even after breathing into the tube for 15-20 minutes. I tried another ph kit and the same thing happened. It's hard to measure the time it takes the solution to turn to acid when it doesn't completely turn yellow. I read something about CO2 and H20 being a weak acid and that the reaction reaches equilibrium with more CO2 molecules than acid in the solution.... it said the ph would be about 6.4 which seems to be what i got (light green) ... any suggestions to make the color change to yellow and more easy to time/measure?
I'm going to test the CO2 output of young (15-20 years) and old (40-50 years) distance runners and need to know how to time the color change.
Erik

You might have more luck with this question on the life sciences forum, but from what little I know, I'd say you'd have a hard time getting CO2 to make something drop below pH 6.4. You might want to search for a more sensitive test kit instead.

I'd guess the basic problem is concentration. You only exhale a really small percentage of CO2 in a normal breath, and it's dangerous to try to increase this concentration by rebreathing the same air over and over again.