Prepare the standard and experimental flasks: make one solution of 200 ml instead of two separate 100 ml solutions.

Add a few drops of 0.5% NaOH to make the solution slightly basic with a medium pink color.

Then divide equally into the standard and experimental flasks; this will ensure that the 2 solutions are exactly the same.

Exhale into flask for 1 minute. Then add 0.5% NaOH until solution returns to standard color.

NaOH calculation: see p. 347:

1. ml of NaOH = # drops of NaOH / 20
2. µmol of CO₂ = ml NaOH * 62.5

Repeat procedure with moderate and strenuous physical activity, 3 trials each.