Prepare the standard and experimental flasks:
Add a few drops of 0.5% NaOH to make the solution slightly basic with a medium pink color.
NOTE: Instead of preparing a second experimental flask, divide the original flask equally into two, then use one as the standard and the other as experimental. This will ensure that the 2 solutions are exactly the same.
NaOH calculation: see p. 337:
Repeat procedure with moderate and strenuous physical activity, 3 trials each.
©Copyleft Peter Chen