Fig. 4.
Standard deviation of the pH value determined with absorption measurements (symbols, top panel) and deviation from predetermined pH value (symbols, bottom panel). Different liquid volumes correspond to the measurement areas shown in the inset of Fig. 2. Solid lines are guides to the eye. The increased uncertainty at low and high pH values is related to low absorption by the acid and base forms of phenol red. The minimum uncertainty is shifted from pKa due to a difference in absorption strength of the acid and base forms.