7.2.10—Select suitable indicators for acid-alkali

An indicator is suitable when its transition range lies within the sharp pH change near the equivalence point. The endpoint colour change should therefore occur close to the stoichiometric endpoint.

Choose from the expected curve, not from the acid name alone. Strong acid/strong base allows a broad choice; weak acid/strong base needs an indicator changing in the basic region.

Phenolphthalein is suitable for a weak acid–strong base titration because its transition is above pH 7; methyl orange is better suited to a strong acid–weak base curve.

An indicator does not measure exact equivalence by magic. If its transition range falls outside the steep section, the endpoint error can be significant.