Indicators in Acid-Base Titrations: Chapter 17.5

An indicator can be used to

determine the equivalence point of a titration.

Most of the indicators used for acid–base titrations are weak acids, where the

protonated (acidic) form is one color and the unprotonated (basic) form is another color.

Phenolphthalein is commonly used for

strong acid–strong base titrations. It is colorless in acidic solution is a pink color at pH ≥ 9

HA(aq) + H2O(l) ⇌ A−(aq) + H3O+(aq)

color 1 color 2

• When A-/HA < 10, color 1; when A−/HA > 10, color 2

Predominates; Predominates

The color change of an indicator occurs near its pKa, where the A−/HA ratio approaches

1

The color change of the indicator (end point of a titration) is different than the

equivalence point of a titration.

pH change is very rapid at the

equivalence point

Phenolphthalein works well for

strong acid–strong base titrations and for most weak acid–strong base titrations.

However, if the weak acid has a very small Ka, phenolphthalein may not be useful and an

indicator with a higher indicator range is used.

Weak base–strong acid titrations require indicators that

change color at lower pH values.

For titrations involving weak acids or weak bases, choose an indicator that changes color within

±1 pH unit of the equivalence point.

For strong acid–strong base titrations, the color change can be within

±2 pH units of the equivalence point.

Phenolphthalein

(colorless) 8.2–10.0 (pink)*