Explain how acid-base indicators work as weak acid equilibria and select an appropriate indicator for a titration

What this dot point is asking

An acid-base indicator is a substance that changes colour depending on the pH of the solution. Most are weak organic acids (or bases) where the protonated and deprotonated forms absorb different wavelengths of light.

How indicators work

Represent the indicator as a weak acid $\text{HIn}$:

where $\text{HIn}$ is one colour and $\text{In}^-$ is another. In acidic solution the high $[\text{H}^+]$ pushes the equilibrium to the left (Le Chatelier), so the $\text{HIn}$ colour dominates. In basic solution $[\text{H}^+]$ is low, the equilibrium shifts right, and the $\text{In}^-$ colour dominates. For example, with litmus the $\text{HIn}$ form is red and the $\text{In}^-$ form is blue.

The colour change range

Because the colour switches over the range where neither form completely dominates, each indicator has a characteristic transition interval. Common examples:

• Methyl orange: red to yellow, range about pH 3.1 to 4.4.
• Bromothymol blue: yellow to blue, range about pH 6.0 to 7.6.
• Phenolphthalein: colourless to pink, range about pH 8.3 to 10.0.

Choosing an indicator for a titration

The key skill is matching the indicator to the equivalence point of the titration. You want the indicator to change colour exactly where the titration curve is steepest, so the colour change is sharp and corresponds to the equivalence point.

• Strong acid with strong base: equivalence point at pH 7, with a very steep curve from about pH 3 to 11. Almost any of the common indicators works.
• Strong acid with weak base: equivalence point is acidic (below 7). Use methyl orange.
• Weak acid with strong base: equivalence point is basic (above 7). Use phenolphthalein.

Why this matters

Choosing the right indicator is essential for accurate volumetric analysis, the quantitative technique used to find unknown concentrations. A mismatched indicator gives a false end point and a wrong result, so this dot point links directly to the titration calculations elsewhere in Unit 3.