As you learned in the preceding section, pH meters can be used to construct pH curves that identify the equivalence point of a titration. Once the equivalence point for a particular acid-base reaction has been determined, that same acid-base reaction can be used for titration analysis. Instead of continuing to identify the equivalence point using a pH meter, however, we can use acid-base indicators to indicate equivalence points in titration analyses. Acid-base indicators are substances that change colour suddenly at a given pH. The point at which an acid-base indicator changes colour is known as its endpoint.

How do acid-base indicators work? To answer this question, recall Le Chatelier's Principle.

An acid-base indicator is a Bronsted-Lowry conjugate acid-base pair. The conjugate acid and the conjugate base are differently coloured.

Consider Bromthymol blue, a commonly used acid-base indicator.

According to Le Chatelier's principle, increasing the concentration of hydronium ions is a stress that causes a shift in equilibrium towards the reactants. In other words, as the solution becomes more acidic, the blue form of the indicator shifts towards the yellow form of the indicator. Conversely, adding a base neutralizes (removes) hydronium ions, causing the equilibrium to shift to the right. This shift favours the blue-coloured form of the indicator, Bb^-.

For any acid-base reaction, the pH curve can be used to choose an appropriate indicator. There are many indicators, however each indicator changes colour over a particular pH range. The chosen indicator must change colour across a pH range that has a central value close to the equivalence point. For a list of possible indicators and their pH ranges of colour change, refer to inside back cover of your textbook and page _ of your data booklet.

To demonstrate how to choose and appropriate indicator, consider again the titration curve for an HCl-NaOH titration. At the equivalence point, the pH is 7. A suitable indicator, therefore, should change colour very close to the equivalence point. Bromothymol blue is a suitable indicator for this particular titration because it has an endpoint pH of 6.8 (assume that bromthymol blue's endpoint is in the middle of its pH range). An endpoint of 6.8 matches closely the reaction equivalence point pH of 7.

pH range: 6.0 - 7.6

Check Your Understanding

Complete Practice Question 12 on page 763 of your textbook. Click the link below to check your answers.