Acid-base indicators

Typical acid-base indicators are organic dyes that change colour at or near the equivalence or end-point. They have the following characteristics:

• They show pH-dependent colour changes.
• The colour change occurs within a fairly narrow pH range (approximately 2pH units).
• The pH at which a colour change occurs varies from one indicator to another, and it is possible to select an indicator which exhibits a distinct colour change at a pH close to the equivalence or end-point.

Selected common indicators together with their pH ranges and colour changes are shown in Table 22.1. Examples for thymol blue and phenolphthalein are shown in Fig. 22.1.

Figure 22.1 Examples of indicators used in acid-base titrations: thymol blue and phenolphthalein

Table 22.1 Colour changes and pH range of selected indicators