How Acid-Base Indicators Work

An acid-base indicator is itself an acid or base whose various protonated species have different colors. The indicator is added at such a low concentration that it has negligible effect on acid-base equilibria of the major components of the solution. In the next chapter, we will learn how to choose an indicator to find the end point for a titration. For now, let s use the Henderson-Hasselbalch equation to understand the pH range over which color changes are observed. 

Consider bromocresol green as an example. We will call p a t f the indicator P Hin to allow us to distinguish it from pK of an acid being titrated. 

At pH = p a = 4.7, there is a 1 1 mixture of yellow and blue species, which appears green. As a crude rule of thumb, we predict that the solution will appear yellow when [Y]/[B ] 10/1 and blue when [B]/[Y] 10/1. (The symbol means is approximately equal to or greater than. ) From Equation 9-4, we predict that the solution will be yellow when pH p nin 1 (= 3.7) and blue when pH p nin + 1 (= 5.7). By comparison, Table 9-3 lists bromocresol green as yellow below pH 3.8 and blue above pH 5.4. Between pH 3.8 and 5.4, various shades of green are seen. Demonstration 9-2 illustrates indicator color changes and Box 9-2 describes an everyday application of indicators. 

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