The pH range of an indicator

The pH range of an indicator is the pH interval in which the indicator changes colour.

To elaborate further, we refer to the Henderson-Hasselbalch equation and use it to study the equilibrium of a pH indicator, which is a weak acid.


Consider a titration using two drops of bromocresol green with pKin = 4.7, so

pH=4.7+log\frac{In^-}{HIn}\; \; \; \; \; \; \; \; (1)

Since the concentration of the indicator in the analyte is very low, H+ from the indicator is negligible and does not affect the total concentration of H+ in the solution. Hence, the value of pH in eq1 is solely due to H+ of the analyte, i.e. the pH of the analyte determines the position of the equilibrium of the indicator.

When bromocresol green is predominantly in the form HIn, it appears yellow to us. If it is predominantly in the form In, we see it as blue.

\begin{matrix} HIn(aq)\\yellow \end{matrix}\rightleftharpoons H^+(aq)+\begin{matrix} In^-(aq)\\blue \end{matrix}

As a rule of thumb, our eyes are able to perceive a complete change of colour of the indicator from its acid form to the conjugate base, and vice versa, when the concentration of one form is ten times greater than the other. So, bromocresol green appears blue when:

[In^-]\geqslant 10[HIn]\; \; \; \; \; \; \; \; (2)

which corresponds to the indicator in a pH environment of pH ≥ 5.7 (by substituting eq2 in eq1). At the other limit, bromocresol green appears yellow when:

[HIn]\leqslant 10[In^-]\; \; \; \; \; \; \; \; (3)

which corresponds to the indicator in a pH environment of pH ≤ 3.7 (by substituting eq3 in eq1). In other words, if a couple of drops of bromocresol green is added to an analyte of pH ≥ 5.7 and another at pH ≤ 3.7, the solutions appear blue and yellow respectively.

We call this difference in pH (3.7 to 5.7 for bromocresol green) the pH range of the indicator.

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