Acid-base indicators

Phenolphthalein stays colourless in any solution below pH 8.3 and turns pink above it.

Boyle noticed in 1680 that acids change the colours of certain plant dyes. Chemists have used that effect ever since to estimate pH without any instrument. An acid-base indicator is a dye whose colour depends on the hydronium ion concentration. Indicators are themselves weak acids or weak bases, and their two forms, with and without the extra proton, have different colours.

How a dye senses pH

Write the indicator as HIn. In water it is in equilibrium with its conjugate base: HIn + H₂O ⇌ H₃O⁺ + In⁻. For methyl orange, HIn is red and In⁻ is yellow, and the acid ionization constant is Ka = 4.0 × 10⁻⁴. Add acid and the extra H₃O⁺ pushes the equilibrium to the left, so the red form takes over. Add base, which removes H₃O⁺, and the equilibrium moves right towards yellow. The colour you see depends on the ratio of In⁻ to HIn.

The Henderson-Hasselbalch equation links that ratio to pH: pH = pKa + log([In⁻]/[HIn]). When the pH equals the pKa the two forms are equal and you see a mixture, orange for methyl orange. The eye notices a change only when one form clearly outnumbers the other, so the visible colour change happens over a range called the colour change interval, roughly pKa ± 1. For methyl orange, with pKa = −log(4.0 × 10⁻⁴) = 3.40, the interval runs from about pH 3.1 (red) to 4.4 (yellow).

Four common indicators

Each indicator is useful only near its own interval. Methyl orange is red below about 3.1 and yellow above 4.4, so it cannot tell a weak acid at pH 5 from a strong base at pH 13: both look yellow. Litmus turns from red to blue over the unusually wide range of about pH 5 to 8, which makes it handy for a quick acid or base test near 7. Bromothymol blue goes from yellow to blue between about 6.0 and 7.6, passing through green near neutral. Phenolphthalein is colourless below pH 8.3 and turns pink to deep magenta up to about 10. Published ranges differ by a few tenths between sources, because the change is gradual and eyes differ.

Using several indicators together narrows the answer. Suppose an unknown solution turns methyl orange yellow, bromothymol blue green and leaves phenolphthalein colourless. Yellow methyl orange means pH above 4.4, green bromothymol blue means between 6.0 and 7.6, and colourless phenolphthalein means below 8.3. Together they place the pH between 6.0 and 7.6. Universal indicator and pH paper go further, mixing several dyes so that the colour keeps changing step by step across a wide range of pH.

The same reasoning decides which indicator to use in a titration, where acid and base are mixed until they exactly cancel. The indicator should change colour where the pH jumps at the end point, not somewhere on the slow climb before it. When a weak acid is titrated with a strong base the equivalence point lies above pH 7 (8.72 for 0.100 M acetic acid), so phenolphthalein works and methyl orange does not. The last unit of this course shows the full curves.