Indicators

What Is an Indicator?

An acid-base indicator is a substance that changes colour depending on the pH of the solution it is in. Indicators are themselves weak acids (or weak bases), where the undissociated form (HIn) and the dissociated form (In⁻) have different colours.

The equilibrium is:

HIn(aq) ⇌ H⁺(aq) + In⁻(aq)

Species	Colour
HIn (acid form)	Colour A
In⁻ (base form)	Colour B

For example, for methyl orange: HIn is red and In⁻ is yellow. For phenolphthalein: HIn is colourless and In⁻ is pink.

---

How Indicators Change Colour

In acidic solution (high [H⁺]), Le Chatelier's principle pushes the equilibrium to the left, favouring HIn. You see colour A.

In alkaline solution (low [H⁺]), the equilibrium shifts to the right, favouring In⁻. You see colour B.

The colour change occurs over a range of about 2 pH units, centred on the pKin of the indicator (where pKin = -log₁₀KIn, and KIn is the dissociation constant of the indicator).

At pH = pKin, the concentrations of HIn and In⁻ are equal, and you see a mixture of both colours.