pH indicators: litmus, universal indicator and phenolphthalein

A pH indicator is a dye that changes colour with pH. Litmus paper, universal indicator and phenolphthalein are the best known. They give a quick answer without an instrument, and they mark the end point of a titration. This guide explains how indicators work, what the common ones show, and how to pick the right one for a titration.

For what pH, acids and bases are, see acids and bases. Terms are defined in the glossary.

What is a pH indicator?

A pH indicator is a weak acid, usually written HIn, that is one colour when it holds its proton and another when it has lost it (In⁻). Which form dominates depends on the pH, just as it does for any weak acid. See buffers and pH for how pKa sets that balance.

• Well below the indicator's pKa, it is almost all in the acid form, so you see the acid colour.
• Well above its pKa, it is almost all in the base form.
• Within about one pH unit either side of the pKa, both forms are present, and the colour is a mix.

So each indicator has a colour-change range about two pH units wide. It tells you which side of that range a solution is on, not an exact pH.

Indicators are used in tiny amounts. A few drops colour a whole flask, so the indicator itself barely changes the pH.

Common indicators

The ranges are from Harvey's Analytical Chemistry 2.1, except litmus. Published ranges vary by a few tenths of a unit between sources.

Litmus paper

Litmus is a mixture of dyes extracted from lichens. It is red in acid and blue in alkali, changing between about pH 4.5 and 8.3. It comes as paper strips:

• Blue litmus paper turns red in an acid.
• Red litmus paper turns blue in an alkali.
• Neutral litmus paper is purple, and turns red or blue.

What colour does litmus turn in acid?

Red. Blue litmus paper turns red in an acid, below about pH 4.5, while red litmus paper stays red (LibreTexts).

Litmus answers one question: acid or alkali? Its change range is wide and sits around neutral, so it can't tell pH 5 from pH 3, or pH 9 from pH 12.

Universal indicator

Universal indicator is a mixture of several indicators chosen so that their colour changes overlap. Together they give a different colour at each step across the pH scale, roughly:

The exact colours depend on the mixture, so always read against the colour chart supplied with that bottle or pad of paper. Universal indicator gives a pH to within about one unit, which is enough to tell a weak acid from a strong one but not enough for making up a buffer.

What colour is universal indicator in neutral solution?

Green, at about pH 7. Acids take it towards red and alkalis towards blue and violet.

pH test strips work the same way, with several indicator dyes on separate pads. Narrow-range strips can read to a few tenths of a unit within their range.

Phenolphthalein

Phenolphthalein (C₂₀H₁₄O₄, CAS 77-09-8, molar mass 318.3 g/mol) is the classic indicator for titrating an acid with sodium hydroxide (PubChem). It is colourless below about pH 8.3 and turns pink, then deeper red, up to about pH 10. In very strongly alkaline solution it fades back to colourless. A faint pink that lasts about 30 seconds is the usual sign that a titration has reached its end point.

Why does phenolphthalein turn pink?

In acid, the middle of the molecule is a closed five-membered ring, which keeps the electrons confined to the three separate benzene rings, so the molecule absorbs only ultraviolet light and looks colourless. When base removes its protons, that ring opens and the molecule becomes flat, with electrons free to move across the whole structure. This form (often called the quinoid form) absorbs visible light, so the solution looks pink (LibreTexts).

Handling phenolphthalein

Solid phenolphthalein carries serious hazard statements, including H350 ("May cause cancer"), in the classifications notified to the European Chemicals Agency (PubChem). That puts it under the health hazard symbol. Handle it under its safety data sheet; see COSHH and safety data sheets.

Titration curves and choosing an indicator

In an acid–base titration, a base of known concentration is added from a burette to an acid, or the other way round. A titration curve plots pH against the volume added. The curve is fairly flat at first, then rises steeply around the equivalence point, where exactly enough base has been added to react with all the acid. It then levels off again.

An indicator works for a titration if its colour-change range falls on the steep part of the curve:

The point where the indicator changes colour is the end point. A well-chosen indicator puts the end point very close to the equivalence point.

Titrations for other reactions use their own indicators. EDTA titrations of metal ions use dyes such as Eriochrome Black T; see EDTA and chelating agents. Karl Fischer titration of water usually detects its end point with an electrode; see Karl Fischer titration.

Indicators vs pH meters

For buffers and anything where the pH must be exact, use a calibrated meter. See pH meters.

Related guides

For what makes an acid strong or weak, see acids and bases. For holding a pH steady, see buffers and pH. For balancing the reactions that titrations rely on, see balancing chemical equations. Every lot has its own COA, published on the product page and on that lot's own page. See Quality.