What Is an Indicator?
An indicator is a substance that changes colour depending on whether a solution is acidic, neutral, or alkaline. Indicators are usually plant-based dyes that contain molecules sensitive to the concentration of H⁺ ions.
Indicators are used to:
- Test whether a substance is acidic or alkaline.
- Measure the approximate pH of a solution.
- Find the end point (neutral point) during a neutralisation reaction.
Litmus
Litmus is the most commonly used indicator in school chemistry. It is extracted from lichens.
| Solution | Colour of litmus |
|---|---|
| Acid (pH < 7) | Turns red |
| Neutral (pH = 7) | Purple (no change) |
| Alkali (pH > 7) | Turns blue |
Litmus comes as solution, paper strips, or damp paper. It is a simple two-colour indicator — it tells you whether a solution is acidic or alkaline, but not the strength.
Litmus has been used as an indicator for hundreds of years. The word "litmus test" in everyday English comes from chemistry — a way of quickly finding out something important (like "that was the litmus test of whether he could be trusted"). The chemical litmus test is just as quick and decisive.
Universal Indicator
Universal indicator is a mixture of several different indicators that changes colour gradually from red to orange, yellow, green, blue, and purple across the full pH range.
| pH Range | Universal Indicator Colour |
|---|---|
| 1–3 | Red |
| 4–5 | Orange |
| 6 | Yellow |
| 7 | Green |
| 8–9 | Blue |
| 10–12 | Indigo / Violet |
| 13–14 | Purple |
Universal indicator is more useful than litmus because it gives an approximate numerical pH, not just "acid or alkali".
Other Common Indicators
| Indicator | Acid colour | Neutral | Alkali colour |
|---|---|---|---|
| Litmus | Red | Purple | Blue |
| Phenolphthalein | Colourless | Colourless | Pink/Red |
| Methyl orange | Red | Orange | Yellow |
| Red cabbage juice | Red/Pink | Purple | Green/Yellow |
Phenolphthalein is widely used in titrations: it is colourless in acid and turns bright pink in alkali, making the endpoint very easy to spot.
You can make a natural indicator at home from red cabbage juice. Boil chopped red cabbage in water for 10 minutes, then cool and filter the liquid. Add a small amount to different household liquids — lemon juice (acid) will turn it red/pink, vinegar turns it pink, whilst baking soda solution (alkali) turns it green or yellow.
Using Indicators Safely
Indicators themselves are generally safe in the quantities used in school, but they are dissolved in the solutions you are testing, which may be strong acids or alkalis. Always:
- Wear safety goggles.
- Handle strong acids and alkalis carefully.
- Wash your hands after experiments.
Real world: Red cabbage as a natural indicator
Red cabbage contains anthocyanin pigments that act as a natural pH indicator — the same principle as laboratory litmus. The purple juice turns red/pink in acid (lemon juice, pH 2), stays purple at neutral, and turns green or yellow in alkali (baking soda solution, pH 8–9). This is a real indicator that works in a home kitchen, demonstrating that pH-sensitive dyes are not limited to chemistry labs.
Real world: Testing swimming pool pH
Swimming pool water must be maintained at pH 7.2–7.8. Below that, the water is too acidic and irritates eyes and corrodes metal equipment. Above pH 8, chlorine disinfection becomes less effective. Pool operators use test kits containing phenol red or other indicators — a few drops turn the water yellow in acid, orange at neutral pH, and red-pink in alkali. The colour is matched against a chart to determine if pH adjustment chemicals need to be added.
Real world: Phenolphthalein in industrial titrations
In food and pharmaceutical manufacturing, titration is used to measure the exact concentration of acids (e.g. to ensure the correct acidity of a product). Phenolphthalein is the most widely used indicator for acid-alkali titrations — it is colourless in acid and turns bright pink the instant the solution becomes alkaline, giving a sharp, unmistakable endpoint. This precision allows industries to maintain consistent product quality.
Where do you see this in real life? Red cabbage juice changing colour when you add lemon juice or baking soda (home experiment), swimming pool test kits (phenol red indicator), soil pH testers for gardeners, pregnancy test strips (pH-sensitive dyes) — indicators are the chemistry of colour change in the service of measurement.
Practice Problems
A student dips blue litmus paper into a liquid and it stays blue. What can she conclude about the liquid?
Show answer
Why is universal indicator more useful than litmus for measuring pH?