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Most indicators answer a yes-or-no question: is this solution above or below a certain pH? Universal indicator answers a much better question: roughly what is the pH? One dropper bottle, one set of colours, and you can place a solution anywhere from strongly acidic to strongly alkaline.
That makes it the most used indicator in schools. It also makes it the one most often misused, especially in titrations. Here’s how it works and how to get the most from it.
What universal indicator is
Universal indicator isn’t one substance. It’s a mixture of several indicators, each changing colour in a different part of the pH range. Their changes are chosen to overlap, so the combined colour shifts smoothly across the whole scale.
A typical recipe (formulations vary by manufacturer) contains some combination of:
| Component | Changes colour around pH |
|---|---|
| Thymol blue | 1.2–2.8 and 8.0–9.6 |
| Methyl red | 4.4–6.2 |
| Bromothymol blue | 6.0–7.6 |
| Phenolphthalein | 8.2–10.0 |
At any pH, some dyes are in their acid form and some in their base form. The colour you see is the blend of all of them. As pH rises, one dye after another switches, and the blended colour moves steadily through the spectrum.
The colour chart
Every brand has its own chart, printed on the bottle or paper pack. Typical colours are:
| pH | Colour | Example |
|---|---|---|
| 0–1 | deep red | concentrated hydrochloric acid |
| 2 | red | lemon juice, stomach acid |
| 3 | orange-red | vinegar, cola |
| 4 | orange | tomato juice |
| 5 | yellow-orange | black coffee |
| 6 | yellow | milk, rainwater (slightly acidic) |
| 7 | green | pure water |
| 8 | blue-green | seawater, baking soda solution |
| 9 | blue | soap solution |
| 10 | navy blue | milk of magnesia |
| 11 | indigo | household ammonia |
| 12–14 | violet-purple | bleach, oven cleaner |
A useful memory aid: the colours follow the order of a rainbow, from red for strong acids to violet for strong alkalis, with green in the middle for neutral.
How to use it
Universal indicator solution
- Put a small sample of the test solution in a test tube or spotting tile.
- Add two or three drops of indicator.
- Mix, then compare the colour against the chart in good white light against a white background.
- Record the pH to the nearest whole number.
Universal indicator paper
- Use a clean glass rod to put a drop of test solution onto a small piece of paper. Don’t dip the paper into the stock bottle: indicator could leach out and contaminate the sample.
- Compare the colour with the chart within a few seconds, before it dries or changes.
Paper is better for coloured or cloudy solutions (the colour shows on the paper, not in the liquid) and for testing tiny amounts. Solution is better for watching colour changes as you add acid or alkali.
Testing gases
Damp universal indicator paper can test gases. Ammonia turns it blue or purple; hydrogen chloride and sulfur dioxide turn it red. The paper must be damp, because gases need to dissolve in water to form ions.
How accurate is it?
Universal indicator gives pH to about ±1 unit in normal use, sometimes ±0.5 with a narrow-range paper and good lighting. The limit comes partly from the dyes, whose colours blend gradually, and partly from human colour perception, which varies between people and lighting conditions. That’s fine for:
- sorting substances into strong acid, weak acid, neutral, weak alkali and strong alkali
- checking that a neutralisation is roughly complete
- quick field tests of soil, ponds or pools
It’s not good enough to distinguish pH 7.2 from pH 7.4, as you’d need for blood samples, or to measure a Ka value. For that, use a calibrated pH meter.
Narrow-range papers are available for specific jobs: for example 0–6, 6–8 or 5.5–9.0. They show more colour steps over a smaller range and can resolve about 0.2–0.5 pH units.
Why you can’t use it for titrations
In a titration, you want one sharp colour change exactly at the equivalence point. Universal indicator changes gradually all the way through the pH range. As you add alkali to acid, the colour shifts steadily from red through orange and yellow, so there’s no single moment you could call the end point. Worse, different people would stop at different shades.
For titrations, use a single indicator such as methyl orange or phenolphthalein, chosen to match the equivalence point. See how to choose an indicator.
Worked examples: reading the result
1. A solution turns orange with universal indicator. What can you say about it? It has a pH of about 4, so it’s a weakly acidic solution, or a very dilute strong acid.
2. Two acids of the same concentration give red and orange colours. Which is stronger? The red one (lower pH), because at equal concentrations a stronger acid releases more H⁺. See strong vs weak acids.
3. During a neutralisation, the indicator goes from purple to green. What has happened? An alkali has been neutralised by adding acid, reaching approximately pH 7.
4. A student adds universal indicator to lemonade and gets a hard-to-read murky brown. The drink’s own colour is interfering. Use universal indicator paper, or a pH meter.
A neutralisation you can watch
Universal indicator makes neutralisation spectacular:
- Put 25 cm³ of dilute sodium hydroxide in a beaker with a few drops of universal indicator. It turns purple.
- Add dilute hydrochloric acid slowly from a burette while stirring.
- Watch the colour sweep backwards through the rainbow: purple, blue, green, yellow, orange, red.
Each colour corresponds to a pH region. It’s a vivid way to see the logarithmic pH scale: it takes a lot of acid to move from purple to blue, then a single drop can jump from blue past green to yellow near the neutral point. Wear eye protection.
Storage and shelf life
- Keep bottles tightly closed. The solution contains ethanol, which evaporates.
- Store papers in their sealed pack away from acid or ammonia fumes, which can change their colour.
- Discard solutions that have faded or changed colour in the bottle.
- Universal indicator solution is flammable because of its alcohol content; keep it away from flames.
Common mistakes
- Using it for titrations.
- Reading the colour in yellow artificial light, which distorts greens and blues.
- Dipping the paper into a stock bottle, contaminating it.
- Reading dried paper, which gives a false colour.
- Reporting pH to decimal places. Universal indicator can’t justify that precision.
Key takeaways
- Universal indicator is a blend of several indicators that together cover the whole pH scale.
- Its colours run like a rainbow: red for strong acid, green for neutral, violet for strong alkali.
- Accuracy is about ±1 pH unit; use a pH meter for precise work.
- It’s ideal for estimating pH and demonstrating neutralisation, but unsuitable for titrations.
- Learn how single indicators work in acid–base indicators.
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