Make a Red Cabbage pH Indicator

Red cabbage is secretly one of the best pieces of chemistry equipment you can buy at a supermarket. Its purple leaves are full of pigments that change colour dramatically with pH, from red through purple and blue to green and yellow. With a knife, some hot water and a few household liquids, you can make your own universal indicator and use it to sort substances into acids and alkalis.

This practical works in a classroom, a kitchen or a science club, and it introduces real chemistry: equilibria, conjugate acids and bases, and the link between molecular structure and colour.

Aim

To extract a natural pH indicator from red cabbage and use it to classify household substances as acidic, neutral or alkaline.

The chemistry in brief

Red cabbage contains anthocyanins, a family of water-soluble plant pigments (the main ones in red cabbage are cyanidin-based). Anthocyanins can gain or lose protons, and each form has a different arrangement of double bonds, so each absorbs a different colour of light.

pH	Approximate colour of red cabbage extract
1–2	red
3–4	pink-red to pink-purple
5–6	purple
7	blue-purple to violet
8	blue
9–10	blue-green to green
11–12	green to yellow-green
13–14	yellow

At very high pH, over time, the pigment starts to break down irreversibly, which is part of why the yellow colour appears. Exact colours vary with the cabbage, the water and how concentrated your extract is, so it’s worth making your own reference scale (see below).

For the general principle of how indicators work, see acid–base indicators.

Equipment and materials

• About a quarter of a red cabbage
• Knife and chopping board (or a blender)
• Heatproof jug or beaker and a kettle (or a pan)
• Sieve or coffee filter
• Clear cups, test tubes or a white spotting tile
• Droppers or teaspoons
• Labels and a pen
• Eye protection

Suggested test substances (all easily available and safe in small amounts, with care): lemon juice, white vinegar, a clear fizzy drink, milk, tap water, distilled water, baking soda (sodium hydrogencarbonate) solution, washing soda (sodium carbonate) solution, liquid hand soap, toothpaste diluted with water, antacid tablet dissolved in water.

Do not use oven cleaner, drain cleaner, bleach or concentrated acids in a home setting. In a school lab, a teacher may demonstrate dilute hydrochloric acid and dilute sodium hydroxide solutions (around 0.1 mol/dm³) to show the extremes, with eye protection for everyone.

Method

Part 1: make the indicator

1. Chop the cabbage finely, or blend it with some water.
2. Put the chopped cabbage in a heatproof jug and cover it with hot water (not boiling in a school setting; about 70–80 °C works well). An adult or teacher should handle the hot water.
3. Leave for 10–15 minutes, stirring occasionally, until the water is deep purple.
4. Strain through a sieve or coffee filter into a clean container. The purple liquid is your indicator.
5. Let it cool. Store it in the fridge and use within a few days, as it can go off.

Part 2: test the substances

1. Label a series of cups or tubes with the names of the test substances.
2. Add the same volume of indicator to each, about 20 cm³ in a cup or 2 cm³ in a test tube.
3. Add a small, similar amount of each test substance, for example 1 teaspoon of liquid or a pinch of solid.
4. Stir and record the colour against a white background.
5. Include a control: indicator plus distilled water, to show the neutral colour.

Part 3: make a reference scale (optional but recommended)

If your school has buffer solutions of known pH (for example, pH 2, 4, 7, 10 and 12), add indicator to each. The colours give you a calibrated scale for your own batch of extract, which is more reliable than any printed chart.

Results table

Draw up a table like this:

Substance	Colour with red cabbage	Estimated pH	Acidic / neutral / alkaline
Distilled water (control)
Lemon juice
White vinegar
Baking soda solution
…

Typical results

Substance	Typical colour	Approximate pH
Lemon juice	red-pink	2–2.5
White vinegar	pink-red	2.5–3
Fizzy drink	pink	3–4
Milk	purple	6.5–6.8
Tap water	purple to violet	6.5–8, depending on area
Baking soda solution	blue-green	about 8.3
Hand soap	blue to green	8–10
Antacid solution	blue-green	8–9
Washing soda solution	green	about 11

Extension: watch neutralisation happen

1. Put some baking soda solution in a cup with indicator. It turns blue-green.
2. Add vinegar a few drops at a time, stirring.
3. The mixture fizzes (carbon dioxide) and the colour shifts back through blue and purple to pink as the solution becomes acidic.

This shows that acids and bases cancel each other’s effects. The reaction is:

NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂

Extension: indicator paper

Soak strips of coffee filter paper or thick kitchen paper in concentrated cabbage extract, let them dry on a clean surface, and cut them into strips. Dip them into test solutions to get a quick colour reading, like homemade litmus paper.

Evaluation: sources of error

• Different amounts of test substance change how far the pH shifts. Use the same volume each time.
• Colour judgement is subjective. Compare against a reference scale and a white background, and have two people agree.
• Coloured test substances (such as cola or orange juice) distort the result.
• Dilution by the indicator means the pH you see is not quite the pH of the original substance.
• Old extract may have changed colour or gone mouldy.

Safety

• Wear eye protection when handling any test substance, including household ones.
• Hot water can scald; an adult or teacher should handle it.
• Don’t taste any of the test mixtures.
• Wash hands afterwards and pour solutions away with plenty of water.
• Washing soda and soap solutions irritate eyes; rinse immediately with water if splashed.

Discussion questions

1. Why do we include distilled water as a control?
2. Which substance was the most alkaline? What evidence supports this?
3. Why is red cabbage indicator less suitable than phenolphthalein for an accurate titration?
4. Suggest why the colours you observed might differ slightly from those of another group.

Hints: (1) It shows the colour at neutral, for comparison. (3) Its colour changes gradually over a wide range, with no single sharp change. (4) Different cabbage, extract strength, amount added, lighting.

Key takeaways

• Red cabbage contains anthocyanins that change colour across the whole pH range.
• The extract acts like a homemade universal indicator: red for acid, purple near neutral, green and yellow for alkali.
• Controls, equal volumes and a reference scale make the results more reliable.
• It’s great for classifying substances, but not precise enough for titrations.
• Compare it with the commercial version in universal indicator.