Natural pH Indicators from Plant Pigments
Summary
A great many fruits, flowers, and vegetables carry pigments that change colour with acidity. In this experiment you’ll extract those pigments, test them against everyday acids and bases, and build a colour-to-pH1 chart — turning the kitchen into a working indicator2 kit. You’ll be able to explain why red cabbage sweeps through a whole rainbow while beet juice barely shifts, and you’ll dry your extract into reusable pH paper.
History
Long before manufactured indicators, dyers and early chemists read acidity from plants. Robert Boyle described using violet and other flower extracts to test “spirits” in the 1660s, and litmus — still the archetypal indicator — is a dye extracted from lichens. The pigments doing the work in red cabbage and berries, the anthocyanins, were named in 1835 (Greek anthos, flower; kyanos, blue). What Boyle saw as a curiosity we can now explain as a shift in molecular structure that follows the concentration of hydrogen ions.
Hazards & preparation
This is one of the safest experiments on the site — the pigments are food, and the test solutions are household staples.
- Wear gloves if you’d rather not stain your hands; beet and cabbage juice are persistent dyes.
- Handle the boiling extraction water with the usual kitchen care.
- Household ammonia and drain-cleaner-strength bases are skin and eye irritants — keep them dilute and wear glasses when testing the alkaline end.
See the Safety page for general lab habits.
Part A — Extract a Red-Cabbage Indicator
Red cabbage is the classic starting point: it holds a high concentration of anthocyanins and shows the widest colour range of any common plant.
Materials:
- Red cabbage — a few leaves
- Water, a pot or kettle, a strainer
- A clear glass or jar
Steps:
- Chop the cabbage finely — more cut surface releases more pigment.
- Cover with water and simmer 10–15 minutes (or steep in freshly boiled water until the liquid is deep purple).
- Strain off the leaves. The purple liquid is your indicator. Cool it before testing.
The extract keeps a few days refrigerated and freezes well in an ice-cube tray.
Part B — Map the pH Scale
Now watch a single pigment paint the whole acid–base range.
Materials:
- Red-cabbage extract from Part A
- A row of clear cups or a white ice-cube tray
- Test solutions spanning the scale: lemon juice or vinegar (acidic), cream of tartar, plain water, baking-soda solution, washing soda, dilute household ammonia (basic)
Steps:
- Put a splash of extract in each cup.
- Add a different test solution to each and stir.
- Line the cups up from most acidic to most basic and record the colours.
You should see something close to this progression:
| pH | Colour | Typical solution |
|---|---|---|
| 1–2 | Red | Strong acid |
| 3–4 | Pink / magenta | Vinegar, lemon juice |
| 5–6 | Purple | Weak acid |
| 7 | Blue-purple | Pure water |
| 8–9 | Blue | Baking soda |
| 10–11 | Blue-green | Washing soda |
| 12–13 | Green | Ammonia |
| 14 | Yellow | Strong base |
Part C — Compare Pigment Sources
Different plants carry different pigment chemistry — and behave differently.
Extract and test the same way as Part A/B:
- Hibiscus (dried flowers, steeped like tea) — anthocyanin; vivid red → purple → blue-green. One of the brightest indicators.
- Butterfly pea flower — anthocyanin; deep blue at neutral, purple in acid, green in base. Famous for the “colour-changing” gin and tonic.
- Turmeric (dissolve in rubbing alcohol) — this pigment is curcumin, not an anthocyanin. It stays yellow through acid and neutral, turning red-brown only above about pH 8 — a base detector rather than a full-range indicator.
- Beet (juice or simmered extract) — this pigment is a betalain, and it barely shifts with pH. In strong base it turns brown and stays that way.
Record the acid / neutral / base colour for each. Two plants can both look red-purple in the jar and yet respond to acid completely differently — the give-away that they are built from different molecules.
Part D — Make Natural pH Paper
Turn the extract into strips you can carry to any solution.
Steps:
- Soak white coffee filters or watercolour paper in strong cabbage or hibiscus extract.
- Let them dry completely (out of direct sun), then cut into strips.
- To test an unknown, dab a drop on a strip and match the colour to your Part B chart.
What you should see
In Part B the row of cups fans out into a full spectrum from a single purple liquid — the clearest possible demonstration that colour is tracking pH, not the pigment. In Part C the anthocyanin plants (cabbage, hibiscus, butterfly pea) all sweep through similar rainbows, while turmeric flips only at the alkaline end and beet stubbornly refuses to play along. The dried strips in Part D reproduce the Part B colours when wetted with the matching solution.
| Symptom | Likely cause | Fix |
|---|---|---|
| Extract too pale to read | Not enough pigment / over-diluted | Use more cabbage, simmer longer, add less water |
| Colours all look purple-ish | Test solutions too close to neutral | Push the extremes — stronger acid and base |
| Green fades to yellow-brown and won’t come back | Anthocyanin destroyed at very high pH | This is irreversible degradation; use fresh extract and milder base |
| Beet juice “doesn’t work” | Betalains are nearly pH-insensitive | Expected — that’s the point of Part C |
The Science
Anthocyanins and pH
Anthocyanins are not one colour of molecule but a set of interconverting forms, and which form dominates depends on the hydrogen-ion concentration:
- Flavylium cation (low pH): red, positively charged
- Quinoidal base (near neutral): purple-blue, neutral
- Carbinol pseudobase (higher pH): nearly colourless
- Chalcone (high pH): yellow, ring-opened
The equilibrium between these forms is what produces the rainbow:
\[\ce{AH+ (red) <=>[$-$H+] A (purple) <=>[$-$H+] A^- (blue) -> chalcone (yellow)}\]
Add acid (more H⁺) and the equilibrium shifts left toward the red flavylium cation; add base and it marches right toward blue and finally the yellow ring-opened chalcone. The last step can be irreversible — which is why an over-alkaline sample fades and never returns.
Betalains are a different molecule
Beet’s red comes from betalains, which contain nitrogen in the colour-producing core and are built on entirely different chemistry from anthocyanins. They are far less pH-sensitive and degrade (rather than reversibly shift) in strong base. This is the lesson of Part C: colour is not composition. Two red-purple juices can hide two unrelated pigment families.
Curcumin
Turmeric’s curcumin is a curcuminoid with an acidic –OH that only loses its proton above roughly pH 8, so the colour change is confined to the basic end — useful precisely because it ignores the acidic range.
Questions to Explore
Why does one purple liquid produce a whole spectrum of colours? What is actually changing as you move from the acidic cup to the basic one?
Hint / answer
The anthocyanin molecule switches between several structural forms, each absorbing different wavelengths. Hydrogen-ion concentration decides which form dominates, so pH — not the addition of any coloured substance — sets the colour.
Why does beet juice barely change colour while red cabbage sweeps the whole scale? They look similar in the jar.
Hint / answer
Beet’s colour is a betalain, a nitrogen-containing pigment that is only weakly pH-sensitive; cabbage’s colour is an anthocyanin whose several forms swap over across the pH range. Same shade in the jar, different molecules.
Why can a green cabbage sample fade to brown and never return to purple? What does that say about the equilibrium?
Hint / answer
At very high pH the anthocyanin opens to the chalcone form and can break down permanently. Most of the colour changes are a reversible equilibrium, but this last degradation is one-way — the pigment is destroyed.
Why is turmeric useful even though it only changes colour once? When would you reach for it instead of cabbage?
Hint / answer
Curcumin only shifts above about pH 8, so a colour change unambiguously signals a base. When you just need to know “is this alkaline?” a single sharp transition is clearer than a full rainbow.
Going further
- Test mystery liquids. Shampoo, soft drinks, antacids, and cleaning products span a surprising pH range — chart them with your strips.
- Bring in metal ions. Adding iron, copper, or alum to the same extracts produces a second family of colour changes through coordination rather than pH. That’s the next experiment: Metal Ions and Plant Pigments.
- In the Acid–Base track: compare this home-made indicator against the buffered, multi-indicator view in The pH Landscape.