Investigating Natural Indicators for Acids and Bases
An indicator is a substance that shows one colour in an acid and a different colour in a base. That is the whole definition. It does not test for anything, it does not react with anything you care about, and it is not used up. It changes colour, and you read the colour.
This page covers what an indicator is, all six you meet in a Grade 11 laboratory with their real colour ranges, and the prescribed practical where you make one yourself out of a cabbage.
What an indicator is, and what it is not
Learners almost always say an indicator "tests for acid". It is close enough to sound right and it costs a mark.
An indicator does not test. It responds. Drop it into a solution and it takes on a colour that depends on how acidic or basic that solution is. You are the one doing the testing, by reading it.
And it is not consumed. Add acid to a tube of red cabbage extract until it turns red, then add base until it turns green, then acid again. It goes back to red. The indicator is still there, still working, doing the same thing it did the first time.
The ones you meet at school split into two groups.
| Examples | Where they come from | |
|---|---|---|
| Synthetic | Litmus, methyl orange, methyl red, phenolphthalein, bromothymol blue, universal indicator | Made in a factory |
| Natural | Red cabbage, red rose petals, beetroot, red onion, mulberries, blueberries, cherries, curcumin from turmeric | Extracted from a plant |
Litmus sits awkwardly between the two. It is a dye from lichen, so it is natural in origin, but it reaches a school as a manufactured paper and everybody treats it as a standard reagent. If a question asks you to name a natural indicator, do not answer litmus.
All six compared
This is the table to know. The colour on its own is not the answer, the pH range over which it changes is what decides which indicator you use for which job.
| Indicator | In acid | In base | Changes around pH | Used for |
|---|---|---|---|---|
| Litmus | Red | Blue | 4,5 to 8,3 | Is it an acid or a base. Nothing more |
| Methyl orange | Red | Yellow | 3,1 to 4,4 | Titrating a strong acid against a weak base |
| Bromothymol blue | Yellow | Blue | 6,0 to 7,6 | Anything near neutral. Also dissolved CO2 |
| Phenolphthalein | Colourless | Pink | 8,3 to 10,0 | Titrating a weak acid against a strong base |
| Universal indicator | Red to orange | Blue to violet | The whole scale | Estimating a pH when you have no meter |
| Red cabbage | Red | Green to yellow | Roughly 2 to 12 | Teaching. And it costs R25 |
Phenolphthalein being colourless in acid is the one that trips people up. An empty-looking flask is not a flask you forgot to add indicator to. Colourless is the acid reading, and the endpoint is the first permanent flush of pink.
Universal indicator is not a single substance. It is a blend of several indicators chosen so that something in the mixture is changing at almost every pH. That is why it runs through a whole rainbow instead of flipping between two colours, and it is also why it is less precise than a single indicator at a titration endpoint.
Natural indicators, and why red cabbage is the one
Plenty of plants work. Beetroot, red onion, mulberries, blueberries, cherries, red rose petals and turmeric all contain pigments that shift colour with pH.
Red cabbage is the one worth using, and it is not close.
| Red rose petals | Red cabbage | |
|---|---|---|
| Solvent | 25 ml ethanol, which is flammable | Hot water |
| Extraction time | Two hours, plus ten minutes warming | Ten minutes |
| Availability | Seasonal, and whatever is in the garden | Any supermarket, all year |
| Cost | Free if you have roses | About R25 a head, which does four classes |
| Colours | Red, green, blue | Red, purple, violet, blue, green, yellow |
The pigments doing the work are anthocyanins. An anthocyanin molecule picks up a proton in acid and loses one in base, and the two forms absorb different wavelengths, so they look like different colours. That is the mechanism, and one sentence of it is all Grade 11 needs.
The red cabbage scale
| pH | Colour | Something at roughly that pH |
|---|---|---|
| 2 | Red | Lemon juice |
| 3 | Red-pink | Vinegar, fizzy drinks |
| 4 | Purple | Tomato juice |
| 6 | Violet | Milk |
| 7 | Blue | Pure water |
| 8,5 | Blue-green | Bicarbonate of soda solution |
| 10 | Green | Soap solution |
| 12 | Greenish yellow | Bleach, oven cleaner |
That is a working pH scale made from a vegetable, and a class that has built one understands what universal indicator is doing far better than a class that was handed a bottle of it.
Where this fits in the curriculum
| Subject | Physical Sciences |
|---|---|
| Grade | 11 |
| Term | 3 |
| Topic | Chemical change, acids and bases |
| Status | An ordinary experiment. Not a formal assessment |
| Marks | 40 on our worksheet. None is prescribed |
The practical
Make an extract, put it in five tubes, and change the pH of four of them.
Apparatus
| Item | Qty |
|---|---|
| Test tubes, 25 x 150 mm, and two racks | 10 |
| Beaker, 250 ml | 2 |
| Beaker, 100 ml | 2 |
| Measuring cylinder, glass 25 ml | 1 |
| Filter paper, 90 mm and a funnel | 1 pk |
| Droppers and disposable pipettes | 1 pk |
| Stirring rod, glass | 2 |
| Red litmus, blue litmus and pH test paper | 1 ea |
| Goggles and gloves | 2 |
You supply the cabbage, the acid and the base. One head of red cabbage from any supermarket, and dilute hydrochloric acid and dilute sodium hydroxide, both of which every Grade 11 laboratory already holds.
Making the extract, and do this on the morning
- Chop a quarter of a red cabbage roughly. A whole head does four classes
- Cover it with about 250 ml of boiling water and leave it ten minutes. It goes deep purple
- Filter it into a clean beaker. You want the liquid, not the leaf
- Test it before the class arrives. One drop of acid into a tube of extract should go red instantly. If it does not, you used too much water
Make about 400 ml for eight groups. It keeps for a day in a fridge and then it starts to smell, so there is no advantage in making it the night before.
The method
- Five test tubes in the rack, about 5 ml of extract in each. They should all look identical. That is the control and it matters
- Tube 1: leave it alone
- Tube 2: add acid drop by drop, swirling, until the colour stops changing. Count the drops
- Tube 3: add base drop by drop, the same way
- Tube 4: two drops of acid, then base drop by drop until it matches tube 1 again
- Tube 5: something from home. Vinegar, lemon juice, bicarbonate of soda, soapy water, a fizzy drink
- Now repeat with litmus and pH paper, so the class can compare a homemade indicator against bought ones
What you should see
| Tube | Contents | Colour |
|---|---|---|
| 1 | Extract only | Purple |
| 2 | Extract + acid | Red or pink |
| 3 | Extract + base | Green |
| 4 | Acid, then base back to the start | Blue or violet |
| 5 | Whatever they brought | Depends |
Tube 4 is the one that teaches. Adding base to an acidified tube walks the colour back down the scale, and a learner who can say why has understood neutralisation, which is the actual curriculum content this practical sits inside.
Neutralisation, which is what tube 4 is really about
An acid and a base react to produce a salt and water. That is a neutralisation reaction, and it is why tube 4 comes back to purple.
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(ℓ)
The acid you added is being cancelled out by the base. As it is cancelled the solution moves back towards neutral, and the indicator, which only ever reports pH, follows it back down the colour scale.
This is exactly how a titration works. In Grade 12 you do the same thing with a burette instead of a dropper, so you can measure how much base it took, and from that work out the concentration of the acid. Same reaction, same indicator, one piece of glassware better.
Getting tube 4 back to exactly the starting colour by counting drops is hard, and that is the point. A group that overshoots has just discovered why a burette exists.
A note on the acid, because the standard method overdoes it
The prescribed version of this practical calls for 6 mol·dm-3 hydrochloric acid and 6 mol·dm-3 ammonium hydroxide.
That is a great deal of acid to change the colour of a dye. Six molar hydrochloric acid burns skin on contact, and six molar ammonium hydroxide fills a closed classroom with ammonia.
Use 0,1 mol·dm-3 of each and swap the ammonium hydroxide for sodium hydroxide. The colour change is identical, because an indicator responds to pH and both solutions are comfortably past the ends of the anthocyanin range. Nothing about the result changes and nothing about the learning changes.
If you are running the rose petal version, warm the ethanol in a water bath, never over a flame. The method does not say how to warm it, and ethanol vapour catches easily.
If it does not work
| What you see | What caused it |
|---|---|
| The extract is nearly colourless | Too much water. Use less water or more cabbage. This is the commonest failure by a wide margin |
| No colour change with the acid | The acid is too dilute, or the bottle is water. Check it on litmus first |
| Every tube goes green straight away | Base contamination. The tubes were not rinsed between uses |
| The colour appears then fades | The extract is more than a day old. Make it fresh |
| Tube 4 will not come back to purple | Normal. Hitting neutral by counting drops is genuinely difficult |
| Rose petal extract barely shifts | The extraction was too short, or the petals were pale. Cabbage does not have this problem |
| Blue litmus stays blue in the acid | It is not an acid, or the paper is old and damp |
| pH paper reads the same for everything | The strip was already wet when it went in |
| The whole class smells of cabbage | Unavoidable. Open a window |
The failure worth keeping
A group that does not rinse between tubes will get nonsense, and they should be left to find it themselves.
A trace of sodium hydroxide left in a tube turns the next sample green before a single drop of acid goes near it. That is contamination changing a result, which is the most useful thing anybody takes out of a practical, and it lands far harder when it is their own tube.
How the 40 marks are made up
| Section | Marks |
|---|---|
| Aim, hypothesis and variables | 6 |
| Results table, all five tubes | 10 |
| Explaining the colour changes | 8 |
| Comparison with litmus and pH paper | 6 |
| Household substances | 5 |
| Conclusion | 5 |
No mark allocation is prescribed. This experiment is set with no questions attached at all, unlike the two either side of it in the same chapter. The worksheet and this split are ours.
Eighteen of the forty marks are for the table and the explanation, because writing down a colour is easy and saying what it means is not.
The mark most often dropped is writing "it went red" instead of "it went red, so the solution is acidic". The colour is the observation. The acid is the conclusion. Two different marks.
If you have time
Make your own indicator paper. Dip strips of filter paper in the extract, let them dry, and test them against the bought litmus. A class that has manufactured a reagent understands what the reagent is.
Blow through a straw into a tube of extract. A minute of steady bubbling shifts the colour towards red, because carbon dioxide dissolves to make carbonic acid. That connects straight to acid rain, which is in the same chapter.
Cycle it. Acid until red, base until green, acid again. It goes back every time, which proves the indicator is not used up better than any sentence about it.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks, with the planning section, the five tube results table, the litmus comparison and the household substances
- Marking memorandum, with expected colours for both extracts, full mark allocation and the six places learners most often drop marks
Related practicals
- Titration, Grade 12. The same indicator chemistry with a burette and an actual calculation
- Prepare a standard solution, Grade 12. The step before a titration
- Percentage yield, Grade 11. The other Grade 11 chemistry practical
- Endothermic and exothermic reactions, Grade 11. A formal assessment, same term
Buy this experiment
We are putting together an Acid-Base Indicators Kit for Grade 11 with the tubes, racks, beakers, filtering gear, droppers and goggles in one box, together with red litmus, blue litmus and pH test paper, plus a printed teacher guide and the marking memo. Coming shortly.
In the meantime every item above is sold separately. The papers are the part to check your store cupboard for, and they are in the pH and litmus test papers range: red litmus, blue litmus, neutral litmus, pH test paper and pH 0 to 14 indicator strips, all in stock.
The cabbage you buy yourself. One head is about R25 and it does four classes, and the supermarket is closer to your school than we are.