A Red Cabbage pH Ladder from Vinegar to Baking Soda
Half a red cabbage makes enough indicator for a full pH ladder of eleven household liquids. The colour changes are large, obvious and repeatable, which makes this the cheapest way to show that acids and bases are a spectrum and not two boxes.
Materials
| Item | Amount | If you do not have it |
|---|---|---|
| Red cabbage | 150 g, about a quarter head | Red onion skins give a weaker but usable indicator |
| Distilled or filtered water | 500 ml | Tap water works unless it is very hard or chlorinated |
| Saucepan and hob | 1 | A microwave and a heatproof jug |
| Clear glasses or white egg cups | 11 | Any small clear container that shows colour |
| Test liquids | see the ladder below | Any clear household liquid with a known rough pH |
| Strainer | 1 | A clean cloth over a jug |
Method
Shred the cabbage finely and simmer it in 500 ml of water for 10 minutes, lid on.
Watch for: The water turns deep violet within three minutes and the kitchen smells faintly of sulphur.
Strain the liquid into a jug and cool it to room temperature. You should have about 400 ml of violet indicator.
Watch for: Hot indicator reads slightly more red than the same liquid at room temperature.
Put 20 ml of indicator into each of the eleven glasses. Line them up in order from most acidic to most alkaline.
Watch for: The eleven glasses look identical; every difference you see later comes from the added liquid.
Add 10 ml of each test liquid to its own glass, swirl once, and photograph the row from a fixed distance.
Watch for: Colours span bright pink for lemon juice through violet for water to yellow-green for baking soda.
Now titrate: add vinegar drop by drop to the baking soda glass until the colour swings back through blue and violet to purple.
Watch for: The colour does not fade through grey; it steps through a distinct blue stage that lasts only a few drops.
Why it works
Red cabbage carries a family of anthocyanin pigments, mainly cyanidin glycosides, whose molecular structure changes with the concentration of hydrogen ions around them. Each structure -- flavylium cation, quinoidal base, chalcone -- absorbs a different band of visible light, so the same molecule reads red, then violet, then blue, then green as the pH rises. The pigment is a genuine indicator rather than a dye: it reports the solution it is sitting in. There are two colour-change regions rather than one, centred near pH 3 and pH 8, which is why the middle of the scale looks like a smooth gradient and the ends change abruptly.
Think of it like: The pigment works like a set of traffic lights wired to a single sensor. Acid keeps the light red, neutral flips it to violet, and base pushes it to green. The light is not changing colour by itself; it is reporting what the sensor reads.
Clean-up and disposal
The indicator is edible but bitter, so pour it down the sink with plenty of water. Anthocyanin stains on cotton lift with a cold water rinse before they dry; on a wooden chopping board, lemon juice lifts the purple back to pink and a light sanding removes the rest. Do not compost the strained cabbage if it has been simmered with anything salty.
Bench notes
The eleven-glass ladder
Line the glasses up left to right and label the bench with tape before you start. The reference order I use is lemon juice, white vinegar, cola, black coffee, milk, tap water, egg white, baking soda solution, washing soda, milk of magnesia, and a pinch of lye-free drain-free household ammonia.
Observed / 实测With 400 ml of indicator, 20 ml per glass leaves enough spare liquid to repeat any single rung twice.
Reading a colour you have not seen before
Keep a strip of the plain indicator on a white tile next to the row. Any glass that matches the strip exactly is neutral; anything pinker is acidic and anything bluer is alkaline.
Observed / 实测The egg white rung is the one that surprises people: it reads distinctly blue-green at about pH 9.
Handling the strong end of the ladder
Variations worth running
- Run the same ladder with tea, which contains different polyphenols, and compare where its colour changes sit.
- Freeze 20 ml of indicator in an ice-cube tray and drop one cube into a glass of vinegar to watch the colour front travel.
- Bubble exhaled breath through 20 ml of indicator with a drinking straw for two minutes and watch it drift from violet toward pink as carbon dioxide forms carbonic acid.
Questions that come up on the bench
- Why does the colour jump instead of changing smoothly?
- Anthocyanins have two separate sets of acid-base equilibria, one near pH 3 and one near pH 8. Between those two regions most of the molecules are already in the same form, so adding more acid or base barely moves the average colour.
- Can I use this to measure pH precisely?
- No. You can bracket a liquid between two colours, which is a range of about one pH unit at best. For a number you need a calibrated electrode or narrow-range indicator papers.
- Does it still work after a week in the fridge?
- Partially. Air and light oxidise the anthocyanins to brownish products, so the endpoints get muddier. A week-old indicator still separates strong acid from strong base but loses the middle of the ladder.