Experiment 8: Red Cabbage pH Indicator

Simmer a red cabbage and the cooking water turns a deep, dramatic purple — and that liquid is a genuine scientific instrument. Red cabbage is loaded with anthocyanin pigments whose molecules change shape, and therefore colour, as the acidity around them changes. In this experiment you brew the indicator yourself and use it to sort household liquids along the pH scale.

The chemistry behind it

pH measures the concentration of hydrogen ions, H⁺, on a logarithmic scale:

pH = -log10[H+]

Each whole pH step is a tenfold change: lemon juice at pH 2 has [H⁺] = 10⁻² = 0.010 mol/L, one thousand times more concentrated than a pH 5 liquid. Anthocyanin flips between molecular structures as H⁺ levels change — protonated forms in strong acid absorb light one way (you see red/pink), neutral liquids leave it purple, and in bases the deprotonated forms shift through blue and green to yellow. One pigment, a whole rainbow of structures:

Variables. Independent variable: the test liquid. Dependent variable: the indicator colour, matched against the scale. Controlled variables: the same batch of indicator, the same indicator-to-liquid ratio, a white background, and the same lighting. Plain water is the essential control — it proves the indicator starts at neutral purple and that any colour change comes from the liquid, not the dye.

Safety first

• An adult does all chopping and boiling; the indicator liquid must cool before you handle it. Wear eye protection when pouring.
• Test only the listed food and soap liquids. Never mix household chemicals together — in particular, never mix bleach with acids or vinegar (it releases poisonous chlorine gas). Bleach appears in this lesson only as a virtual-lab preset.
• Do not taste test liquids, even familiar ones, once they are in lab cups.

Doing the investigation

Hands-on. An adult simmers chopped red cabbage in water for 10 minutes; strain and cool the purple liquid — that is your indicator. Set out six labelled cups: lemon juice, white vinegar, plain water, baking-soda solution (1 tsp in ½ cup water), soap solution, and milk. Add about a teaspoon of indicator to each, stir gently, and record the exact colour against a white card.

In the virtual lab below (Red Cabbage pH Indicator) drag the pH slider slowly from 0 to 14 and watch the anthocyanin run through its colours, or pick a preset liquid from the dropdown. Read off the pH, the [H⁺] value and the classification. Follow its guided procedure: find the pure-purple point (about pH 6–7 — the neutral zone), visit lemon juice and bleach at the extremes, and log the colours of three liquids in your observation log. The bar chart lines the preset liquids up by pH so you can see the tenfold staircase.

Worked investigation: sorting the kitchen by pH

Hands-on results with one batch of indicator:

Processing — putting numbers on the extremes:

Lemon juice, pH 2:    [H+] = 10^-2    = 0.010 mol/L
Soap solution, pH 9.5: [H+] = 10^-9.5 = 3.2 x 10^-10 mol/L
Ratio: 0.010 / 3.2x10^-10 = about 3 x 10^7
Lemon juice has roughly 30 million times more H+ per litre than the soap solution.

Conclusion. The colour order — red/pink for the two kitchen acids, purple for neutral water, blue-green then green as the bases strengthen — matches the virtual lab's presets within about one pH unit throughout. Milk sits just on the acid side of neutral, which fits its real pH of 6.6. And the mixture prediction from the original lesson checks out: stir lemon juice into baking-soda solution and it fizzes (the acid–carbonate reaction from Experiment 1) while the colour marches back toward purple as the acid is neutralised.

Uncertainty. Matching colours by eye is good to about ±1 pH unit — that is a factor of ten in [H⁺], so treat cabbage estimates as sorting, not measuring. Two traps widen the error: milk's own whiteness dilutes the colour you see, and each new batch of indicator brews to a different strength.

Evaluation: how good is this evidence?

• No calibration scale (the main weakness). Colours were judged from memory of the scale. Improvement: first build a reference row of known-pH liquids (the virtual lab's presets give you the targets), photograph it, and match unknowns against that row in the same light.
• Indicator dose varied. A teaspoon "about" is not a measurement; a stronger dose deepens every colour. Improvement: pipette the same volume of indicator and test liquid every time.
• The indicator is itself slightly acidic, so it nudges very dilute samples. Improvement: use the smallest indicator volume that still shows a clear colour.
• Coloured test liquids (cola, juices) tint the result. Improvement: note the liquid's own colour in a no-indicator cup and subtract it mentally — or choose colourless test liquids.

Common mistakes

• Testing a liquid before the indicator has cooled or with a dirty stirrer — carry-over between cups smears the whole series.
• Calling every green result "a strong base": green starts around pH 9; strength needs the comparison row.
• Forgetting the water control, then being unable to prove the indicator started neutral.
• Ranking by how *pretty* the colour is instead of matching to the scale in order.

Exam Experience

Question 1 — calculation [4 marks]. Liquid A has a pH of 3 and liquid B a pH of 6. (a) How many times greater is the [H⁺] of A than of B? (b) A third liquid has [H⁺] = 1 × 10⁻⁴ mol/L. Calculate its pH and classify it as acid, neutral or base. (c) What colour would cabbage indicator show in it?

Scoring guide:

• Each pH step is ×10, and 3 steps = 10³ = 1,000 times greater [1]
• pH = −log₁₀(10⁻⁴) = 4 [1]
• pH 4 < 7, so it is an acid [1]
• Pink/red (between lemon juice and vinegar on the scale) [1]

Question 2 — design [4 marks]. A student wants to use cabbage indicator to estimate the pH of an unknown cleaning spray to the nearest pH unit. Describe how they should first calibrate the indicator, and how they should then test the spray fairly.

Scoring guide:

• Prepares a reference series of liquids of known pH (buffer solutions or the virtual lab's preset values) with indicator added [1]
• Uses the same indicator batch and the same indicator:liquid ratio for references and unknown [1]
• Compares colours against a white background in the same lighting, immediately after mixing [1]
• Reports the estimate as a value ±1 pH unit, recognising the method's limit [1]

Multiple choice 1. pH is a measure of:

• A. How much liquid there is
• B. The concentration of hydrogen ions, on a log scale
• C. The temperature of a solution
• D. How strongly a liquid is coloured

Answer: B. pH = −log₁₀[H⁺]; the log scale compresses an enormous range of concentrations into 0–14.

Multiple choice 2. Why is plain water an essential cup in this experiment?

• A. It dilutes the indicator to the right strength
• B. It shows the indicator's neutral colour, proving colour changes in other cups come from the test liquids
• C. It cleans the stirrer between tests
• D. It is the most acidic liquid available

Answer: B. Water at pH 7 is the control: purple is the "no effect" baseline against which every acid or base shift is judged.

Going deeper: from cabbage juice to a calibrated scale

Your cabbage brew is one member of a large family. Universal indicator is a cocktail of several dyes chosen so their colour changes tile the whole 0–14 range — red, orange, yellow, green, blue, violet — trading the cabbage's pretty subtlety for readability. Litmus, the oldest indicator of all, comes from lichens and makes a simpler binary decision: red in acid, blue in base. Laboratories then graduate to pH papers (indicator dried onto strips, good to about ±0.5) and finally to the electronic pH meter, which measures the voltage across a glass electrode and reads to ±0.01 — the same quantity your eyes estimate to ±1.

The calibration habit is what separates these instruments from a lucky guess. Before trusting an unknown, a chemist runs standards: solutions whose pH is guaranteed (buffer capsules are sold for exactly this). Your reference row of known liquids in the evaluation section is the kitchen version of the same discipline — and the virtual lab's preset list (lemon 2.0, vinegar 2.8, cola 3.2, milk 6.6, water 7.0, baking soda 8.4, soap 9.5, bleach 12.5) is a ready-made set of standards to calibrate against.

Anthocyanins show up far beyond this lesson. The red of pickled red cabbage is the same pigment held in its acid form by vinegar; cook the cabbage in hard (alkaline) water and it slumps to an unappetising blue-grey — chefs add a splash of acid to rescue the colour. Gardeners test soil with indicator kits because soil pH decides which nutrients plants can absorb, and blueberries — full of anthocyanins themselves — demand acidic soil around pH 4.5–5.5. The pigment that sorted your kitchen liquids is quietly running gardens, kitchens and dye works all at once.

Watching neutralisation happen in colour

The indicator also lets you *watch* Experiment 16's neutralisation in slow motion. Take the vinegar cup (pink, about pH 3) and add baking-soda solution one spoonful at a time, stirring: the pink softens to rose, then mauve, then — at one particular spoonful — flips to purple, the neutral colour; the next spoonful pushes it to blue-green and you have overshot. That flip point is the moment the moles of base added match the moles of acid present, the same equivalence idea the antacid spoonful-counting uses. Two lessons hide in the flip: indicators do not just classify, they can *count* (each spoonful is roughly equal, so the flip spoonful estimates the acid amount), and endpoints are easy to overshoot — which is why titration technique adds the last part drop by drop.