Indicators and the pH scale sim
Test eleven real solutions and three mystery tubes with universal indicator, litmus, methyl orange, thymolphthalein, phenolphthalein and a pH meter, and build your own pH scale.
Acids, bases, salts and ammonia · covers the O-Level, IGCSE, GCSE topic
What the sim shows
Each tube holds a fresh 2 cm³ sample, and every indicator test uses a new sample, because two indicators in one sample would mix their colours. Nothing happens until you act: each drop of a tool or button press is one step (two drops, one dip, one rinse) on the tube ringed in the rack (choose it by dragging or with ← →); only the colour spreading and the meter's 1.5 s settle run on after it. Universal indicator gives a colour for every pH: red in strong acid, through orange, yellow, green at 7 and blue, to indigo and violet in strong alkali. Read the approximate pH from the chart, but chart colours differ slightly between brands. The other indicators each change over a short range, and inside it the colour is a blend (drawn here as a straight-line mix): methyl orange is red below about pH 3.1 and yellow above 4.4, so it can't tell neutral from alkaline. Thymolphthalein is colourless below 9.3 and blue above 10.5. Phenolphthalein (used in AQA) is colourless below 8.2 and pink above 10. Litmus paper turns red in acid and blue in alkali, and doesn't change in neutral solutions; the used strip is laid on the tray in front of its tube. The pH meter was calibrated with buffers of pH 4, 7 and 10, and reads to ±0.1 (a fixed, repeatable scatter for each sample). Rinse the probe with distilled water between solutions, or the last solution carried over makes the reading wrong. The model pulls the reading towards the last solution by 40% of the difference in unbuffered neutral solutions and 15% in the others, at most 2.5 pH units: water straight after hydrochloric acid reads 7.0 + 0.4 × (1.0 − 7.0) = 4.6, give or take the meter's 0.1. The pH values are typical, at 25 °C: pure water is 7.0 only when freshly boiled; in air it dissolves carbon dioxide and falls to about 5.6. Real vinegar (about 0.8 mol/dm³) is about pH 2.4; the tube here is 0.1 mol/dm³ ethanoic acid. Limewater is calcium hydroxide, a strong alkali, but only about 0.02 mol/dm³ dissolves. Strength isn't the same as pH: compare solutions at the same concentration. 0.1 mol/dm³ hydrochloric acid has pH 1.0 but ethanoic acid only 2.9; sodium hydroxide 13.0 but ammonia 11.1. [H⁺] = 10⁻ᵖᴴ mol/dm³ and [H⁺][OH⁻] = 1.0 × 10⁻¹⁴, so each pH unit is a tenfold change, which is why the H⁺/OH⁻ bars use a log scale.
- pH meter display: pH to 0.1, STABLE after 1.5 s (close-up below)
- Universal indicator chart, pH 0–14
- Fresh 2 cm³ sample in each 12 × 100 mm tube; the ringed tube is the one you test
Use it in your lessons
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