pH Curves Virtual Lab: Weak Bases and Polyprotic Acids

This pH curves virtual lab follows the A-level and IB practical where students measure a titration curve with a pH meter. They calibrate the probe with pH 4, 7 and 10 buffers, titrate ammonia with hydrochloric acid, record the pH after each addition and find the end point from ΔpH/ΔV. The curve gives pKb of ammonia at half-equivalence and shows which indicators would work. Phosphoric acid then gives two jumps on one curve. Every value below comes from the simulation.

Curriculum links

• AQA A-level Chemistry 7405, 3.1.12.5 (pH curves, titrations and indicators) and required practical 9: how pH changes when a weak acid reacts with a strong base and when a strong acid reacts with a weak base.
• IB Chemistry (first assessment 2025), R3.1: pH curves for strong and weak acids and bases, and the choice of indicator.
• AP Chemistry Unit 8, topic 8.5 Acid–Base Titrations (SAP-9.E), including polyprotic acids.

Simulic is not affiliated with or endorsed by AQA, the International Baccalaureate or the College Board.

Before the lab (5 min)

Ask students to commit to a prediction, on paper or as question 1 of the class link:

"20 mL of 0.10 M ammonia is titrated with 0.10 M HCl. Will the pH at the equivalence point be below 7, exactly 7 or above 7?"

Students who know weak acid curves often say "above 7".

Method in the simulation

1. Set View to pH-probe lab. Keep Flask + burette on NH₃ + HCl (weak base, strong acid), C(NH₃) 0.10 M, V(NH₃) 20 mL and C(HCl) 0.10 M.
2. Calibrate: press Buffer pH 4, Buffer pH 7 and Buffer pH 10, note each reading, then press Calibrate.
3. Press Record at 0 mL. Add HCl with +1 mL, pressing Record after each addition. Between 19 and 21 mL, use the V(HCl) added slider to add 0.5 mL at a time.
4. Read the analysis under the results table: the end point and the pH at half that volume. Check the indicator table.
5. Change Flask + burette to H₃PO₄ + NaOH (triprotic acid) (the probe stays calibrated) and record at 0, 10, 19, 20, 21, 30, 39, 40, 41 and 50 mL.

V(HCl) (mL)	0	5	10	15	19	19.5	20	20.5	21	25	30
pH (NH₃)

Expected results

Calibration. The uncalibrated probe reads 4.46, 7.28 and 10.10 in the buffers. After Calibrate, readings are correct.

Ammonia with HCl:

V(HCl) (mL)	0	5	10	15	19	19.5	20	20.5	21	25	30
pH	11.12	9.73	9.25	8.77	7.97	7.66	5.28	2.91	2.61	1.95	1.70

• The equivalence point is at 20.00 mL and pH 5.28: NH₄⁺ is a weak acid, so the solution is acidic.
• At half-equivalence (10.00 mL) pH = 9.25 = pKa(NH₄⁺), so pKb(NH₃) = 14 − 9.25 = 4.75. From the readings above, the analysis line gives an end point of 19.75 mL and pKb ≈ 4.74.
• Indicator table: methyl red changes at 20.00 mL (0.0%) and methyl orange at 20.02 mL (+0.1%), but phenolphthalein changes at 18.36 mL (−8.2%).

Phosphoric acid with NaOH: 1.63 at 0 mL, 2.29 at 10, 3.49 at 19, 4.70 at 20, 5.92 at 21, 7.20 at 30, 8.48 at 39, 9.66 at 40, 10.83 at 41 and 11.86 at 50 mL. There are two jumps, at 20 and 40 mL. Halfway between them, at 30 mL, pH = 7.20 = pKa₂. There is no third jump: HPO₄²⁻ (pKa 12.35) is too weak an acid.

Questions for students

1. (Prediction, asked again after the lab) Is the equivalence pH for NH₃ with HCl below, at or above 7?
2. Which is a control variable?
3. What does the calibrated probe read after 10.00 mL of HCl?
4. What is the pH at 30.00 mL in the H₃PO₄ titration?
5. Why is methyl red a better indicator than phenolphthalein for NH₃ with HCl?

Answers for teachers: (1) Below 7 (5.28). (2) The HCl concentration. (3) 9.25 (accept 9.20–9.30), so pKb = 4.75; uncalibrated, it reads 9.39. (4) 7.20 (accept 7.15–7.25), equal to pKa₂. (5) The jump runs from about pH 7.7 to 2.9. Methyl red changes at about pH 5, inside it (20.00 mL); phenolphthalein changes at 8.2, before the jump (18.36 mL, 8% early).

Common misconceptions

• "A pH meter always reads the true pH." Uncalibrated, it reads 9.39 at half-equivalence instead of 9.25, giving pKb 4.61.
• "Every weak species gives an equivalence point above 7." Only a weak acid with a strong base. A weak base with a strong acid gives 5.28.
• "A triprotic acid gives three jumps." H₃PO₄ gives two, and citric acid only one, because its pKa values (3.13, 4.76, 6.40) are too close together.

Extension

• Sodium carbonate: choose Na₂CO₃ + HCl (two equivalence points). Phenolphthalein marks the first end point (20.13 mL) and methyl orange the second (39.80 mL).

FAQ

How is this different from the titration virtual lab?

The titration virtual lab uses the Classic titration view (HCl or ethanoic acid with NaOH). This one uses the pH probe, a weak base and polyprotic acids.

Can the class link open in the pH-probe lab?

Yes. In the link's starting values, set View to pH-probe lab (more systems, results table) and Titration system (lab view) to NH₃ + HCl (weak base, strong acid). Calibration is not a starting value, so students calibrate at the start.

Why does the jump look smaller than for HCl with NaOH?

The NH₃/NH₄⁺ buffer region has already brought the pH down to 7.97 by 19 mL. From 19 to 21 mL it falls 5.4 units, against 8.8 units (2.59 to 11.39) for HCl with NaOH.

Related simulations and guides

Buffer Solutions – Why Does the pH Hardly Change? Acid–base indicators – litmus, phenolphthalein, red cabbage, universal indicator

For buffers, see the buffer virtual lab; for unknown concentrations, the titration unknowns virtual lab.