pH Curve Virtual Lab: Weak Acid–Strong Base and Strong Acid–Weak Base
AQA Required Practical 9 in the browser. Calibrate the pH meter with pH 7 and pH 4 buffers, titrate ethanoic acid with sodium hydroxide or ammonia with hydrochloric acid, choose smaller additions near the end point, and plot the curve as you go. Mark the equivalence point, read the pKa at half-equivalence and see which indicator's colour change falls on the steep section.
About this simulation
- What
- An interactive Chemistry simulation of pH Curve Virtual Lab: Weak Acid–Strong Base and Strong Acid–Weak Base.
- Who
- Designed for AP, IB, and A‑Level Chemistry students.
- How
- Runs in any modern browser — drag, adjust, and explore in real time.
Updated 2026-10-11
Key Concepts
Calibrating the pH meter
Set the meter in a pH 7.00 buffer, rinse the electrode, then set the slope in a pH 4.00 buffer. Without this, every reading — and the pKa you calculate — is shifted.
Small additions near the end point
The pH changes by several units within a fraction of a cm³ of equivalence. Add 0.10 cm³ at a time there, or the steep section will not be captured.
pKa from half-equivalence
Halfway to equivalence, [HA] = [A⁻], so pH = pKa. For ethanoic acid this gives about 4.76; for ammonia titrated with HCl, the pKa of NH₄⁺, about 9.25.
Choosing an indicator
Pick an indicator whose colour-change range lies within the steep section: phenolphthalein for a weak acid with a strong base, methyl orange for a strong acid with a weak base.
What This pH Curve Lab Teaches
A pH curve shows how the pH of a solution changes as acid or alkali is added from a burette. Its shape tells you the equivalence point, the strength of the acid or base, and which indicator would work. Measuring it with a pH meter is AQA A-level Required Practical 9.
The lab follows the standard method. Calibrate the meter with buffers, then add titrant and record the pH after each addition. Far from the end point large additions are fine; near it the pH jumps by several units within a fraction of a cm³, so you must switch to 0.10 cm³ additions. The model solves the exact charge balance at every point, including dilution.
In the analysis, mark the equivalence point at the middle of the steep section. Half that volume gives a buffer with equal amounts of acid and conjugate base, where pH = pKa — 4.76 for ethanoic acid, 9.25 for the ammonium ion. Then test the indicators against the steep section, and repeat without calibration to see the systematic error it causes.
Frequently Asked Questions
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