Acid–Base Titration

Run a virtual titration for a range of acids, alkalis and indicators, and plot the pH curve.

Teaching notes

Choose a substance for the burette (titrant) and the conical flask (analyte), set their concentrations and the flask volume, then press Run to titrate — the burette empties, the flask's colour shifts with universal indicator, and the pH curve builds up point by point as it goes. Drag the volume added slider at any time to scrub to a specific point instead.

Reading the curve

The x-axis is a fixed 0–50 cm³, matching a standard burette's capacity — it never rescales. Changing a concentration doesn't zoom the graph; it slides the steep jump left or right along that fixed scale, exactly as it would on paper with a real burette. Two dashed gold lines mark the titre — the volume at which moles of acid equivalents added equal moles of base equivalents originally in the flask (or vice versa) — and ½ titre. At ½ titre, the pH equals the pK_a of whichever weak species is present — a genuinely useful way to read Ka off a graph rather than just being told the formula. If your choices push the titre past 50 cm³, the burette would run dry before reaching equivalence, and the stats panel says so directly.

The five combinations worth trying

Strong acid + strong base (default: HCl in the flask, NaOH in the burette). Nearly flat until right at the titre, then an enormous jump — several pH units within a single drop — before flattening out again. Equivalence pH = 7.00 exactly.
Weak acid + strong base (switch the flask to ethanoic acid). A shallower "buffer region" appears well before the titre, where pH resists change as acid and its conjugate base coexist. The jump at the titre is smaller, and the equivalence point sits above pH 7 (the leftover ethanoate ion is a weak base).
Weak base + strong acid (ammonia in the flask, HCl in the burette). The mirror image — equivalence point below pH 7, because the leftover ammonium ion is a weak acid.
Diprotic acid (H₂SO₄ in either vessel). Needs twice the moles of base to reach the titre, because each mole releases two H⁺. Concentrated H₂SO₄ also shows a little flattening near pH 2, from its second, genuinely weak dissociation (HSO₄⁻ ⇌ H⁺ + SO₄²⁻) — not something a "diprotic acid = 2× monoprotic" shortcut would show.
Two acids, or two bases, in both vessels. No neutralisation happens, so the curve just drifts gently with dilution — no sharp jump anywhere. This is itself the useful result: a jump only appears when the two vessels can actually react.

Suitable indicator

The stats panel computes, from the model, how much the pH changes over a single 0.05 cm³ drop either side of the titre, and checks whether that range covers methyl orange (pH 3.1–4.4) or phenolphthalein (pH 8.2–10.0). For strong-strong titrations both work; for a weak acid + strong base, only phenolphthalein works (methyl orange would change colour too early, well before the titre); for a weak base + strong acid, only methyl orange works. For anything without a sharp jump, neither does — a real limitation of indicator titrations, not just this simulation.

Suggested use

Leave the defaults (0.100 mol dm⁻³ HCl in the flask, 0.100 mol dm⁻³ NaOH in the burette, 25.0 cm³ flask) and press Run. Watch the pH sit near 1 for most of the run, then leap past 7 in a single drop.
Reset, switch the flask to ethanoic acid, and run again. Compare the shape: a flatter buffer region, a smaller jump, and an equivalence point you can read as being above 7.
Use the ½ titre marker to check a Ka or Kb calculation: read the pH there, and confirm it matches the pKa printed in the model notes.
Try H₂SO₄ against NaOH and check the titre stat is double what the same concentration of HCl would need.
Set both vessels to acids (or both to bases) and confirm the curve has no jump at all — then use the swap button to put the same two substances the other way round and see the mirror-image curve.

Key ideas

A titration's "sharp jump" happens because, right at the equivalence point, a single extra drop swings the solution from being dominated by one reactant to being dominated by the other — away from that point, there's a large buffering reservoir of unreacted material to absorb the change.
The steeper/larger the jump, the more precisely the equivalence point can be found with an indicator or a pH meter — which is why strong-strong titrations are the easiest to do accurately, and weak-weak titrations are essentially unusable with an indicator.
For a weak acid or weak base, the pH at half-titre equals its pKa (or the pKa of its conjugate acid, for a base) — the Henderson–Hasselbalch relationship, visible directly on the graph rather than derived separately.
The equivalence point of a weak acid vs strong base titration is above pH 7, and a weak base vs strong acid titration is below pH 7, because the "leftover" conjugate ion is itself a weak base or weak acid respectively.
Choosing the right indicator means choosing one whose colour-change range falls inside the sharp jump — not just "changes colour somewhere near pH 7".