ScienceExplain

Acid–Base Titration Curve Lab

A titration is the chemist’s way of counting molecules by volume. You add one solution to another until they exactly neutralise each other — and if you record the pH all the way through, the shape of the curve tells you a surprising amount about the acid.

Acid–Base Titration Curve

Add base drop by drop and watch the pH jump

What the curve shows

Start with the strong acid (hydrochloric). The pH begins around 1, rises very slowly, and then — right at the equivalence point — shoots almost vertically from about pH 3 to pH 11. One extra drop of base can move the pH by several units.

Now switch to the weak acid (ethanoic acid). The curve looks completely different:

  • It starts higher, because a weak acid only partially ionises in water.
  • It climbs gently through a buffer region, where the solution resists pH change.
  • At exactly half the equivalence volume, pH = pKa. This is how chemists measure pKa in the lab: run the titration, find the half-equivalence point, read off the pH.
  • The equivalence point itself sits above pH 7, because the product is a weak base (the conjugate base of the acid).

The buffer region follows the Henderson–Hasselbalch relationship: pH = pKa + log([A⁻]/[HA]). When exactly half the acid has been neutralised, the two concentrations are equal, the log term is zero, and the pH equals the pKa.

Try this

  • Select the weak acid and press Add base slowly. Watch the flat buffer region — that is the acid and its conjugate base working together to soak up added hydroxide.
  • Stop at the half-equivalence point and check the readout: the pH should sit right at the pKa, about 4.74.
  • Press Jump to equivalence and compare the pH there for the strong acid (7.00) and the weak acid (well above 8).
  • Change the acid concentration. The curve shifts, but the equivalence pH barely moves for the strong acid.

Why it matters

Titration is how the concentration of an unknown solution is measured, how the acidity of soil and water is checked, and how the purity of food and pharmaceutical products is verified. Choosing the right indicator depends entirely on where that steep jump sits — pick one that changes colour outside the vertical region and the titration tells you nothing.

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