Acid–Base Titration Lab
Add base to acid (or acid to base) drop by drop, watch the pH curve build, and pick the indicator that changes colour right at the equivalence point.
The equivalence point
A titration finds an unknown concentration by adding a solution of known concentration until the moles of acid and base are exactly equal: the equivalence point. Near it the pH jumps steeply, by several units for a single drop. A strong acid with a strong base gives pH 7 there; a weak acid with a strong base gives a basic solution (pH ≈ 8.7 here), because the salt it forms (acetate) is itself a weak base.
Buffers and indicators
Halfway to equivalence in a weak-acid titration, [HA] = [A⁻], so pH = pKa (4.74 for acetic acid). Around there the pH hardly moves: the mixture is a buffer. An indicator is a weak acid whose two forms have different colours; it changes over about 2 pH units, so choose one whose range sits inside the steep jump.
The burette holds 0.100 M titrant; 1 drop = 0.05 mL; 25 °C (Kw = 1.0 × 10⁻¹⁴, Ka(CH₃COOH) = Kb(NH₃) = 1.8 × 10⁻⁵). pH is found by solving the full charge balance, not by the usual shortcuts.