Chemistry / Unit 10 Chem 1884 worked example, 60 to 90 seconds
Reading Key Features of a Titration Curve
The idea
Reading a titration curve means locating the equivalence point at the steepest section's midpoint, reading the half-equivalence volume at exactly half that point's volume, and, for a weak acid titration, reading pKa directly off the pH value plotted at the half-equivalence point; for a weak base titration, that same pH value gives the conjugate acid's pKa (equivalently, pOH there gives pKb).
In practice
A weak acid titration reaching equivalence at 20.00 mL has its half-equivalence point at 10.00 mL, where the curve's pH equals the acid's pKa directly, by the Henderson-Hasselbalch relationship holding at equal acid and conjugate base concentrations there, a fact worth checking against any curve read this way.
Where people slip
Reading pKa from any point other than exactly half-equivalence, where acid and conjugate base concentrations are not yet equal, gives an incorrect value entirely.
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