Acid–Base Titrations and Titration Curves

Chemistry

1. Fundamental Concepts

Titration: A lab technique to determine concentration of an unknown acid/base by reacting it with a standard solution of known concentration (titrant).
Equivalence point: Moles of acid = moles of base (stoichiometrically neutralized).
Endpoint: Observed pH change via indicator color change, near equivalence point.
Titration curve: Graph of pH vs. volume of added titrant, showing pH changes during reaction.

2. Key Concepts

Strong acid–strong base: Equivalence at pH 7; steep vertical jump near pH 7.
Weak acid–strong base: Equivalence pH > 7; buffer region before equivalence; initial pH higher than strong acid.
Strong acid–weak base: Equivalence pH < 7; buffer region before equivalence.
Polyprotic acids: Multiple equivalence points (one per dissociable H⁺).
Buffer region: Flat portion of curve where pH changes little with added titrant.

3. Examples

Easy

Titrate 0.10 M HCl (strong acid) with 0.10 M NaOH (strong base).

Curve starts at low pH, rises sharply at equivalence (pH 7).

Medium

Titrate 0.10 M CH₃COOH (weak acid) with 0.10 M NaOH.

Shows buffer region; equivalence pH > 7; indicator: phenolphthalein.

Hard

Titrate H₃PO₄ (triprotic acid) with NaOH.

Three distinct equivalence points and buffer regions; pH differs at each step.

4. Problem-Solving Techniques

Identify strong/weak acid/base and reaction type.
Use stoichiometry to find moles before/after reaction.
For mixtures: determine excess H⁺/OH⁻ or buffer formation.
Use Henderson–Hasselbalch for buffer pH.
Locate equivalence point(s) on curve to find volume and pH.
Match indicator pKa to equivalence pH for accurate endpoint.