Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 14.21 Titration curves (Ka & buffer region)-Study Notes - New Syllabus

14.21 Using Titration Curves (Weak–Strong Systems)

Titration curves for weak acid–strong base or weak base–strong acid systems provide important information about buffer action and allow determination of \( \mathrm{K_a} \) (or \( \mathrm{pK_a} \)).

(i) Demonstrating Buffer Action from the Curve

In a weak acid–strong base titration, a buffer region is clearly visible before the equivalence point.

Features of the Buffer Region

• Occurs before equivalence point.
• Contains mixture of \( \mathrm{HA} \) and \( \mathrm{A^-} \).
• pH changes gradually despite addition of base.

Explanation

• Added \( \mathrm{OH^-} \) reacts with weak acid:

\( \mathrm{HA + OH^- \rightarrow A^- + H_2O} \)

• This forms conjugate base, maintaining buffer system.
• Result: small pH change.

Conclusion

• The flat (gradual) region of the curve demonstrates buffer action.

(ii) Determining \( \mathrm{K_a} \) from the Curve

Key Concept: Half-Equivalence Point

• At half-neutralisation:
  • \( \mathrm{[HA] = [A^-]} \)

From Henderson–Hasselbalch:

\( \mathrm{pH = pK_a} \)

Method

1. Find equivalence point volume from curve.
2. Calculate half of this volume.
3. Read pH at this point.
4. This pH = \( \mathrm{pK_a} \).
5. Convert if needed:

\( \mathrm{K_a = 10^{-pK_a}} \)

Why This Works

• At half-equivalence:

\( \mathrm{K_a = \frac{[H^+][A^-]}{[HA]}} \)

• Since \( \mathrm{[A^-] = [HA]} \):

\( \mathrm{K_a = [H^+]} \)

Therefore:

\( \mathrm{pH = pK_a} \)

Extension: Weak Base–Strong Acid

• Same principle applies.
• Half-equivalence gives \( \mathrm{pK_a} \) of conjugate acid.

Key Features

• Buffer region shows resistance to pH change.
• Half-equivalence point gives \( \mathrm{pK_a} \).
• Enables experimental determination of \( \mathrm{K_a} \).