Weak Acid and Strong Base Titration Curve: Half-Equivalence Point and Buffer Region

When a weak acid (HA) is titrated with a strong base, the curve has four landmarks. The most tested one is the half-equivalence point, where pH = pKa.

The four regions

Why pH = pKa at the half point

Half of the HA has been converted to A^-, so the ratio [A^-]/[HA] is 1. In pH = pKa + log([A^-]/[HA]), log(1) = 0.

This is also where the curve is flattest, so the solution resists pH change best (maximum buffer capacity).

Why equivalence is above 7

At equivalence all the acid is now its conjugate base. A^- accepts protons from water, making OH^-. So the equivalence point of a weak acid with a strong base is basic. For a weak base with a strong acid it is acidic. Strong with strong gives pH 7.

Using the curve

• Read the equivalence volume from the steepest part of the jump.
• Half that volume gives the half-equivalence point; the pH there is the pKa.
• Moles of acid = moles of base added at equivalence (for a monoprotic acid).
• A polyprotic acid shows one half-equivalence point and one equivalence point per proton.

Common traps

• Assuming equivalence is at pH 7. That holds only for strong acid with strong base.
• Using the equivalence volume for pKa. Use half of it.
• Calling the start a buffer. Before any base is added there is only HA.

One-minute recap

• Buffer region lies between start and equivalence.
• Half-equivalence: pH = pKa.
• Equivalence: only conjugate base, pH above 7.