Half-Equivalence Point vs Equivalence Point: What Is the Difference?

The equivalence point is where moles of titrant equal the moles of analyte you started with. The half-equivalence point is where you have added exactly half of that titrant. They sit at different volumes and tell you different things.

Side by side (weak acid, strong base)

Why pH equals pKa at the half-equivalence point

Half of the acid has been converted to conjugate base, so [HA] = [A^-]. In the Henderson-Hasselbalch equation the log term becomes log(1) = 0, leaving pH = pKa.

Why the equivalence point is above 7 for a weak acid

All the acid has become its conjugate base, which is a weak base and raises the pH. For a strong acid with a strong base the equivalence point is exactly 7.

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Common traps

• Reading pKa at the equivalence point. Use the half-equivalence point.
• Assuming the equivalence point is always pH 7. That holds only for strong acid with strong base.
• Forgetting a diprotic acid has two of each. Each proton has its own half-equivalence and equivalence point.

One-minute recap

• Half-equivalence: half the titrant, [HA] = [A^-], pH = pKa.
• Equivalence: full titrant, only conjugate base, pH above 7 for a weak acid.
• Use the first to find pKa and the second to find concentration.

Related: see the weak acid and strong base curve.