Weak Acid/Base Titration

When a weak acid is titrated with a strong base, four distinct regions appear on the pH curve:

Initial pH — Set by the weak acid's Ka and concentration. Use ICE table: pH = ½(pKa − log c)
Buffer Region — Between 10% and 90% neutralization. Henderson-Hasselbalch applies: pH = pKa + log([A⁻]/[HA])
Half-Equivalence Point — Exactly half the acid is neutralized: [HA] = [A⁻], so pH = pKa. This is the key to finding Ka experimentally
Equivalence Point — All acid neutralized. The conjugate base (A⁻) undergoes hydrolysis, so pH > 7. Use phenolphthalein, not methyl orange

For strong acid + weak base, the curve is inverted: equivalence pH < 7 (conjugate acid hydrolysis). Use methyl orange.

Indicator selection — The indicator's pKin must fall within the steep part of the curve near equivalence. Phenolphthalein works for WA/SB (equiv pH ~8.7), methyl orange for SA/WB (equiv pH ~5.3)
Reading pKa from graph — Find the half-equivalence volume, read pH at that point → that's pKa. Then Ka = 10⁻ᵖᴷᵃ
Buffer capacity — Maximum at the half-equivalence point. The flatter the curve, the better the buffer
Weak/weak titrations — No sharp inflection → no suitable indicator. Equivalence must be found via conductometric methods
Common error — Students assume equivalence = pH 7. Only true for strong/strong. Weak acid/strong base → pH > 7, strong acid/weak base → pH < 7

Feature	SA + SB	WA + SB	SA + WB	WA + WB
Initial pH	~1 (low)	~3 (higher)	~1 (low)	~3 (higher)
Buffer Region	None	Yes ✓	None	Yes (both sides)
Equiv. pH	7.0	> 7 (~8.7)	< 7 (~5.3)	≈ 7 (no sharp change)
Steep Jump	Very sharp	Moderately sharp	Moderately sharp	Very gradual
Best Indicator	Any	Phenolphthalein	Methyl Orange	None — use conductometry
½ Equiv. Point	Not useful	pH = pKa	pOH = pKb	Complex