3.1.13 (HL)—pH curves for weak acids/bases

Compare all four strong/weak combinations by starting pH, buffer region, equivalence-point pH and steep-section position. For a weak acid titrated with strong base, half-equivalence gives pH = pKa and the equivalence solution is basic. For a weak base titrated with strong acid, half-equivalence gives pOH = pKb (then pH = pKw − pOH) and the equivalence solution is acidic. Weak–weak curves often lack a sufficiently steep indicator region.

A buffer region contains appreciable weak acid and conjugate base, so the pH changes relatively slowly as titrant is added.

A weak acid–strong base curve starts at a higher pH than an equally concentrated strong acid, contains a buffer region, has pH = pKa at half-equivalence, and has an alkaline equivalence point from conjugate-base hydrolysis. Weak–weak titrations often lack a sufficiently steep jump for a simple indicator endpoint.