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The initial pH is calculated using the Ka (acid dissociation constant) and an ICE table, since the weak acid is the main pH contributor. For example, a 0.10 M acetic acid solution has an initial pH of 2.87. This method applies whenever a weak acid or base dominates the pH before any titrant is added.

When sodium hydroxide is added to a weak acid, hydroxide ions react with the acid to form its conjugate base, creating a buffer solution. The pH can be calculated using the henderson hasselbalch equation calculating buffers. As more base is added, the buffer resists pH change until the equivalence point is reached.

At the half-equivalence point, exactly half of the weak acid has been neutralized to form its conjugate base. Since the concentrations of weak acid and conjugate base are equal, the Henderson-Hasselbalch equation simplifies to pH = pKa. For acetic acid titration, this occurs at 12.50 mL of added sodium hydroxide.

At the equivalence point, all weak acid has been converted to its conjugate base, which is basic. The pH is calculated using the Kb of the conjugate base and an ICE table. For acetic acid titrated with sodium hydroxide, the equivalence point pH is 8.72, significantly higher than 7 due to the basic nature of acetate ions.

The relationship between Ka and Kb is given by Kw = Ka × Kb, where Kw is the water ionization constant (1.0 × 10⁻¹⁴). Rearranging gives Kb = Kw / Ka. For acetate ion, Kb = 1.0 × 10⁻¹⁴ / 1.8 × 10⁻⁵ = 5.6 × 10⁻¹⁰.

After the equivalence point, excess strong base (sodium hydroxide) is present in solution. The strong base is a much stronger contributor to pH than the conjugate base, so the final pH is determined by the concentration of excess hydroxide ions. For example, adding 70 mL of 0.10 M NaOH results in a pH of 12.22.

In weak acid-strong base titrations, the equivalence point occurs in the basic region (pH > 7) because the conjugate base is basic. The initial pH is higher, and a buffer region exists before the equivalence point. This contrasts with titration calculations strong acid strong base, where the equivalence point is at pH 7.