Interpret titration curves for the four acid-base combinations and select a suitable indicator.

What this dot point is asking

TASC expects you to read titration curves, distinguish the equivalence point from the end point, recognise the buffer region, and select an appropriate indicator with justification.

Equivalence point and end point

The end point is where the indicator changes colour. A well chosen indicator gives an end point that coincides with the equivalence point. These are separate ideas: equivalence is a chemical fact about moles, while the end point is what the indicator shows.

The four combinations

• Strong acid with strong base: the curve starts low, rises gently, then jumps steeply over a wide pH range (roughly $3$ to $11$), with the equivalence point at pH $7$. The salt formed is neutral.
• Strong acid with weak base: the equivalence point is below $7$ (acidic), because the salt contains the conjugate acid of the weak base.
• Weak acid with strong base: the equivalence point is above $7$ (basic), because the salt contains the conjugate base of the weak acid. The curve also has a flatter buffer region before the jump.
• Weak acid with weak base: the jump is small and gradual, so there is no sharp equivalence point and indicators are unreliable; use a pH meter.

The buffer region and half-equivalence

In a weak acid against strong base titration, the early flat region is a buffer of the weak acid and its conjugate base. At the half-equivalence point exactly half the acid is neutralised, so $[\text{acid}] = [\text{base}]$ and therefore $\text{pH} = pK_a$. This is a quick way to read $pK_a$ straight off a curve.

Choosing an indicator

The indicator must change colour within the steep vertical part of the curve, so that one drop of titrant carries the pH through the colour change. Match the indicator's $pK_a$ (its working range, roughly $pK_a \pm 1$) to the equivalence pH.

• Phenolphthalein (range about $8.3$ to $10$) suits weak acid with strong base.
• Methyl orange (range about $3.1$ to $4.4$) suits strong acid with weak base.
• Either suits strong acid with strong base, because the jump spans both ranges.

Reading features off a curve

A titration curve carries more information than just the equivalence point. The initial pH reflects the strength and concentration of the analyte: a strong acid starts very low, a weak acid starts noticeably higher for the same concentration. The half-equivalence point lies at half the equivalence volume and, for a weak acid, gives $\text{pH} = pK_a$ directly. The equivalence volume itself lets you calculate the unknown concentration through the mole ratio, exactly as in volumetric analysis. A polyprotic acid (such as carbonic or phosphoric acid) shows two or more separate steep regions, one per proton removed, each with its own equivalence point.

Why the jump is steep

Near equivalence almost all the analyte has reacted, so a tiny further addition of titrant produces a large relative change in the amount of excess acid or base, and therefore a large change in pH. This near-vertical region is what makes an indicator end point sharp. For weak-weak combinations the jump is shallow because neither partner is fully ionised, so no indicator gives a clean colour change and a pH meter must be used instead.

In the exam, identify which partner is weak, predict whether the equivalence point is acidic, neutral or basic, then select an indicator whose range sits inside the steep part of the curve and justify the choice.