IB Chemistry Titrations & pH Curves: FAQ

Most marks vanish on three things: misreading the equivalence point (not the same as the half-equivalence point), forgetting units on pKa/Ka answers, and picking the wrong indicator for a weak acid–strong base curve. Examiners also penalise vague curve descriptions that don't reference actual pH values or volumes shown on the axes.

The equivalence point is where moles of acid exactly equal moles of base — a theoretical point calculated from stoichiometry. The endpoint is where the indicator actually changes colour during the real experiment. They should be close but rarely identical, and IB examiners frequently ask you to explain why a chosen indicator gives an endpoint near the equivalence point.

Identify four features in order: the starting pH (tells you acid/base strength), the buffer region (flat-ish middle section for weak acid/base titrations), the steep vertical jump (equivalence point), and the final plateau (excess titrant). Reading these in sequence stops you jumping straight to guessing the equivalence point.

At half-equivalence, exactly half the weak acid has been converted to its conjugate base, so [HA] equals [A⁻]. By the Henderson–Hasselbalch equation, pH = pKa at this point — it's the fastest way to read Ka straight off a titration curve without any further calculation.

A weak acid starts at a higher pH than a strong acid of the same concentration, shows a buffer region (a gentle slope) before equivalence, and has a smaller vertical jump at equivalence — often not reaching pH 12. A strong acid–strong base curve starts low, has almost no buffer region, and jumps sharply through several pH units at equivalence.

pH 7 only occurs at equivalence when both acid and base are strong. If a weak acid reacts with a strong base, the salt formed hydrolyses to give a slightly basic solution, so equivalence sits above pH 7. The reverse — weak base with strong acid — gives equivalence below pH 7.

Pick an indicator whose colour-change pH range falls entirely within the steep, near-vertical section of the curve at equivalence. For strong-strong titrations almost any indicator works because the jump spans many pH units; for weak acid or weak base titrations, the jump is smaller, so the indicator range must match the equivalence pH closely.

Yes. HL students additionally need Henderson–Hasselbalch calculations, buffer capacity reasoning, and polyprotic acid titration curves with multiple equivalence points — none of which appear on the SL paper. According to the IB Chemistry guide (first assessment 2025), buffer solutions and their quantitative treatment sit specifically within the AHL content of Topic 8.

Yes, and it's one of the more reliable IA choices because the data (pH vs volume added) is continuous and easy to process statistically. Strong candidates vary concentration, temperature, or use a weak acid/weak base pair to compare buffer capacity, then justify their research question against a genuine gap in knowledge rather than just repeating a textbook demonstration.

A pH meter or data logger gives continuous, more precise readings than universal indicator and lets you find the equivalence point using the first-derivative (steepest gradient) method rather than colour estimation. Examiners reward the more precise method in the Tool and Methodology criteria, since it directly reduces random error in your final pH curve.

Practise past-paper data-based questions until reading a curve becomes automatic: identify equivalence, buffer region and starting pH within seconds, then attach the correct number to each. A 7 comes from precision — quoting exact pH and volume values from the graph, not describing trends vaguely.

Paper 2 data-based questions are the main format — you'll be given a titration curve or table of pH readings and asked to identify equivalence, calculate Ka or pKa, and justify an indicator choice. Paper 1 (SL and HL) tests indicator theory and curve-shape recognition through multiple-choice items.

Acids, bases and pH curves form a core part of Topic 8, which regularly contributes multiple marks across both papers in every exam session — not an optional extra. Because it combines calculation, graph interpretation and practical work, it's also a frequent source of IA topics, so time spent mastering it pays off in more than one assessment.