IB Chemistry Titration Curves: Shapes, Key Points and Exam Tips

A titration curve plots the pH of the solution against the volume of titrant added. In IB Chemistry you need to do four things with it: sketch the curve for any strong or weak acid and base pairing, find the equivalence point, read the pKa from the buffer region, and justify an indicator. This guide takes you through each one, with the reasoning examiners reward.

TL;DR

• A titration curve is pH against titrant volume. The equivalence point is where the reaction is stoichiometrically complete, and its pH depends on the strength of the acid and base.
• Strong acid with strong base: equivalence at pH 7. Weak acid with strong base: equivalence above pH 7. Strong acid with weak base: equivalence below pH 7.
• At the half-equivalence point of a weak acid, pH = pKa. This is the fastest way to get a pKa from a graph.
• Choose an indicator whose colour-change range sits on the steep part of the curve, then say so in your answer.

Table of Contents

• What a titration curve shows
• The four curve shapes
• The buffer region and the half-equivalence point
• Choosing an indicator
• How to sketch a curve in an exam
• Common mistakes to avoid
• Our perspective
• Practise with exam-style material
• FAQ
• Sources

What a titration curve shows

As base is added to acid (or the reverse), the pH changes slowly at first, then jumps sharply near the equivalence point, then levels off. LibreTexts describes the curve as pH plotted against either the volume of titrant or the degree of titration, where a degree of 1.0 means exactly enough titrant has been added to complete the reaction. That is the equivalence point.

Two habits help straight away:

• Label the axes: pH on the vertical, volume of titrant (in cm³) on the horizontal.
• Do not force the curve to start or end at a neat whole number. The starting pH depends on the initial concentration and strength of the solution in the flask. The final pH depends on the excess titrant.

The four curve shapes

Learn these as a set. Once you understand why each one differs, you can draw them without memorising.

Strong acid and strong base

The curve starts low, rises gently, then climbs almost vertically through pH 7 and flattens at a high pH. The salt formed does not hydrolyse, so the equivalence point is neutral. The vertical section is long, which makes indicator choice easy.

Weak acid and strong base

The curve starts higher because the acid only partly ionises. It rises quickly at first, then flattens into the buffer region, then rises steeply. Equivalence is above pH 7 because the conjugate base of the weak acid is left in solution and accepts protons from water. The vertical section is shorter and sits higher.

Strong acid and weak base

This is the mirror image. The curve starts high, falls through a buffer region, and passes through a steep section whose midpoint is below pH 7, because the conjugate acid of the weak base is left in solution.

Weak acid and weak base

There is no sharp vertical section, so no ordinary indicator gives a clear end point. If a question asks you to comment, say exactly that.

The buffer region and the half-equivalence point

The flat part of a weak acid curve exists because the flask now holds both the weak acid and its conjugate base. This mixture resists pH change when small amounts of titrant are added, as LibreTexts explains.

At exactly half the equivalence volume, half the acid has been converted, so [acid] = [conjugate base]. The Henderson-Hasselbalch equation then gives pH = pKa. The same source confirms this relationship at a degree of titration of 0.5.

Here is how to use it:

1. Read the equivalence volume from the middle of the steep section.
2. Halve it.
3. Find that volume on the horizontal axis and read the pH from the curve.
4. That pH is the pKa. Convert to Ka with Ka = 10^(−pKa) if the question asks.

Worked example: a weak acid reaches equivalence at 24.0 cm³. Half is 12.0 cm³. If the curve reads pH 4.76 there, the pKa is 4.76 and Ka is about 1.7 × 10⁻⁵ mol dm⁻³.

The buffer region is also where dilution matters. As titrant is added, the total volume grows, so concentrations fall. Keep this in mind when a question asks you to calculate a pH partway through.

Choosing an indicator

This is where marks are often lost. An indicator changes colour over a pH range, typically one to two units wide, and that range should fall within the steep section of your curve. Vanderbilt’s titration guide lists the ranges you will meet most often:

For a strong acid with a strong base the pH jump is so large that several indicators work. For a weak acid with a strong base the equivalence point is alkaline, so phenolphthalein fits.

Do not simply memorise that phenolphthalein goes with weak acids. The UCalgary chemistry textbook explains that the indicator’s pKa should match the pH at the equivalence point. A mismatch causes a systematic error. In an exam, write one sentence that names the equivalence pH, the indicator’s range, and says the range lies within the vertical section.

Also remember that the visible end point only approximates the true equivalence point. Research on colour indicators and systematic titration error shows how the choice of indicator shifts the result. It is a useful point for evaluation in your internal assessment.

The IB itself tests indicator reasoning. A question in the IB specimen papers for Chemistry asks which statements are correct about the colours of an indicator HInd in acidic and alkaline solution. It rewards understanding of the equilibrium, not recall of a colour chart.

How to sketch a curve in an exam

1. Decide the strengths of the acid and base. This fixes the equivalence pH.
2. Calculate the starting pH from the initial concentration. For a strong acid, pH = −log[H⁺]. For a weak acid, use Ka and an equilibrium expression.
3. Mark the equivalence volume. Use n = cV to find it from the known concentrations.
4. Draw the steep section centred on that volume, at the correct equivalence pH.
5. For a weak acid or base, draw the buffer region and mark the half-equivalence point.
6. Finish by levelling off towards the pH of the excess titrant.

Check the sketch against the question. If it gives concentrations and volumes, examiners expect your equivalence point to sit at the right volume, not just roughly in the middle.

Common mistakes to avoid

• Putting the equivalence point at pH 7 for every titration.
• Drawing a vertical line for a weak acid with a weak base.
• Reading the pKa from the equivalence point instead of the half-equivalence point.
• Choosing an indicator without referring to the curve.
• Forgetting that the volume axis is the titrant volume, not the total volume.

Our perspective

We see the same pattern with titration curves again and again. Students who treat each curve as a separate picture to memorise struggle. Students who ask “what is in the flask at this point?” find that the starting pH, buffer region, equivalence pH and final pH follow naturally.

That is why we write our IB Chemistry material around the reasoning. Our resources are written by IB examiners, so the questions, mark schemes and notes reflect how marks are actually awarded, including the justification sentences for indicator choice. The notes, flashcards, topic tests and mock exams link to one another, so an error on a titration question leads you straight to the explanation behind it. Our performance analytics then show whether acids and bases is a real weak spot or a one-off slip.

We believe no other IB STEM platform joins these pieces together this way, and with examiners writing the content, we think it is the best value for money available to IB students.

— Oliver

Practise with exam-style material

Reading about curves helps, but sketching them under timed conditions is what builds confidence. Our IB Chemistry resources include exam-style questions with examiner mark schemes, and our IB Chemistry mock exams let you test the topic in context. For a structured plan, see our guide to IB Chemistry acids and bases in three weeks.

You can start with a free 7-day trial. After that, the Per-Subject Plan is $9 per month and the All-Access Plan is $19 per month. See the pricing page for the details.

The IB updates its subject guides, so check the IB chemistry curriculum updates to confirm what is assessed in your exam session.

FAQ

Where is the equivalence point on a titration curve?

It is the midpoint of the steep, near-vertical section. At that volume the moles of acid and base have reacted in exactly the ratio of the balanced equation. Its pH is 7 only when both the acid and base are strong.

How do I find the pKa from a titration curve?

Find the equivalence volume, halve it, and read the pH at that half volume. At this half-equivalence point the acid and its conjugate base are present in equal amounts, so pH = pKa. This works for a weak acid titrated with a strong base.

Why is the equivalence point above pH 7 for a weak acid?

The salt formed contains the conjugate base of the weak acid. This base reacts with water to produce hydroxide ions, which makes the solution slightly alkaline at equivalence. For that reason, phenolphthalein is a suitable indicator.

Which indicator should I choose?

Pick the one whose colour-change range lies within the steep section of your curve. Then state the equivalence pH and the indicator range in your answer. Examiners look for that justification, not just the name of the indicator.

Sources

• 13.5: Acid/Base Titration, LibreTexts
• Titration, Vanderbilt University
• Acid-Base Colour Indicators, UCalgary Chemistry Textbook
• Colour maps of acid-base titrations with colour indicators, Springer
• IB Chemistry specimen papers, IBO
• Chemistry updates, IBO

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