Hand an examiner four unlabelled titration curves and they can tell you instantly which acid met which base. The clues are all in the shape: where the curve starts, whether it has a flat shoulder near the bottom, how tall the cliff is, and where the middle of that cliff sits. Learn to read those four things and this becomes one of the easiest HL topics.
📘 What you need to know
There are four combinations: strong/strong, weak acid with strong base, weak base with strong acid, and weak with weak.
Strong acid + strong base: equivalence at pH 7, longest vertical section, starts at about pH 1.
Weak acid + strong base: equivalence above pH 7 (about 8.7), starts higher at about pH 2.9, and has a buffer region.
Weak base + strong acid: equivalence below pH 7 (about 5.3), starts at about pH 11, also has a buffer region.
Weak + weak:no proper vertical section, so the equivalence point cannot be found with an indicator.
The half-equivalence point is where half the weak acid has reacted. There, pH = pKa (or pOH = pKb for a weak base).
The equivalence pH is set by the salt left in the flask, not by the reaction being incomplete.
All four curves side by side
Each of these is 25.0 cm3 of a 0.100 mol dm−3 solution in the flask, with 0.100 mol dm−3 of the other reagent added from the burette. The equivalence volume is 25.0 cm3 every single time, because the moles always match at the same point. Only the pH at that point moves.
These are solved from the charge-balance equations rather than sketched, so the heights and the positions of the cliffs are the real ones you would measure in the lab.
Reading the four clues
1. Where does it start?
The very first point, before you add anything, tells you the strength of what is in the flask. A 0.100 mol dm−3strong acid starts at pH 1.00, because it is fully split. The same concentration of weak acid starts near pH 2.9, because only about 1% of it has split. Weak bases start near pH 11 rather than the 13 of a strong base.
2. Is there a shoulder?
Look just after the start. On the two weak curves the line rises quickly for a moment and then flattens into a shoulder that lasts for a long stretch. That shoulder is a buffer region: the flask now contains both the weak acid and its conjugate base, which soak up whatever you add.
Strong-strong curves have no shoulder, because there is no conjugate pair in the flask to do the soaking.
3. How tall is the cliff?
Strong with strong gives the tallest vertical section, roughly pH 4 to 10. Each weak partner you introduce chops a piece off one end. Two weak partners and there is barely a cliff left at all, which is why weak-with-weak titrations are almost never done.
4. Where is the middle of the cliff?
That is the equivalence point, and its pH depends entirely on what salt is now sitting in the flask.
This is why the equivalence point is not at pH 7 for the weak cases. The neutralisation itself is complete — it is the salt that then shifts the pH.
Students often assume the equivalence point of a weak acid is above 7 “because the acid did not fully react”. It did fully react. The ethanoate ion left behind is a base, and it is that ion pulling protons off water afterwards that raises the pH.
The half-equivalence point
Halfway to equivalence — at 12.5 cm3 in our curves — exactly half of the weak acid has been converted to its conjugate base. So the flask contains equal amounts of the two.
Put that into the Ka expression and something very tidy drops out.
At half equivalenceKa = [H+][A−] ÷ [HA] and since [A−] = [HA], Ka = [H+] so pH = pKa
Our ethanoic acid curve reads exactly 4.76 at 12.5 cm3, and pKa for ethanoic acid is 4.76. That is not a coincidence — it is a standard way of measuring a Ka in the lab. For a weak base the same argument gives pOH = pKb at half equivalence.
Feature
Strong acid + strong base
Weak acid + strong base
Weak base + strong acid
Starting pH
1.00
2.88
11.12
Buffer region
none
yes, before equivalence
yes, before equivalence
pH at half equivalence
no special meaning
= pKa = 4.76
pOH = pKb, so pH 9.25
pH at equivalence
7.00
8.73
5.28
Salt left behind
NaCl, neutral
CH3COONa, alkaline
NH4Cl, acidic
Range of steep section
about pH 4 to 10
about pH 7 to 11
about pH 3 to 7
🧩 Identifying a curve you have never seen
Read the starting pH. Low and near 1 means a strong acid; near 3 means a weak acid; near 11 means a weak base; near 13 means a strong base.
Look for a shoulder shortly after the start. A shoulder means a weak species is in the flask.
Find the middle of the vertical section and read its pH. Above 7 means the acid was weak; below 7 means the base was weak.
If there is no clear vertical section, both were weak.
Worked examples
WORKED EXAMPLE
A curve starts at pH 2.9, flattens into a shoulder, then rises steeply to an equivalence point at pH 8.7. Identify the acid and the base, and explain the equivalence pH.
Step 1: use the starting pHA 0.1 mol dm−3 strong acid would start near 1.0, so 2.9 means the acid is weak.Step 2: use the finish and the shoulder
The curve ends high and there is one buffer region, so the base is strong.
Step 3: explain the equivalence pH
The salt formed contains the conjugate base of a weak acid, e.g. CH3COO−.
CH3COO− + H2O ↔ CH3COOH + OH−
That reaction makes hydroxide ions, pushing the pH above 7.
Weak acid titrated with a strong base; the salt hydrolyses to give OH−quote the hydrolysis equation — “because the acid is weak” alone rarely gets full marks
WORKED EXAMPLE
25.0 cm3 of a weak acid needs 22.0 cm3 of NaOH to reach equivalence. The pH at 11.0 cm3 is 4.20. Find Ka.
Step 1: check what 11.0 cm3 means22.0 ÷ 2 = 11.0, so this is the half-equivalence point.Step 2: apply the half-equivalence rule
Half the acid has become its conjugate base, so [HA] = [A−] and pH = pKa.
pKa = 4.20Step 3: convert to KaKa = 10−4.20 = 6.31 × 10−5Ka = 6.31 × 10−5spotting that 11.0 is half of 22.0 is the whole question
WORKED EXAMPLE
Explain why ammonia is not titrated against ethanoic acid in school laboratories.
Step 1: classify both reagents
Ammonia is a weak base and ethanoic acid is a weak acid.
Step 2: describe the curve shapeThe pH drifts gradually from about 2.9 to about 9.2 with no vertical section.Step 3: link the shape to the practical problem
An indicator needs a sharp pH jump to change colour at one point.
There is no steep section, so no indicator can show the end point accuratelya pH meter can still find it from the steepest part, but colour indicators cannot
💡 Exam tip
Learn the four equivalence pH values as a set: 7, above 7, below 7, and no useful value.
Whichever partner is weak pulls the equivalence point towards its own side: weak acid pushes it up, weak base pulls it down.
When sketching, mark starting pH, equivalence volume, equivalence pH and, for a weak acid, the half-equivalence point.
The equivalence volume is the same in all four cases. Only the pH moves — a favourite exam check.
To explain a non-neutral equivalence point, write the hydrolysis equation of the ion in the salt.
pKa read off a curve at half equivalence is a standard experimental method. Be ready to describe it.
⚠ Common mix-up
Thinking a weak acid needs less alkali. Same moles of acid means the same volume of alkali. Only the pH profile changes.
Saying the equivalence point is above 7 because the reaction is incomplete. It is complete; the salt is what raises the pH.
Putting the buffer region after the equivalence point. It comes before, while both the weak acid and its conjugate base are present.
Reading pH = pKa at the equivalence point. That rule belongs to the half-equivalence point.
Drawing a strong-strong curve starting at pH 0. At 0.100 mol dm−3 it starts at 1.00.
Assuming weak-weak titrations are impossible. They happen, they just cannot be followed with an indicator.
Up next: The pOH Scale. We keep flipping between hydrogen ions and hydroxide ions, so it is time to give the hydroxide side its own scale and learn the shortcut that connects the two.
Want this explained one-to-one?
Book a free session with an experienced IB Chemistry tutor and get your trickiest topics made simple.
