Two pages ago you had curves. One page ago you had indicators with two-unit windows. Put the windows on top of the curves and the choice makes itself: the window has to sit inside the vertical drop. Get that wrong and you stop the burette at the wrong moment.
📘 What you need to know
The equivalence point is chemistry: exactly enough of one solution has been added.
The end point is the indicator: the moment its colour changes and you stop.
The rule: the indicator’s pH range must lie inside the vertical section of the curve.
If it does, the end point and the equivalence point are within a fraction of a drop of each other.
Strong acid + strong base jumps from about 4 to 10, so several indicators work.
Weak acid + strong base jumps from about 7 to 11, so phenolphthalein is the choice.
Strong acid + weak base jumps from about 4 to 7, so methyl red is the choice.
Weak acid + weak base has no sharp jump, so no indicator works.
Indicator data is in section 18 of the data booklet. You do not have to memorise the ranges.
Why the range has to sit inside the jump
Think about what happens if it does not. Say the vertical section runs from pH 7 to 11, and you pick an indicator that changes between 3.1 and 4.4. The flask passes through pH 4 long before the equivalence point, so the colour changes while there is still plenty of acid left. You would stop the burette early and record a volume that is too small.
Now put the range inside the jump. The pH races through that window in a fraction of a drop, so the colour change and the true equivalence point happen at effectively the same reading.
Trace the green curve with a finger. It crosses the yellow band gently at around 20 cm3, nowhere near equivalence — that is exactly the early colour change you must avoid.
The vertical sections, side by side
Another way to see it: forget the curves and just plot the pH range each jump covers, then check which indicator windows fall inside.
Look along the red bar at the top: methyl red and phenolphthalein both sit inside it, which is why a strong acid and strong base titration is so forgiving.
Titration
pH range of the jump
Suitable indicator
Why
strong acid + strong base
about 4 to 10
methyl red or phenolphthalein
both ranges fall inside the jump
weak acid + strong base
about 7 to 11
phenolphthalein
its 8.3 to 10.0 window sits in the jump
strong acid + weak base
about 4 to 7
methyl red
its 4.4 to 6.2 window sits in the jump
weak acid + weak base
no sharp jump
none
the colour change would spread over cm3
Methyl orange still turns up in school labs for strong acid titrations, even though its range only just reaches the bottom of the jump. The reason is practical rather than theoretical: the red to yellow change is easy to see, and the error it introduces is small when the jump is a full six units long.
🧩 Picking an indicator in an exam
Decide which reactant is strong and which is weak.
Work out the pH range of the vertical section from that, or read it off the graph you are given.
Look up the candidate indicators in section 18 and find one whose range lies inside it.
Justify it by quoting both ranges: the jump and the indicator. A name on its own is only half an answer.
Worked examples
WORKED EXAMPLE
Ethanoic acid is titrated with sodium hydroxide. Choose a suitable indicator and justify your choice.
Step 1: classify the two reactantsEthanoic acid is weak; sodium hydroxide is strong.Step 2: work out where the jump sits
A weak acid with a strong base gives an equivalence point above 7, so the vertical section runs roughly from pH 7 to 11.
Step 3: match an indicator to that windowPhenolphthalein changes over 8.3 to 10.0, which lies inside 7 to 11.
Methyl orange changes at 3.1 to 4.4, far too low.
Phenolphthalein, because its range falls within the steep part of the curvequote both number ranges — “phenolphthalein” alone rarely gets full marks
WORKED EXAMPLE
Explain what would go wrong if methyl orange were used for that titration.
Step 1: compare the two rangesMethyl orange changes over 3.1 to 4.4. The jump runs from about 7 to 11.
The indicator range is nowhere near the jump.
Step 2: work out when the flask is at pH 3.1 to 4.4
That happens early on, in the gently sloping buffer region, long before equivalence.
Step 3: say what the experimenter would do
The colour would creep from red to yellow over several cm3, and they would stop far too early.
The end point would come well before equivalence, so the titre would be too smallsay the titre is too small — you stop early, so less alkali went in
WORKED EXAMPLE
A titration curve has a vertical section running from pH 4.0 to pH 7.0. Which of methyl orange (3.1–4.4), methyl red (4.4–6.2) and phenolphthalein (8.3–10.0) is best?
Step 1: test each range against 4.0 to 7.0Methyl orange 3.1 to 4.4: starts below 4.0, so part of it is outside.Methyl red 4.4 to 6.2: entirely inside. Phenolphthalein 8.3 to 10.0: completely above the jump.Step 2: identify the titration while you are here
A jump ending at pH 7 with an equivalence point below 7 means a strong acid with a weak base.
Methyl red, because 4.4 to 6.2 lies completely inside 4.0 to 7.0phenolphthalein would never change colour at all in this titration
💡 Exam tip
Justify with two number ranges: the jump and the indicator. That is where the marks are.
You are given section 18 of the data booklet, so do not waste revision time memorising ranges. Learn the rule instead.
If a graph is provided, read the jump off it rather than guessing from the reactants.
An indicator that changes too early gives a titre that is too small; too late gives one that is too large.
For weak with weak, the answer is no suitable indicator — and you can add that a pH meter would still work.
Only a few drops of indicator go in, so it never affects the pH it is reporting.
⚠ Common mix-up
Choosing an indicator with pKa closest to 7. It has to match the equivalence point of that titration, which is often not 7.
Naming an indicator with no justification. Half the marks are for the reasoning.
Saying phenolphthalein always works. It is useless below pH 8, so it fails for a weak base titration.
Treating equivalence point and end point as the same thing. The whole point of choosing well is to bring them close together.
Thinking universal indicator is a good choice. It changes gradually across the whole scale, so it gives no sharp end point.
Adding too much indicator. It is a weak acid itself and will start affecting the pH.
Up next: Buffer Solutions. The flat shoulder on the weak acid curve has been sitting there unexplained since page 9. It turns out that region is doing something genuinely useful, and living things depend on it.
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