Acid-base titration: aliquot, burette and endpoint
Explain how a fixed aliquot, measured variable delivery and a suitable indicator work together, including how to recognise an endpoint without overshooting.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Distinguish a fixed liquid aliquot from variable delivered volume.
- Recognise an acid-base neutralisation and indicator colour changes.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which item transfers the fixed 25.0 cm³ aliquot?
What you will learn
- Describe the roles of a burette, volumetric pipette and suitable indicator in acid-base titration.
- Identify the endpoint from the stated indicator change and explain steps that improve the reading.
Assign the measuring roles
A volumetric pipette, used with a suitable filler, transfers a fixed aliquot into the conical flask. The burette holds the other solution and measures the variable amount delivered. A suitable indicator in the flask gives a visible change close to the intended neutralisation endpoint. A measuring cylinder's approximate volume is not an equivalent replacement when the aliquot must be accurate.
In a supervised titration method, rinsing the pipette with its intended solution and the burette with its intended solution avoids unintended dilution by retained water. The conical flask can be rinsed with distilled water: extra water changes concentration in the flask but does not add reacting solute to the measured aliquot. Avoid residual reactant in the flask, which would change the amount to be neutralised. A burette tip containing air rather than solution can also make the first delivery reading misleading.
Use the indicator change, not a guessed pH
Swirl to mix as solution is delivered. A preliminary rough trial locates the endpoint region; subsequent measured trials approach it slowly, adding dropwise near the first persistent specified colour change. A white background helps distinguish the change. Record initial and final burette readings at eye level, using the bottom of the concave meniscus for a suitable clear aqueous solution.
Choose an indicator suitable for the stated acid/base system. An endpoint is the observed indicator change; it is not a universal claim that every titration ends at pH 7. For a supplied methyl-orange scenario starting in alkaline yellow solution with acid added, the first persistent orange endpoint is the target described here. Continuing to strongly acidic red can overshoot it. Universal indicator gives a range of gradual colours and is generally less suitable for a sharp endpoint than an appropriate single indicator.
Sources for this lesson
Pause and explain
Why does a small distilled-water rinse in the conical flask not add reacting solute to the measured aliquot?
Worked example
A supplied titration uses 25.0 cm³ alkali, acid in the burette and methyl orange. Explain the three main apparatus/reagent roles and the stated endpoint.
Show the worked solution
- The volumetric pipette transfers the fixed 25.0 cm³ alkali aliquot.
- The burette delivers a measured variable acid volume while the flask contents are mixed.
- For this supplied indicator scenario, approach the first persistent orange change from yellow, adding dropwise near it rather than continuing to excess acidic red.
Answer Pipette: fixed aliquot; burette: measured acid delivery; methyl orange: visible endpoint, yellow to the first persistent orange in the stated scenario.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain the roles of the volumetric pipette, burette and suitable indicator in the supplied titration.
Give me a hint
Connect fixed aliquot, varying delivery and visible endpoint.
Compare my reasoning
- The pipette measures and transfers one fixed aliquot.
- The burette measures the volume delivered from its initial and final readings.
- The suitable indicator changes visibly near the required endpoint.
Pipette: fixed aliquot; burette: variable measured delivery; indicator: visible endpoint.
Look for these in your work
- All three required roles are distinct.
- The indicator is suitable for the stated titration rather than universally interchangeable.
In the stated yellow-to-orange methyl-orange scenario, a student rapidly adds acid until the flask is strongly red. Identify a likely problem and an improvement.
Give me a hint
Use the stated endpoint rather than any obvious late colour.
Compare my reasoning
- The strong acidic red suggests the first persistent orange endpoint has been passed.
- Use the rough trial to locate the region and add dropwise near the target.
- Mix and observe against a white background to recognise the specified persistent change.
Likely overshoot; approach the first persistent orange endpoint slowly with mixing and dropwise addition.
Look for these in your work
- The issue is linked to the supplied indicator sequence.
- The improvement directly limits further delivery beyond the endpoint.
Compare rinsing a flask containing a measured alkali aliquot with a little distilled water and with extra alkali of the same concentration. Which changes the amount needing neutralisation?
Give me a hint
Distinguish adding solvent from adding reacting solute.
Compare my reasoning
- Distilled water adds solvent and dilutes the aliquot in the flask without adding alkali solute.
- Extra alkali adds reacting solute beyond the measured aliquot.
- The extra alkali can require additional acid and make the result inconsistent with the assumed original aliquot.
Extra alkali changes the reacting amount; the small water rinse changes concentration without adding alkali solute.
Look for these in your work
- Amount and concentration are distinguished.
- The predicted titration consequence follows from added reactant.
Common mistakes
- Replacing the fixed accurate aliquot with a rough beaker volume.
- Assuming the endpoint must always be pH 7.
- Continuing far beyond the specified colour change.
- Rinsing the conical flask with extra reactant and ignoring the added reacting amount.
What can you explain now?
Why add solution dropwise near the endpoint?
Compare with the explanation
To limit overshooting the specified persistent colour change.
A large extra volume can pass the endpoint before it is recognised.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Sketch the pipette, flask and burette roles, then explain a suitable indicator endpoint, slow final addition and the different effects of flask rinses.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.