Chemistry · 12.2 Acid–base titrations · Paper 5/6 practical
Acid–Base Titration. Find the unknown.
Pipette 25.0 cm³ of an alkali of unknown concentration into a conical flask, add indicator, then run in a standard acid from the burette until the indicator just changes colour. Repeat for concordant titres, then calculate the unknown concentration from c₁V₁ = c₂V₂.
0620 Topic 12.2 — Acid–base titrations
Burette · pipette · indicator
Paper 5/6 — Practical
0.00 cm³
colour —
Shortcuts Space fast/close · D one drop · Enter record · R refill.
Apparatus & reagents
—
in flask: NaOH
25.0 cm³
Burette readings
Initial reading
0.00
Final reading
0.00
Titre this run
0.00 cm³
Acid conc.
0.100
Read the burette to the nearest 0.05 cm³, eye level with the bottom of the meniscus.
Results table — rough + concordant
Run the rough titration first, then two accurate runs.
Calculation — find the unknown
Mean titre
— cm³
Mol acid
—
Computed c(NaOH)
— mol/dm³
True c(NaOH)
hidden
📋 Method (Cambridge practical procedure)
- Rinse the pipette with the alkali, then pipette 25.0 cm³ of the alkali into a clean conical flask.
- Add 2–3 drops of indicator. Place the flask on a white tile under the burette.
- Rinse the burette with the acid, fill it, and record the initial reading to 0.05 cm³ (read the meniscus at eye level).
- Run in acid quickly, swirling, for a rough titration to find the approximate end-point.
- Refill and repeat slowly, adding the acid dropwise near the end-point until the indicator just changes colour permanently. Record the final reading.
- Repeat until you have two concordant titres (within 0.10 cm³). Average only the concordant results.
- Calculate: mol acid = c × V/1000; use the mole ratio to find mol alkali; then c(alkali) = mol ÷ (25.0/1000).
⚠ Sources of error & precautions
- Overshooting the end-point — add dropwise near the end and swirl constantly; the last drop should give a permanent colour change.
- Rinsing — rinse the burette with acid and the pipette with alkali so residual water does not dilute them.
- Parallax — read the bottom of the meniscus at eye level on a white background.
- Air bubble in the burette tip — run some liquid through to expel it before the initial reading.
- Indicator amount — use only 2–3 drops; too much indicator itself reacts and shifts the end-point.
- Use only concordant titres for the mean — discard the rough and any anomalous run.
🧪 Apparatus list
- 50 cm³ burette + small funnel for filling
- 25 cm³ volumetric pipette + pipette filler
- Conical flask (250 cm³), white tile, wash bottle of distilled water
- Indicator (methyl orange / thymolphthalein / screened methyl orange)
- Retort stand, boss and clamp
🎯 Syllabus reference (0620)
- 12.2 Acid–base titrations — describe a titration using a burette, volumetric pipette and a suitable indicator; describe how to identify the end-point of a titration using an indicator.
- 12.1 Experimental design — name apparatus for measuring volume (burette, pipette, measuring cylinder).
- 7.1 — the characteristic neutralisation reaction of acids with alkalis.