Required practical · A-level chemistry
RP 1: Volumetric solution & acid–base titration
Hazards
⚠Dilute acids/alkalis — irritant
⚠Wear eye protection throughout
Method
- Make the standard solutionWeigh the solid by difference on a 2 d.p. balance, dissolve fully in a beaker with distilled water, transfer with rinsings to a 250 cm³ volumetric flask, fill to the mark so the bottom of the meniscus sits on the line at eye level, stopper and invert ~10 times.
- Rinse and fill the burette properlyRinse the burette with the solution it will hold (not just water), then fill and remove the air bubble from the jet below the tap.
- Pipette 25.0 cm³ — and only 2–3 drops of indicatorPipette 25.0 cm³ of the other solution into a conical flask using a pipette filler; add 2–3 drops of indicator only.
- Rough run, then accurate dropwise runsDo a rough titration, then accurate runs: add dropwise near the endpoint with swirling, rinsing the flask walls with distilled water.
- Stop at two concordant titresRepeat until you have two concordant titres within 0.10 cm³; use their mean in the calculation.
Mistakes examiners see every year
| The mistake | Why it costs marks |
|---|---|
| Rinsing the burette with water only | Water left on the walls dilutes your solution and shifts every titre. Rinse burette with the titrant and the pipette with what it will measure (AQA handbook). |
| Leaving the funnel in the burette | Drips from the funnel change the reading mid-run. Remove it after filling (RSC titration guidance). |
| Blowing out the pipette tip | The pipette is calibrated to retain the liquid left in the tip — drain under gravity only. |
| Reading the top of the meniscus, or reading at an angle | Read the bottom of the meniscus at eye level, on a white tile background. Burettes read to the nearest 0.05 cm³. |
| Overshooting the endpoint | AQA's 2019 examiner report: the endpoint is the first permanent colour change — with phenolphthalein a lasting pale pink, not purple. Add dropwise near the endpoint, swirling. |
| Too much indicator | Indicators are themselves weak acids — a large amount shifts the result and muddies the colour. 2–3 drops. |
| Averaging non-concordant titres | Only titres within 0.10 cm³ of each other go into the mean; discard the rough. |
| Air bubble in the burette jet | It escapes mid-run and inflates the apparent titre. Open the tap fully to flush it before starting. |
What to expect
With both solutions near 0.100 mol dm⁻³ and 1:1 stoichiometry, expect a titre around 25 cm³. Uncertainty per burette reading ±0.05 cm³ (±0.10 cm³ per titre), so % uncertainty ≈ (0.10 ÷ 25.0) × 100 ≈ 0.4% — one reason a titre below ~10 cm³ is poor design. Safety note (CLEAPSS SSS 31): dilute NaOH is still an irritant, and alkali in the eye is more damaging than acid of the same concentration — eye protection throughout.
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