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TL;DR The 50 cm³ Class B burette is the most mark-sensitive piece of glassware in O-Level Chemistry Paper 3. It is graduated to 0.10 cm³ and must be read to the nearest 0.05 cm³ at eye level, reading the bottom of the meniscus. Rinse it with the solution it will deliver (never water), bleed air from the jet before recording the initial reading, and practise the half-turn stopcock technique so you can stop within a single drop of the endpoint. Two concordant titres within 0.20 cm³ is the SEAB accuracy target.
A burette is a long, narrow glass tube fitted with a stopcock (tap) at the bottom. In O-Level Chemistry, you use a 50 cm³ Class B burette to deliver a measured volume of solution into a conical flask during acid--base titrations.
Key specifications to memorise:
Total capacity: 50.00 cm³.
Smallest graduation: 0.10 cm³ (the lines etched on the glass).
Required reading precision: the nearest 0.05 cm³ - you estimate halfway between two graduation lines.
Scale direction: the 0.00 cm³ mark is at the top; the 50.00 cm³ mark is at the bottom. The burette measures how much solution you have delivered, not how much remains.
SEAB lists the burette first in its Paper 3 practical-techniques table and pairs it with the 25 cm³ pipette, conical flask, and wash bottle as the standard titration apparatus set (SEAB 2026 syllabus, p. 28).
2 | Setting up the burette
Follow this sequence every time. Examiners award MMO (Making, Managing, and Observing) marks for correct setup technique.
2.1 Clamp the burette vertically
Attach a burette clamp to a retort stand and secure the burette so it hangs straight. A tilted burette gives inaccurate readings because the liquid surface is not perpendicular to the scale.
2.2 Check the stopcock
Turn the stopcock through a full 360-degree rotation. It should move smoothly without sticking or leaking. If it leaks, apply a thin layer of stopcock grease or ask for a replacement burette. A dripping stopcock makes concordant titres impossible.
This is the step most students forget, and it costs marks.
Pour a small quantity of the solution the burette will contain (usually the acid) into the burette. Tilt and rotate the burette so the solution wets the entire inner surface, then drain through the jet into a waste beaker. Repeat once more.
Why: If the burette is wet with distilled water from washing, the water dilutes your solution and lowers its effective concentration. The titre you record will be too large, producing a systematic error that cannot be corrected by repeating the experiment.
2.4 Fill above the zero mark
Use a funnel to pour solution into the burette until the meniscus sits a few centimetres above the 0.00 cm³ mark. Remove the funnel - leaving it in during the titration can cause drips that change the delivered volume unnoticed.
2.5 Remove air bubbles from the jet
Open the stopcock fully and let solution rush through the jet (the narrow glass tube below the stopcock) into a waste beaker. Watch the jet: if you see a trapped air bubble, keep the flow going until the bubble clears. An air bubble that dislodges during the titration adds extra volume that was not really delivered, giving a falsely low titre.
2.6 Record the initial reading
Adjust the meniscus so it sits at or just below 0.00 cm³, then take a reading using the technique described in Section 3. Write this value in the "Initial reading" row of your results table immediately - do not rely on memory.
3 | Reading the burette correctly
Burette-reading errors are the single largest source of lost marks in Paper 3. Three rules eliminate nearly all of them.
Rule 1 - Read at eye level
Position your eyes so they are level with the meniscus. If you look down, refraction makes the meniscus appear lower than it really is (you read a volume that is too small). If you look up, the opposite happens. This optical distortion is called parallax error.
Practical tip: for most students, this means bending your knees or lowering the retort stand - do not tilt the burette to bring the reading to your eye.
For colourless or lightly coloured solutions, the liquid curves upward where it touches the glass, forming a concave meniscus. Read the scale at the lowest point of that curve.
For deeply coloured solutions such as potassium permanganate, the meniscus is difficult to see. In that case, read the top of the meniscus. At O-Level, the titrating solution is almost always colourless, so the bottom-of-meniscus rule applies in the vast majority of exams.
Rule 3 - Record to the nearest 0.05 cm³
The graduation lines are 0.10 cm³ apart. You must estimate whether the meniscus falls on a line or halfway between two lines. This means every valid burette reading ends in one of these digits:
.00,.05,.10,.15,.20,.25,.30,.35,.40,.45,.50
Valid examples: 12.50, 12.55, 12.60, 24.05, 0.00.
Invalid examples: 12.52, 12.57, 24.03. These values imply a precision of 0.01 cm³, which the burette cannot deliver.
If a candidate writes 12.5 instead of 12.50, the examiner may deduct marks for insufficient decimal places. Always record two decimal places.
4 | Delivering solution during the titration
4.1 Hand positioning
If you are right-handed, use your left hand to operate the stopcock and your right hand to hold and swirl the conical flask. This arrangement keeps the stopcock from accidentally loosening (left-hand turns tighten a standard glass stopcock). Left-handed students reverse the setup.
4.2 Rough titre - full flow
The first run is a rough titre to find the approximate endpoint. Open the stopcock to allow a steady stream and watch for the indicator colour change. Record the result, but label it "Rough" in your table. It is not used in the average.
4.3 Accurate titres - drop-by-drop near the endpoint
For subsequent runs:
Add solution at a moderate flow rate until you are about 2 cm³ below the rough titre.
Switch to drop-by-drop additions. After each drop, swirl the flask and check the indicator.
Within the final 0.50 cm³, use the half-turn technique: turn the stopcock just enough that a partial drop forms on the tip of the jet without falling. Use the wash bottle to rinse that partial drop into the flask with a small jet of distilled water. This gives you control at the 0.05 cm³ level.
4.4 Swirl continuously
Swirl the conical flask throughout the addition. Continuous swirling ensures the titrant mixes instantly with the analyte, so the indicator responds to the true concentration in the flask rather than a localised pocket of excess acid or alkali. If you stop swirling to read the burette, you risk missing the endpoint.
5 | After the endpoint
5.1 Record the final burette reading
Apply the same three rules from Section 3: eye level, bottom of meniscus, to the nearest 0.05 cm³. Write the value in your table immediately.
5.2 Calculate the titre
Titre=Final reading−Initial reading
For example, if the initial reading is 0.50 cm³ and the final reading is 25.00 cm³, the titre is 24.50 cm³.
5.3 Repeat for concordant results
SEAB requires at least two concordant titres - titres that agree within 0.20 cm³ of each other (SEAB 2026 syllabus, p. 27). Average those concordant values (not the rough) and use the mean for your concentration calculation.
If your first two accurate titres differ by more than 0.20 cm³, perform a third run. Do not fabricate results - examiners can identify suspiciously identical values.
6 | Common mistakes and how to avoid them
Mistake
Why it matters
Fix
Not rinsing the burette with solution
Residual water dilutes the titrant, inflating the titre
Flush the jet at full flow before the first reading
Overshooting the endpoint
One extra drop can push the titre past concordance
Switch to half-turn additions within 2 cm³ of the rough titre
Reading at the wrong angle (parallax)
Systematic shift in every reading
Bend to bring eyes level with the meniscus
Forgetting to record the initial reading
Cannot calculate the titre; must restart
Write the initial reading before touching the conical flask
Recording to 0.01 cm³ or 0.1 cm³
Marks deducted for incorrect precision
Always record two decimal places ending in 0 or 5
7 | How burette marks are scored in Paper 3
Paper 3 divides practical marks into several categories. Two are directly affected by your burette technique (SEAB 2026 syllabus, pp. 25--26):
MMO (Making, Managing, and Observing) - Examiners look for evidence that you:
Used the correct rinsing procedure for burette and pipette.
Read the burette at eye level with the meniscus technique.
Controlled the stopcock competently near the endpoint.
Obtained concordant titres within the 0.20 cm³ tolerance.
PDO (Presenting Data and Observations) - Your results table must:
Show initial reading, final reading, and titre for every run (including the rough).
Record all burette readings to exactly two decimal places, with the last digit being 0 or 5.
Clearly label the rough titre and identify the concordant set used for averaging.
Present the mean titre to the correct number of decimal places.
Good burette technique feeds both categories simultaneously: clean readings produce clean tables, which produce accurate calculations in the ACE (Analysis, Conclusions, and Evaluation) section.
