Burette

Description and structure

A burette is a long, graduated glass tube with a tap (stopcock) at the bottom. It is used to deliver precise, variable volumes of liquid, most commonly in titrations.

Key features:

• Total volume: usually 50.00 cm3
• Graduations: marked every 0.10 cm3
• Reading accuracy: to the nearest 0.05 cm3 (estimated between the 0.10 cm3 lines)
• Scale direction: 0.00 at the top, 50.00 at the bottom
• Tap: glass or PTFE stopcock that controls the flow rate

The burette is held vertically in a clamp attached to a retort stand.

How to use

Preparation

1. Rinse the burette with a small volume of the solution it will contain. This prevents dilution by residual water. Tilt and rotate to wet all inner surfaces, then drain through the tap.
2. Fill the burette above the 0.00 cm3 mark using a funnel. Open the tap briefly to fill the space below the tap and remove air bubbles.
3. Remove the funnel (it could drip extra liquid and affect readings).
4. Adjust the liquid level to near 0.00 cm3 or to a convenient starting point.

Taking readings

1. Position your eye level with the meniscus. Use a white card behind the burette to make the meniscus easier to see.
2. Read at the bottom of the meniscus.
3. Record to the nearest 0.05 cm3. If the meniscus sits exactly on a 0.10 line, record as x.x0. If it sits halfway between two lines, record as x.x5.

Delivering liquid

1. Record the initial reading.
2. Open the tap and allow solution to flow into the receiving flask. Swirl the flask as you add.
3. Near the end point (in a titration), slow to half-drops: partially open the tap so a drop hangs at the tip, then wash it into the flask with distilled water from a wash bottle.
4. Record the final reading.
5. Volume delivered = final reading - initial reading.

Accuracy

The burette has a reading precision of +/- 0.05 cm3. Since you take two readings (initial and final), the total uncertainty in the volume delivered is +/- 0.10 cm3.

This makes the burette far more precise than a measuring cylinder for small to moderate volumes. It is the standard apparatus for titrations where accuracy matters.

Common errors

Air bubbles below the tap: An air bubble occupies volume. When it eventually rises out, the reading jumps, making the delivered volume appear larger than it really is. Check for bubbles before starting.

Not rinsing with the correct solution: Rinsing with water dilutes the first portion of solution. Always rinse with the solution the burette will contain.

Recording to one decimal place: Writing 23.5 instead of 23.50 loses the precision mark on Paper 6. Always use two decimal places.

Leaving the funnel in: The funnel can drip additional liquid, changing the volume. Remove it after filling.