Pipette

Description and types

A pipette is a narrow glass tube designed to transfer a precise volume of liquid. The most common type in IGCSE Chemistry is the volumetric pipette, which has a bulge in the middle and a single graduation mark near the top.

Volumetric pipette: Delivers one fixed volume (e.g., 25.0 cm3) with an accuracy of approximately +/- 0.06 cm3. Used in titrations to transfer a precise volume of solution to the conical flask.

Graduated pipette: Has markings along its length and can deliver variable volumes. Less commonly used at IGCSE level but appears in some practical setups.

How to use a volumetric pipette

1. Rinse the pipette with a small amount of the solution it will contain. Draw the solution into the pipette using the filler, wet the inner surface, then discard. This prevents dilution.
2. Draw up the solution above the graduation mark using a pipette filler.
3. Adjust the level: Remove the filler and place your index finger over the top. Slowly release finger pressure to let the liquid level drop to the graduation mark. The bottom of the meniscus should sit exactly on the line.
4. Deliver: Touch the tip of the pipette to the inside wall of the receiving flask and release your finger. The liquid drains under gravity.
5. Do not blow out the last drop. The pipette is calibrated to account for the small amount that remains in the tip. Only blow out if the pipette is specifically marked “blow out” (rare in school laboratories).

Accuracy

A 25.0 cm3 volumetric pipette has an accuracy of +/- 0.06 cm3. This makes it more precise than a measuring cylinder for that specific volume. However, it can only deliver one volume, so it is not suitable for measuring arbitrary amounts.

Common errors

Pipetting by mouth: Dangerous and explicitly penalised. Always use a pipette filler.

Not rinsing with the correct solution: Water left inside the pipette dilutes the first portion of solution, reducing its concentration and affecting the results.

Blowing out the last drop: Unless marked “blow out”, the calibration accounts for the retained drop. Blowing it out delivers more than the stated volume.

Not touching the tip to the flask wall: A drop hanging on the outside of the tip is not delivered. Touching the wall ensures clean transfer.

Air bubbles in the tip: If air enters while drawing up, the volume delivered will be less than intended. Check that the tip is fully submerged before drawing up.