Glass Pipettes

Use a volumetric pipette for a single fixed volume at highest accuracy. Use a graduated pipette for flexible volumes at slightly lower accuracy. Use a Pasteur pipette for dropwise transfer without a volume requirement. Use a burette for variable volume dispensing during titration.

Volumetric pipettes range from 1 ml to 100 ml fixed sizes. Graduated pipettes range from 1 ml to 25 ml. Burettes are 25 ml or 50 ml. Use at least 50 percent of a pipette or burette capacity per measurement for best accuracy.

Never use mouth suction. Use a rubber pi-pump or three-way bulb for graduated and volumetric pipettes. Pi-pumps allow more precise control and are standard in teaching and professional labs. For Pasteur pipettes, a small rubber teat is sufficient. Use PTFE-tipped fillers for strong acids or organic solvents.

Class A pipettes meet the tightest ISO tolerances, required for official methods and traceable analyses. Class B tolerances are approximately twice as wide and suitable for teaching and routine bench work. For educational use, Class B is generally adequate and more cost-effective.

A graduated pipette delivers any volume up to its maximum and is more flexible but less accurate. A volumetric pipette delivers one specific volume to the highest accuracy. Use graduated for different volumes; use volumetric when transferring a fixed volume as accurately as possible.

Fit the filler securely then draw liquid past the calibration mark. Bring the meniscus down to the mark at eye level. Wipe the tip before dispensing. Never use mouth suction. Wash the filler with water between uses with different chemicals.

A burette is a long graduated glass tube with a stopcock at the base, mounted vertically, used to dispense variable volumes during titration. Unlike a pipette which delivers in a single transfer, a burette allows slow dropwise dispensing with the volume calculated from start and end readings.

Borosilicate Pasteur and graduated pipettes can be autoclaved at 121 degrees C for 15 minutes in a metal canister or foil wrap. Volumetric pipettes and burettes should not be autoclaved as the heat cycle can alter calibration. For sterile microbiology work, autoclaved Pasteur pipettes are standard.

Position your eye level with the liquid surface to avoid parallax error. Read the bottom of the meniscus for colourless liquids; read the top for strongly coloured solutions like potassium permanganate. After dispensing, touch the tip to the vessel wall and wait 3 to 5 seconds. Do not blow out the last drop from a volumetric pipette.