Overview
This lecture covers the types, classifications, and proper usage techniques of pipettes for liquid transfer in laboratory settings.
Types and Classifications of Pipettes
- Pipettes are used to transfer liquids, generally ≤20 mL; automatic pipettes are used for >20 mL.
- Pipettes can be glass or automatic, with glass pipettes consisting of a tip, stem, and mouth.
- Pipettes are classified by type: transfer pipette, measuring (graduated) pipette, micropipette, and automatic pipette.
- Transfer pipettes dispense a single, fixed volume without subdivisions.
- Transfer pipette subtypes: Ostwald-Folin (bulb near tip, used for viscous fluids) and Volumetric (bulb in middle, used for aqueous solutions).
- Measuring (graduated) pipettes dispense several different volumes; they have graduation lines along the stem.
- Types of measuring pipettes: Mohr (no graduation to tip, calibrated between two marks) and Serologic (graduation to tip, calibrated to the tip).
- Micropipettes transfer volumes <1 mL, e.g., Sahli-Hellige, Lang-Levy, RBC, WBC, and Kirk and Overflow pipettes.
- Automatic pipettes have mechanisms for aspirating and dispensing; types: air displacement, positive displacement, and dispenser/deliver pipettes.
Automatic Pipette Mechanisms
- Air displacement pipettes use a piston to create suction and require a disposable tip for each use.
- Positive displacement pipettes have a piston that moves within the tip or barrel, similar to a syringe, and do not need a separate disposable tip each time.
Proper Pipetting Technique
- Always hold pipettes upright; immerse the tip sufficiently in the liquid without touching container walls.
- Use a pipette bulb to apply suction until the liquid rises above the graduation mark, then stop suction.
- Adjust the meniscus: lower meniscus for colorless/aqueous liquids, upper meniscus for viscous/colored liquids.
- After drawing liquid, line up the meniscus with the desired mark and wipe off any drops.
- Transfer liquid by letting it drain while holding the pipette vertically and touching the tip to the vessel side.
Key Terms & Definitions
- Meniscus — the curved surface of a liquid in a pipette; read from the bottom for clear solutions, top for colored/viscous ones.
- Transfer pipette — dispenses a single, fixed volume without subdivisions.
- Measuring (graduated) pipette — can dispense multiple volumes, marked with graduations.
- Micropipette — used to transfer less than 1 mL of liquid.
- Automatic pipette — uses mechanical action for liquid transfer, often with disposable tips.
Action Items / Next Steps
- Practice correct pipetting technique as outlined.
- Review types and classifications of pipettes before the next lab session.