Graduated pipette

Graduated pipettes commonly come in 5, 10, 25 and 50 mL volumes. To avoid accidental ingestion of potentially harmful substances, a variety of propipetters have been developed, both entirely manual and electrically assisted:

Originally pipettes were made of soda-lime glass, but currently many are made of borosilicate glass; disposable graduated pipettes are often made of polystyrene. Graduated pipettes are often graduated in one of two ways:

• Mohr, backward or drain-out pipettes have a 0 mL mark just above the end of the pipette. A volume is pulled into the pipette, sometimes to the maximum volume, then the needed volume is aliquoted out.
• Serological, forward or blow-out pipettes have no 0ml mark as that corresponds to an empty pipette. The volume needed is pulled into the pipette and then dispensed.

Top: A Mohr, backward or drain-out pipette.
Bottom: Serological, forward or blow-out pipette.

Historically, the accuracy of a graduated pipette was not as good as that of a volumetric pipette (accuracy of 3 significant fig); however, with improved manufacturing methods, the accuracies listed by the manufacturer can equal volumetric pipettes.^[1] Graduated pipettes have +/- tolerances that range from 0.6% to 0.4% of the nominal volume when measured at 20 °C (68 °F). Graduated pipettes are manufactured according to ISO specifications for accuracy and the arrangement of the graduations. A-grade pipettes are more accurate than B-grade pipettes.

There are number of lines that are located at specific intervals throughout the pipette which allows the user to adjust the volume of the liquid to the specific amount that they want. Graduated pipettes are considered to be more precise than Pasteur pipette but not as precise as volumetric pipette.

Standard technique to use a graduated pipette

1. Hold the pipette in the solution and don't let it touch the bottom on the beaker.
2. Use a pipettor/pipette aides to draw the liquid into the pipette (Never use your mouth to suck up the liquid into the pipette, it is considered unsafe).
3. Use your forefinger to control the volume of the liquid in the pipette.
4. Release the excess liquid that is in the pipette to get your desired amount or fill the liquid up to 0ml and release only the desired amount.

References

1. ↑ http://qorpak.com/Store/pages/PipetGuide.aspx

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