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. Graduated pipettes have +/- tolerances that range from 0.6% to 0.4% of the nominal volume when measured at 20 °C (68 °F). It works by creating a partial vacuum above the liquid-holding chamber and releasing the partial vacuum created to draw up or deliver liquid.^[1] Graduated pipettes are manufactured according to ISO specifications for accuracy and the arrangement of the graduations. Grade A and AS pipettes has the highest accuracy and S stands for swift delivery. It has identical error limits as the DIN EN ISO standards. Grade B pipettes generally has twice the error as compared to grade A and AS pipettes. ^[2]

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 its needed. Graduated pipettes are considered to be more precise than Pasteur pipette but not as precise as volumetric pipette. The pipette are either adjusted to "TC"/"IN" or "TD"/"EX". TC is shorten for 'to contain' and the last drop of liquid in the pipette should be blown out. TD is shorten for 'to deliver' and it is manufactured in a way that the last drop of liquid should not be blown out.^[3][4]

Graduated pipette: 10 ml

10 ml Graduated pipette

The abbreviation "TD" stands for to deliver which means that the last drop of liquid in the pipette should not be blown out.

Grade AS pipettes has the highest accuracy and the S stands for swift delivery. It has identical error limits as the DIN EN ISO standards

Grade B pipettes generally has twice the error as compared to grade A and AS pipettes

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 rubber bulb to draw the liquid into the pipette (Using mouth to suck up liquid into the pipette is not recommended and it is considered unsafe).
3. Using forefinger is recommended to control the volume of the liquid in the pipette.
4. Release the excess liquid to acquire the amount needed.

References

1. "Pipet Guide". qorpak.com. Retrieved 2016-03-14.
2. "Graduated pipettes". marienfeld-superior.com. Retrieved 2016-03-10.
3. "Volumetric Measurement in the Laboratory" (PDF). brand.de. Retrieved 2016-02-17.
4. "Pipets". cmi2.yale.edu. Retrieved 2016-02-17.