Pipetting

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Pipetting involves drawing a liquid into a pipet and allowing liquid to drain from the pipet in a controlled manner. Pipetting is used to quantitatively transfer exact volumes of a liquid from one container to another.

Two types of pipets commonly used are the Mohr pipet and the volumetric pipet. The Mohr pipet has graduations that allow it to deliver a range of volumes. The volumetric pipet has a single graduation that allows it to deliver one specific volume accurately. How do I choose which pipet to use?

Figure 1. A standard white-tipped pipet bulb, a 10 mL Mohr pipet and a three-way safety pipet bulb. (Click on each name for more information.)

Figure 2. A volumetric pipet

Which of the items in Figure 1 is meant to hold liquid? Answer

Draw a picture of a pipet. Label the mouth, the tip, the 0.00 mL mark and the 10.00 mL mark, and the mark below which you should not allow liquid to drain. Answer

Before using your pipet, be sure it is free of chips or cracks, is clean, and has been rinsed several times with the solution you will pipet. This will help ensure accurate volume measurements and avoid contamination of the liquid.

Video: Rinsing pipet with solution ( 9.87 M ) Text description

T or F? When rinsing the pipet, it is important to wet the pipet mouth. Answer

Two types of bulbs are commonly used to draw liquid into a pipet: the standard pipet bulb and the three-way safety bulb. NEVER use your mouth to draw liquid into a pipet!

While drawing up solution, make certain that the pipet tip remains below the surface of the liquid; this will prevent drawing liquid into the pipet bulb.

Video: Drawing in with standard pipet bulb ( 4.16 M ) Text description

Video: Drawing in with three-way safety bulb ( 6.87 M ) Text description

Video ( 1.20 M ) Explain what the student in the video could have done to avoid this error. Answer

Determine the volume of solution in a pipet by reading the bottom of the meniscus at eye level. Record the volume using all certain digits and one uncertain digit. Certain digits are obtained from calibration marks. Uncertain digits (the last digit in the number) are estimated between calibration marks. Some examples are shown below:

4.49 mL reading	4.88 mL reading

5.00 mL reading

Accurately transferring a specified volume of liquid from one container to another requires careful attention to detail. Pay special attention to the level of liquid in the pipet and carefully monitor the meniscus.

Video: Transfer procedure using standard bulb ( 4.19 M ) Text description

Video: Transfer procedure using 3-way bulb ( 2.80 M ) Text description

When transferring solution, do not allow the liquid to drain past the 10.00 mL mark. The pipet can accurately deliver up to 10.00 mL, but the liquid below the 10.00 mL mark is not included in this measurement and its volume cannot be specified.

Video ( 4.17 M ) What volume of liquid was transferred? Answer

What is wrong with this pipet reading? Answer

The volumetric pipet is used in a manner similar to the Mohr pipet. Like the Mohr pipet, the first step in using a volumetric pipet is to prepare the pipet for use. Next, draw liquid into the pipet using either a standard bulb or a three-way safety bulb. Draw liquid past the graduation on the neck of the pipet, and then drain liquid to align the meniscus with the graduation.

Video: Filling the volumetric pipet ( 5.75 M ) Text description

Next, transfer the liquid to the receiving vessel. Unlike the Mohr pipet, which should not be completely drained when a liquid is transferred, the liquid in the volumetric pipet should be drained completely into the receiving vessel. However, DO NOT blow out the last little bit of liquid from the pipet tip.

Video: Liquid transfer ( 2.40 M ) Text description

Miscellaneous Exercises

Video ( 1.04 M ) Suggest a more effective way to clean a pipet. Answer

Video ( 715K ) The student in the video believes he transferred exactly 10.00 mL of liquid. Explain his mistake. Answer

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