Volumetric Pipets

Unlike graduated cylinders, volumetric pipets are

designed to deliver only a specific volume of liquid.

If 10 mL of liquid is needed, a 10 mL volumetric

pipet is used, but if 25 mL of liquid is needed, then

a completely different 25 mL volumetric pipet is

used. A volumetric pipet is a specialist in that it can

deliver only the one specific volume that it was

designed to deliver. A volumetric pipet has only one

mark on it. That mark is the line for the volume of

liquid the pipet was designed to deliver.

A pipet bulb is used to draw liquid into the pipet

from a separate container (typically an Erlenmeyer

flask or beaker). To use a volumetric pipet:

1. Place the tip of the pipet into the liquid.

2. Take the pipet bulb not attached to the pipet and

squeeze it.

3. Attach the squeezed bulb to the top of the pipet.

4. Slowly release the bulb while making sure to

keep the tip of the pipet submerged in the liquid.

If the tip of the pipet becomes dislodged from

the liquid, air will be introduced into the pipet

and potentially make a big mess to clean-up.

5. Continue to allow the liquid to rise in the pipet until it rises above the mark

but has not reached the pipet bulb. It is important to keep liquid from entering

the pipet bulb.

6. Remove the pipet bulb and place a finger (thumb or pointer finger work best)

on top of the pipet to keep the liquid level constant in the pipet.

7. Slowly release pressure on your finger so that the liquid level slowly drops.

8. Get the bottom of the meniscus to the mark (make sure you read it at eye

level to avoid parallax error!). This can take some practice. If you go past the

line, you will need to use the pipet bulb again to draw more liquid into the

pipet.

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Figure 3.8: Various sizes of

volumetric pipets.