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Measuring the Volume of a Liquid with a Pipet

Adapted from "Science Lab Techniques"
Frontier High School
Red Rock, OK
http://scienceroom.frontierok.com

Pipets are much more accurate for measuring the volume of a liquid than graduated cylindars. Reading the volume in a pipet is just like reading a graduated cylinder, however using a pipet involves some additional technique. The diameter does not allow a liquid to be poured into a pipet - the liquid must be drawn in by suction created by a standard pipet bulb.

Much of the "professional" lab work today is done with automatic pipets. These are expensive little gadgets that come in many different volumes, each delivering exactly its assigned volume with one click of a button. Those of us who don't have access to such gear need to know the basic techniques of pipetting, which uses glass pipets and a standard pipet bulb.

The standard pipet bulb requires manual dexterity that improves with practice. It may take a few tries before you can accurately transfer a measured volume of liquid. Here's how it works:

• Pour slightly more liquid than needed into a beaker using the rough graduations on the beaker. Never pipet directly from a reagent bottle.
• Place the tip of the pipet below the surface of the liquid in the beaker.
  
  
  
• Squeeze the pipet bulb and press it firmly over the top of the pipet. Do not force the bulb onto the pipet. The soft plastic collar of the bulb is tapered inside to insure a good seal as long as you keep a steady pressure between the pipet and the bulb.


• Gradually release the presure of the squeeze on the bulb and allow the suction to pull liquid into the pipet. Draw more liquid than needed, but do not allow the liquid to enter the bulb.
• With the tip of the pipet still below the liquid's surface, quickly remove the bulb and place your finger or thumb over the top of the pipet to prevent the solution from draining back into the container.
  
  


• Record the exact volume of liquid in the pipet (remember the meniscus).

(Note: this is the step that requires the most practice). To release the liquid, slowly roll your finger to the side just enough to break the seal over the top of the pipet and allow the liquid to flow. To stop the flow of liquid, roll your finger back.

Photo credits: Trustees of Dartmouth College, Copyright 1997Ð2000

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