Measuring the volume of a liquid with a pipette

Pipettes are much more accurate than graduated cylinders. Reading the volume of liquid in a pipette is just like reading a graduated cylinder, however there is one additional technique needed with a pipette. The diameter does not allow a liquid to be poured into a pipette - the liquid must be drawn into the pipette. regular bulb on top and safety bulb on bottom This picture shows two of several types of pipette bulbs used to draw a liquid into a pipette. The red bulb is known as a standard pipette bulb. The black bulb is known as a safety pipette bulb. Much of the "professional" lab work today is done with automatic pipettes. These are expensive little gadgets that come in many different volumes, each delivering exactly its assigned volume with one click of a button. High school chemistry students need to know the basic techniques of pipetting, so you will be using glass pipettes and the standard pipette bulb.

The standard pipette bulb requires manual dexterity that is improved by repetitive use. You may have to practice using the bulb with a pipette before you are able to accurately transfer a measured volume of liquid. The technique goes something like this:

* Pour slightly more liquid than needed into a beaker using the "ballpark" graduations on the beaker. Never pipette directly from a reagent bottle.
* Place the tip of the pipette below the surface of the liquid in the beaker.
* Squeeze the pipette bulb and press it firmly over the top of the pipette. Do not force the bulb onto the pipette. 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 pipette and the bulb.
* Gradually release the pressure of your squeeze on the bulb and allow the liquid to be drawn into the pipette. Draw more liquid than needed, but do not allow the liquid to enter the bulb.
* With the tip of the pipette still below the liquid's surface, quickly remove the bulb and place your finger or thumb over the top of the pipette to prevent the solution from draining back into the container.
* Record the exact volume of liquid in the pipette (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 pipette and allow the liquid to flow. To stop the flow of liquid, roll your finger back.