Measuring with Pipettes

Graduated pipettes made of glass or plastic are used for accurate measurement of liquid volumes. Use them for measuring volumes between 1 ml and 10 ml: For larger volumes, you would use a graduated cylinder, and for smaller volumes, a micro-pipettor is more convenient.
Several kinds of pipette fillers are available, from simple bulbs to fancy electric pumps. In Biology labs, you’ll most often use a Pipet-Pump ® filler which is both easy to use and reliable. This is a plastic tube which fits over the blunt end of the pipette and has a plunger sort of like a hypodermic syringe (left). To fill the pipette, insert its tip into the liquid and use the wheel to raise the plunger. Go slowly, so you don’t get bubbles. Remember to read the bottom of the meniscus of the liquid to ensure an accurate measurement. To empty the pipette, simply press the plunger back down. Some Pipet-Pumps have a lever on the side which you can press to let in air and allow the pipette to empty. One problem you may have with the Pipet-Pump is how to blow out that very last drop from the pipette. Here’s a hint: don’t start with the plunger all the way down. Raise the plunger ½” or so before you put the tip in the liquid. That little extra distance will allow you to expel the last drop.
You will encounter two main types of pipettes in Biology labs (right). Most are the type called serological pipettes. These are graduated all the way to the tip (A). For example, if you look at a 10-ml serological pipette, there will be a line at the top marked zero. When you fill the pipette to this line, it contains 10 ml. Below this line, there are ten smaller, unmarked graduations, representing 0.1 ml each, and then a line marked 1 ml. Below that are more 0.1 ml graduations and marked lines for 2 ml, 3 ml, etc. The last marked line will be 9 ml, but there will be 0.1-ml marks below that and down on the tip to indicate that to dispense the full 10-ml volume, you have to go all the way to the tip.
The other type of pipette (B) is a measuring pipette. Notice that this one isn’t graduated all the way to the tip, and there’s actually a line for 10 ml. That means that if the pipette is filled to the zero line, it actually contains more than 10 ml, and to dispense 10 ml, you don’t empty the pipette completely but instead just let out fluid until it reaches the 10-ml line.
In order to measure the volume you want, you may have to do a little mental math. Suppose you need 8 ml of water. You could fill the pipette all the way to the zero line and then dispense 8 ml by letting the water out until it reaches the 8-ml line. However, you then wind up with 2 ml extra in the pipette, and you have to either put it back (not a good idea if you’re dealing with sterile solutions) or discard it. For a serological pipette, a better way would be to fill the pipette to the 2-ml line and dispense all the way to the tip: no waste. Some pipettes have a second scale that runs in the reverse direction to make this easier. You can use whichever scale is more convenient.
Some students seem to think that micro-pipettors are so much more accurate that they would rather measure 3 ml by using a 1-ml micro-pipettor three times than by using a 10-ml pipette. Not so! In fact, any time you have to make multiple measurements, accuracy decreases dramatically. Use the 10-ml pipette and just measure once. Also, think about efficiency: if you have to measure 10 1-ml volumes, it will be much faster to fill a 10-ml pipette and dispense 1 ml into each tube than to use a micro-pipettor 10 times.

Leave a Reply

Your email address will not be published. Required fields are marked *