BIOL 1406

PreLab 1.7

When measuring the volume of a liquid, how do I know which measuring device to use?

Erlenmeyer and volumetric flasks

You will often be required to measure the volume of liquids in lab.  The choice of a suitable device for measuring volume will depend on both the amount of liquid you want to measure, and the level of accuracy needed.  In general, you should use the smallest measuring device available that is large enough to hold the volume of liquid you wish to measure.  Using a measuring device several times (because it is too small to measure the full amount required) compounds errors in the measurement each time it is used.  Conversely, using a measuring device that is too big also reduces accuracy. 

  1. Beakers and Flasks are designed primarily to hold or store liquids, not to measure them.  Although these containers sometimes have calibration marks on them, these marks are not as accurate as calibration marks on graduated cylinders or pipettes.  Calibration marks on beakers and flasks are generally only “friendly suggestions” of the true volumes, and should not be used to estimate volumes unless a very low level of accuracy is needed.
     
  2. Graduated Cylinders come in a variety of sizes, ranging from 10 mL up to about 2 L.  In general, a graduated cylinder should not be used to measure volumes that are less than 50% of its capacity.  Since the smallest graduated cylinders have a capacity of 10 mL, graduated cylinders are not suitable for measuring volumes of less then 5 mL.
     
  3. Pipettes commonly come in sizes of 1 mL, 2 mL, 5 mL, and 10 mL.  Like graduated cylinders, a pipette should not be used to measure volumes that are less than 50% of its capacity.  Therefore, pipettes are suitable for measuring volumes of as little as 0.5 mL up to as much as 10 mL.
     
  4. Micropipettors (or automatic pipettors) are generally used to measure volumes of 1 mL or less.  Because they measure such small volumes, they are calibrated in microliters (mL) rather than milliliters.

REMEMBER: When you measure water or aqueous solutions in beakers, flasks, graduated cylinders, or pipettes, the top surface of the liquid will rise slightly towards the edges, forming a curved surface called the meniscus.  You should always line up the lowest point of the meniscus with the appropriate calibration mark on the measuring device.  Make sure that your eye is level with the meniscus, as well, so that the bottom of the meniscus lines up properly with the calibration mark.


 

 

How do I choose the correct micropipettor to use?

When using micropipettors, it is essential to choose the correct size for the job.  Three sizes are available:

¨      pipettors that measure volumes between 2 mL and 20 mL

¨      pipettors that measure volumes between 20 mL and 200 mL

¨      pipettors that measure volumes between 100 mL and 1,000 mL

Click the play arrow on the left to watch a short video about how to select and use the appropriate micropipettor.

If you have problems seeing the video, you can view it in an External Viewer
 

 


   IMPORTANT: Never attempt to use a micropipettor to measure a volume beyond the range it was designed to measure.


 

Use the interactive exercise below to practice how to choose and set the
appropriate micropipettor to measure a given volume.

 

Your Turn

Write 2 – 20 mL,  20 – 200 mL,  or 100 – 1,000 mL to indicate which micropipettor should be used for each of the following measurements:     

50 mL        Hint Check your answer.
300 mL     Hint Check your answer.
0.015 mL  Hint Check your answer.
0.6 mL      Hint Check your answer.
1.0 mL      Hint Check your answer.
0.05 mL    Hint Check your answer.

 

Your Turn

Assume that you have the following measuring devices available:

                50 mL beaker                                     5 mL pipette

                50 mL graduated cylinder                   100 – 1,000 mL automatic pipettor

                25 mL graduated cylinder                   20 – 200 mL automatic pipettor

                10 mL pipette                                     2 – 20 mL automatic pipettor

Fill in the name of the device that will most accurately measure each of the following volumes. You can copy and paste the names from the list above.
0.25 mL   Check your answer.
37 mL      Check your answer.
2.5 mL     Check your answer.
0.08 mL   Check your answer.
21 mL      Check your answer.
8.5 mL     Check your answer.
0.007 mL  Check your answer.
0.016 mL  Check your answer.
0.8 mL     Check your answer.
0.04 mL   Check your answer.

 

Close this browser window to return to Blackboard and complete the practice quiz and assessment quiz.