Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Proper Pipette Usage
Name:  Sarah
Status: other
Grade: other
Location:MI
Country: N/A
Date: 7/8/2005


Question:
After you discharge a liquid from a pipette a small amount remains in and on the tip. After a few seconds you touch the tip to the side of the vessel into which you discharge the pipet solution. What does this do? if a liquid remains in the pipette after this operation should it be "blown out"?


Replies:
The procedure you describe is the "accepted" procedure. No, you should not "blow out" the remaining liquid. You should never make contact with the pipette with your mouth under any conditions. Pipettes are calibrated assuming this small amount of liquid that remains because of surface tension. If you wish to do your own calibration, deliver the prescribed amount of liquid into a tared vessel and weigh the amount of liquid that was delivered. Repeat the delivery 5-10 times so that you get some idea of the reproducibility. For most solutions you can take the density of the solution to be equal the density of water, or if the solute is a solid/liquid of known density approximate the density by assuming the densities are additive ( a good approximation for dilute solutions). Weight measurements are more precise and accurate than most volume measurements. You can also weigh the amount of liquid that remains in the pipette by doing a weight difference on it too, if your balance allows you to get the pipette on the pan.

Vince Calder


No, most pipettes are calibrated to deliver their stated content with that liquid at the tip remaining in the pipet. The touching of the tip to the side of the container is to ensure that any drop that is hanging at the tip is delivered to the container, the action is not to try and release the last bit of liquid from within the pipette.

Greg (Roberto Gregorius)


Sarah,

The answer to your question depends on the sort of pipette that you are using. Typical serological pipettes used in microbiology labs are "blow-out" pipettes where the gradations of volume inscribed in the side of the pipette include the volume to the tip; touching the pipette end to the side of the container will help the liquid exit the pipette by breaking the surface tension of the liquid. Other pipettes do not include the volume at and about the tip in the gradations on the side of the pipette, instead rely on the movement of the meniscus between two identified points on the gradations to determine the volume delivered. For detailed information, including what different pipettes look like and best practices for using them, please read the set of slides prepared by The Science Learning Center of the University of Michigan at Dearborn, which you can download from:

www.umd.umich.edu/casl/natsci/slc/slconline/PIPET/PPT%20pipette%20slides-RL D4.PPT.

Of greatest importance to your health when using a pipette, never, and I mean NEVER, use your mouth in taking up liquid into a pipette. Even the most careful of us make errors, whether of a chemical or biological sort, either of which can result in unhappy tastes at minimum, or, worse, visits to emergency rooms, even stays at the hospital. A fellow graduate student in a microbiology lab came down with a dangerous Salmonella infection: Turned out that the Salmonella in him had the very same genetic markers as the laboratory strain of Salmonella on which he was experimenting -- in all likelihood, he infected himself when pipetting an aliquot of a culture. That fellow fully recovered, and kept a propipette (a safety pipette bulb) as a standard included item in his lab coat from then on. It is a good practice.

Don Silvert


When you quickly empty a glass pipette, some liquid remains wetting the interior of the pipette. The higher the viscosity of the liquid, the thicker this wetting-film. Your "few seconds" waiting time allows the film to slowly flow a little further downhill, towards the tip, where it coalesces and blocks the opening at the tip. Once blocked, the air inside can again be used to squeeze a little more liquid out the tip. Once outside, the liquid will not detach and fall. There often is not enough liquid remaining for gravity to overcome surface tension, and drop another droplet. But if the tip is touching the wall, the liquid will adhere to that wall in preference to the pipette tip, and eventually work down into the bottom of the container. (The droplet outside pipette is convex, the container-wall is concave, surface tension...) This way you can deliver more of the liquid that tries to stay behind in your pipette. If you are trying to be quantitative, it is often enough to help. With viscous liquids, sometimes I do it two or three times.

Some liquid will usually remain after this. What you then do depends on what you are trying to accomplish. Sometimes your accuracy has been sufficiently served, and then you may get a neutral liquid (water?) to rinse out your pipette, so it is clean for later uses. Other times your lost delivery can be reduced another factor of ten or more by sucking up some of the solution you just dumped into, and expelling that back into the container. Rinsing with the product, so to speak. If you are delivering 1ml of "A" into a 100ml beaker of mix "ABC", and you know your pipette will keep roughly 0.1ml of liquid wetted on its walls, you have a choice: to lose 10% of "A", or 0.1% of the mix "ABC". Usually the latter is much preferable. Then rinse with clean liquid, so none of the product mix gets back into your source chemical, next time you use the pipette. Usually I assume there is a next-time, even if there will not be. Sometimes the rinse-liquid gets dumped forwards into the mix, too.

I have rarely used my mouth on the pipette, even blowing-only, and my bulbs or plungers do not have a brisk airflow. You can see I have developed other methods than "blowing out" the pipette.

It depends on your particular job. If you think for yourself, you can come up with your own best disciplines.

Jim Swenson


It depends on the pipet. If the pipet is glass and has a double ring around end, it is considered a "blow out" and is calibrated to contain that drop. It may also say "TD" or "TC". TD means to deliver and TC means to contain.

vanhoeck


Hi Sarah,

You will find that most pipettes are marked with the letters "TD" -- that is "To Deliver". That means that if you use the method you describe (touch the end of the pipette to the side of the vessel you are transferring the fluid to) it will actually transfer the desired amount of the liquid. Blowing the remaining liquid out of the pipette would deliver too much to your reaction vessel.

Other glassware, such as beakers, might be marked "TC", for "To Contain". When you fill the glassware to a mark the mark indicates how much is actually in the glass (not how much will be transferred to another glass when it is poured out).

Greg Bradburn



Click here to return to the Chemistry Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory