Ask A Scientist Chemistry Archive

name         Sarah
status       other
grade        other
location     MI

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"?
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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
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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
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