Volumetric Glassware
There are several types of glassware used to measure the volume of a liquid accurately.
These include graduated cylinders, volumetric flasks, burets, and pipets. Graduated cylinders and volumetric flasks are designed to contain an accurately known volume. Burets and
pipets are designed to deliver, or dispense, an accurately known volume.
Reading the volume of a liquid in a cylinder or other apparatus is complicated by the fact
that liquids do not form flat surfaces in glass. Rather, the surface has a marked curvature,
called a meniscus. The liquid level is read at the bottom of the meniscus. An illustration of a
meniscus is shown below.
Graduated Cylinders:
A graduated cylinder is just what its name implies. Cylinders are designed to contain an
accurately known volume, but are usually used to dispense that volume. Therefore, they are
not as accurate as a volumetric flasks, but the fact that they are graduated makes them
more versatile than volumetric flasks. They are the workhorse volumetric apparatus for most
lab tasks, and are available with capacities from 10 mL to 2 L.
Use of a graduated cylinder is straightforward. If one wishes to fill it to a desired volume, the
liquid is added to just below the target volume. The last few drops are added with a dropper
or Pasteur pipet to bring the bottom of the meniscus to the desired graduation. If one wishes
to read the volume contained in a cylinder, one locates the bottom of the meniscus. The
volume is read “between the lines” or graduations, and the last digit contains the uncertainty
of the measurement.
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Volumetric Glassware
Volumetric Flasks:
Volumetric flasks are designed to contain a very accurately known volume of liquid. They
are used for the preparation of solutions in which concentrations must be known to four
significant figures. Volumetric flasks have a wide base, and a narrow neck with a single
scribe mark which marks the fill level.
To use a volumetric flask one adds the solute (a carefully weighed solid, or a liquid
transferred by a volumetric pipet) and then fills the wide base with solvent. The solvent is
swirled to dissolve and mix the solute. Finally, the neck of the flask is filled until the bottom
of the meniscus is at the scribe mark. One then caps the flask and inverts it several times to
completely mix the contents. The solution is then transferred to a bottle or other container
for use. Volumetric flasks are not generally used for storage of solutions.
Burets:
A buret is long, narrow tube with a stopcock at its base. It is used for dispensing accurately
known volumes of liquids or solutions. It is graduated in 0.1 mL increments, with the 0.00 mL
mark at the top.
Just before use, a buret should be "conditioned" to ensure that any water adhering to the
inside walls is removed. Add ~ 5 mL of the liquid that is to be used into the buret. Tip the
buret sidewise and rotate it so the liquid rinses the walls of the buret, then drain the liquid
through the stopcock.
The buret can now be filled with solution. Do this carefully and avoid trapping air bubbles in
the tube. The liquid level can be above the 0.00 mL mark. Clamp the filled buret in place if
this was not done prior to filling; it's some-times easier to hold the buret while filling it. Open
the stopcock and drain enough liquid to fill the buret's tip. Have a beaker for waste solution
handy for this and similar operations. There should be no bubbles in the tube or tip of the
buret. These will lead to volume errors. If there are bubbles in the tube, carefully tap the
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Volumetric Glassware
buret to free them. Use the stopcock to force bubbles out of the tip. It may be necessary to
empty and refill the buret.
When the buret is clean and bubble free, drain the liquid until the meniscus (the bottom of
the curved surface of the liquid) is at or slightly below the 0.00 mL mark. It is not necessary
to align the meniscus exactly at the 0.00 mark since the difference between the initial and
final volumes is the desired measurement. If there is a drop of liquid clinging to the buret tip,
remove it by gently touching the tip to a glass surface, such as the edge of the waste
beaker. The volume of a drop is about 0.1 mL, the same volume as the buret's graduations.
Find the bottom of the meniscus, and read the liquid level in the buret to the nearest 0.10
mL at that point. This will take a little practice. Remember, you're reading from the top down.
Record this value as the initial volume. Although it's tricky to "read between the lines,"
remember that the last digit of a measurement is expected to have some uncertainty! Onefifth (1/5) of a division (0.02 mL) can be reproducibly estimated if the meniscus is between
calibration marks, after a little practice.
Now dispense the liquid you need. If you are using the buret to measure a set amount of
liquid, determine what the final reading should be to obtain that amount. Dispense the liquid
slowly into the receiving vessel. After closing the stopcock, catch any hanging droplet in the
receiving vessel. It's part of the measurement at this point, so don't catch it in the waste
container. Wait a few seconds for the meniscus to stabilize, then read and record the final
volume to the nearest 0.01 mL. The difference between the initial and final readings is the
volume you dispensed. When using a buret, it's easier to work with the exact volume
dispensed than to try to dispense an exact volume. Plan your work with this in mind.
Although burets are sometimes used as dispensers, they are far more frequently used in
procedures called titrations. In a titration, one attempts to determine an equivalence point as
exactly as possible. This usually involves the first persistent color change of an indicator.
With a little practice, one can dispense fractions of drops (less than 0.1 mL) into the titration
vessel, and reproduce results within 0.10 mL or less. Reading the buret and recording
volumes is handled the same way as in dispensing liquids.
When one is finished with a buret, drain the remaining liquid and clean it carefully. Finish
with several rinses of deionized water, and make sure the stopcock and tip get their fair
share. If solute dries on the buret, it can be very difficult to remove.
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Volumetric Glassware
Pipets:
Pipets are calibrated tubes that are used for transferring and measuring liquids. There are
many types of pipets, some for specialized analyses and tests. They can be made from
glass or plastic. The following types of glass pipets (plastic varieties are also available) are
common:
Volumetric pipets
Volumetric pipets are used for dispensing known volumes of liquids. They are
glass tubes that usually have an expanded midsection and a single scribe mark
for the volume they are designed to deliver. Volumetric pipets are made in many
sizes, from 0.5 to 200 mL. They are the most accurate pipets, delivering volumes to +/- 0.03 mL or so, depending on the volume of the pipet. They are designed to drain with gravity, leaving a small amount of liquid in the tip. Do not
blow or otherwise force this liquid out of the pipet.
Measuring pipets
Measuring pipets are graduated, usually to 0.1 mL. Typical sizes are 1 to 25 mL
total volume. There are two types, serological and Mohr.
Serological pipets are designed to be filled to the desired volume, then emptied
completely (usually blown out). They are less accurate than volumetric pipets,
because the tubes have larger diameters. However, they are more flexible than
volumetric pipets because they can be used for a wide range of volumes.
Mohr pipets are very similar to serological pipets except that they are not designed to be completely drained. One should inspect graduated pipets to determine which type they are before use. The photograph shows a serological pipet
on top, and a Mohr pipet below it.
There are many types of automated, semiautomated, and multiple-tipped pipets for specialized medical and biochemical uses. Most of these have plastic disposable tips. They
cannot be used with organic liquids or with strong aqueous acids and bases. Glass pipets
remain the workhorses for such applications.
Pipets are filled by suction, usually from a rubber bulb or other pipet-filling device.
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Volumetric Glassware
Glass pipets should be “conditioned” by rinsing them with a few milliliters of the solution to
be measured before actual measurements are made. The liquid is drawn into the pipet (for
a volumetric pipet, stop at the expanded part of the tube), then the pipet is tipped on its side
and turned so the liquid coats the inner surface at least to the top graduation. The wash
liquid is discarded.
To measure liquid with a pipet, use the pipet bulb to pull liquid into the pipet until the liquid
level is above the graduation. Use the index finger of your dominant hand to cover the top
of the pipet. Liquid will not drain while your finger is in place. Wipe the tip of the pipet to
remove any adhering droplets of the liquid in which it was immersed. Carefully allow excess
liquid to drain from the pipet until the bottom of the meniscus is at the scribe mark. Now
hold the pipet so its tip is above the vessel to be filled and allow its contents to drain. The
amount of liquid that should drain from the pipet depends on its type, as described above.
Cleaning of Volumetric Glassware:
Most volumetric glassware is designed to be accurate to four significant figures. Obtaining
that accuracy requires some care. Details on how to use various types of glassware have
been given in the discussions above. Cleanliness and careful storage of volumetric glassware is general, and will be discussed here.
Chemically clean glassware supports a uniform film of water, with no “beads” as one would
see on a newly waxed car. Maintaining that level of cleanliness is fairly simple, but getting it
back once it’s gone requires work, and cleaning with fairly harsh chemicals. When you have
finished using a piece of glassware, immediately rinse it three or four times with a small
volume of distilled water. This will remove the chemical you measured, and leave no
residue. If a piece of glassware is not supporting a uniform film of water, it should be set
aside for cleaning. Your instructor will do this or show you how.
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