Reading: Burets
Revised 3/20/12
BURETS
A buret is a piece of volumetric glassware used to deliver variable liquid amounts typically for
titrations. It is a long glass cylinder open at one end and fitted at the opposite end with a
stopcock valve (Figure 1). The stopcock valve controls the flow of liquid from the cylinder
through the buret tip. The buret used in the General Chemistry lab has a 50-mL volume and a
Teflon stopcock requiring no lubrication. The buret is etched with calibration markings at 0.10mL intervals allowing the estimation of variable liquid levels to the hundredths of a milliliter
(0.00 mL). Record all numbers from markings and one number estimated between markings
(significant figures) to indicate the sensitivity of the volume measurement.
Figure 1. Buret Use.
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Reading: Burets
Revised 3/20/12
The numbering of the markings begins at the top (open end) of the buret. Therefore, the volume
delivered can be determined from the difference between initial and final liquid levels. Refer to
Figure 1 and note the correct way to read the liquid level in a buret (from the top down).
Before use, a buret should always be cleaned and prerinsed with the solution to be delivered.
Do NOT attempt to fill a buret precisely to the 0.00-mL mark at the top. This is a waste of time,
as all volumes delivered by a buret are determined from the difference between initial and final
readings. After filling the buret with solution, drain a few milliliters of liquid through the
stopcock into a beaker to remove any air bubbles trapped in the valve. Close the stopcock, tap
the buret sides gently, and allow the liquid to stand for a few minutes to clear gas bubbles
dissolved in the liquid. Remove any drop hanging from the buret tip by touching the tip to the
side of the beaker or wiping the tip with a tissue. Read the initial volume to the hundredths place
(0.00 mL).
To transfer the liquid, open the stopcock valve until the desired amount of liquid is slowly
drained into a container. Close the valve and touch off the hanging drop of liquid on the buret tip
adding it to the liquid in the container. This step is important because the hanging drop is part of
the volume delivered by the buret. Allow a moment for liquid on the cylinder walls to settle
before recording the final volume to ±0.01 mL (again from the top down). The difference
between the initial and final volumes is the amount of liquid transferred to the container.
Buret Care
• Wash the buret with a mild dilute soap solution. Rinse first with tap water, and then
deionized (DI) water. If beads of water form on the walls, rewash.
•
Never dry a buret in an oven! The heat will distort the glass and change the calibrated
volumes. Never dry a buret using air jets! The air system contains oil droplets and fine dust,
also the compressed air can break the glass.
•
A buret should always be rinsed with a small amount of the solution to be used. This step
prevents contamination or dilution of the solution from water or other chemicals on the
glassware’s inside walls and removes the need to dry the buret.
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Reading: Burets
•
Revised 3/20/12
After an experiment is completed, drain all chemicals from glassware, pouring them into
marked collection containers in the hoods. Glassware may become irreversibly etched from
prolonged contact with some solutions (especially NaOH or KOH solutions). Wash the
glassware, rinse thoroughly, and return to common storage area at the front of the laboratory.
•
Return all broken or cracked glassware to the stockroom for replacement.
Review Questions:
•
When reading a buret, are the numbers read from the bottom up or the top down?
•
When recording a buret volume, how many places past the decimal point should be
recorded?
•
Why are the sides of the buret tapped after filling it?
•
Why are a few milliliters of liquid drained through the tip after filling the buret?
•
After delivering a certain volume into a flask, a drop of liquid is hanging from the tip. Is it
part of the delivered volume? Should it be added to the flask?
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