Introductory Experiment:
Calibration of Volumetric Glassware
Introduction
Volumetric glassware, for the beginning and accomplished analytical chemist, is probably
one of the most important "tools of the trade". You will succeed or fail on your ability to
effectively use glassware, which is designed to contain or to deliver a known volume accurately
and precisely. This experiment is designed to acquaint you with volumetric glassware, both in
terms of its practical use and with regard to its volume calibration.
Most take for granted the calibration of volumetric glassware. For truly exacting wet
chemical analyses, however, the importance of volume calibration cannot be overstated. As
discussed in several current quantitative analysis texts*, one should compensate for temperature
effects on diluent density and (in rare cases) the thermal expansion of glass. In tandem with
these effects, is the non-negligible effect of buoyancy in mass measurements. You will learn
how to compensate for thermal induced density changes for both diluent and air, appropriate for
high accuracy analytical measurements.
There are three general approaches to the calibration of volumetric glassware that are in
wide use and with which the student should be familiar.
1. The direct, absolute calibration is based on the direct mass measurement of water delivered
by a buret or pipet or contained in a volumetric flask and conversion to volume with the density,
at the specified temperature.
2. The indirect, absolute calibration, or calibration by comparison, whereby a calibrated vessel
is used as a reference in the calibration of an unknown vessel, is sometimes used when many
glassware items are to be calibrated.
3. A relative calibration is based on the measured volumetric relationship between two items of
glassware, without knowledge of the absolute volume of either one. Accuracy is compromised
in favor of precision.
You will perform a direct, absolute calibration of a buret in this introductory experiment. Study
the following procedure and understand the measurement techniques and calculations before
coming to the laboratory period.
* Quantitative Chemical Analysis, 7th Ed., 2007, by D.C. Harris (Chapter 2)
Analytical Chemistry 6th Ed. by G.D. Christian (2004) Wiley, (Chapter 2)
Procedure
1.
Thoroughly clean a buret (drying is not necessary). Make sure that water drains well and
that the stopcock is lubricated. Fill the buret with distilled water, approximately to the 0.0
mL mark. Read the volume to the nearest 0.01 mL. Check for leaks by re-measuring the
volume after a few minutes.
2.
During this time weigh a clean, dry, stoppered 125 mL Erlenmeyer flask to the nearest mg.
Record this "initial" weight.
3.
Again, read the buret to the nearest 0.01 mL. Record this "initial" volume. Make sure there
is no liquid hanging from the buret tip. Now, dispense about 10 mL of water into the
weighed Erlenmeyer flask. Do not allow the flow rate to exceed 30 mL/min. Ensure that
no liquid spatters out of the flask.
4.
After about 10 mL close the stopcock and carefully touch the buret tip to the inner neck of
the flask. Wait a minute or so, stopper the flask and read the buret. Record this "final"
volume. Record the temperature. Weigh the stoppered flask to the nearest mg and record
this "final" weight. Refill the buret with water using a wash bottle.
5.
Repeat steps 3 and 4 at least 3 times. Do not empty and dry the Erlenmeyer flask between
measurements, but make sure that the flask is stoppered when not receiving liquid.
6.
Repeat steps 1 through 6 with a clean, dry 125 Erlenmeyer flask and 10 mL aliquots of
water per measurement, starting at the 20 mL mark on the buret. Perform at least 2
additional measurements at this volume.
7.
Choose a pipet in your drawer and perform a series of calibration measurements. Construct
a table for the pipet data that is similar to the one you prepare for the buret calibrations.
Calculations
Enter your data and calculated results into the attached table. Use the table to the right to obtain
the correction factor (or density) appropriate for the temperature at which your measurements
were made.
Temperature =
Density =
Initial Buret
Reading
(mL)
Final Buret
Reading
(mL)
Apparent
Volume (mL)
Mass of
flask (g)
Mass of
flask plus
water (g)
Water
dispensed
(g)
Actual
Volume
((mL)