As part of the procedure for the determination of the sulfate content of an unknown sample it
be necessary to accurately determine the empty weights of a set of crucibles. This is
accomplished by first cleaning the crucible and then heating them in the flame of a Tirrel burner.
They are then cooled and weighed. This process of heating and cooling is repeated until
successive weighings agree to within 0.2mg. The procedure will, in addition, acquaint the student
with the use of the analytical balance and the proper use of the desiccator. Be sure that you read
the appendix section dealing with the use of desiccator.
Wash and rinse four 10 ml crucibles and their lids. Identify each of the crucibles by means of
existing visual differences. Carefully note in your lab notebook what these identifying marks are
and assign a number to each crucible. Do not use marking pens or pencils to mark your crucibles.
The high heat which will be used to take them to constant weight will obliterate such markings
and more importantly will affect their weight in an unpredictable manner. The lids to the crucibles
do not need to be taken to constant weight, however, they should be clean.
Support a crucible on its side in a wire triangle
(see the demonstration
the Web, "Heating a Crucible to Constant Weight.")
Place a Tirrel burner under the crucible and adjust the flame of the burner to give a non-luminous flame with a full gas supply. The tip of the flame cone should be just below but not touching the crucible. Heat the crucible at red heat in this manner for 5-10 minutes. Allow it to cool for a few minutes and then place it in the desiccator. When it has reached room temperature weigh the crucible accurately, to ± 0.1 mg, and then put it back into the desiccator. Reheat and reweigh it until successive weighings agree to within 0.2 mg.
For additional instruction on the use of balances in the laboratory, click here.
1. Both the hot and cool crucibles should only be handled with tongs. It is therefore
imperative that the tips of the tongs be very clean. If they appear dirty or rusty be sure
that you clean them. Dipping them into some dilute HCl is helpful in removing rust.
Never allow the tips to come in contact with the desk top.
2. Never touch a red hot crucible with the tongs. The high heat used will soften the enamel
enough so that some it will stick to the tongs and thereby lead to erratic weights.
3. Crucibles should never be placed directly on your desk. Obtain a piece of aluminum foil
which can be used to place the crucibles and tongs.
4. Make sure that the plate in the desiccator is completely clean. Also check to see that crucibles sitting in the holes of the plate do not touch any part of the wire gauze which separates the desiccant from the rest of the desiccator.