Apparatus for Facilitating Dumas Bulb Measurements In order to familiarize beginning physical chemistry students with the detailed techniques of error analysis and error propagation calculations, the Dumas bulb determination of molecular weights is utilized providing a n intellectually simple experiment capable of yielding results of fair accuracy and precision. In order to achieve the best results by this method, the drawn out tip version of a Pyrex Dumas bulb (left) is employed making possible more reliable weight and volume determinations. The problem af filling and checking the tip off pump seal an the bulb is eliminated by the apparatus shown (right). By means of the bulb filling apparatus the desired quantity of sample, usually 3-10 ml, can b e h a d e d into the Dumas bulb with no difficulty and little waste. The volatile nature of the samples (unknowns) enables one to clean the apparatus simply by pumping on it and simultaneously turning the stopcocks around a couple of times to clean out the dead space and remove any sample that might be in the ground area. The open end of the apparatus is plugged, for cleaning, with a drawn down glass rod. A single stage pump of the type used for distillation is eminently iatiifaetory. Once the sample is vaporized and the bulb is sealed, the bulb may be reinserted into the apparatus and pumped on. If there is a leak in the seal, air will be seen bubbling into the bulb when the repumped bulb is held so that the condensed sample is down in the tip off. Restarting the experiment a t this point, if a leak exists, enables one t o still make two or three separate, good measurements in a 3-hr laboratory period. Of course, the various tips must be preserved in the event of a restart, in order to maintain knowledge of the bulb's weight. In addition, the sealing pracedure, done with a well defined bunsen burner flame or a torch, should be done so as to leave the Dumas bulb tip straight, for reinsertion into the bulb filling apparatus, and the glass "balled up" to help insure proper sealing. fi a more elaborate experiment is desired, the expanded Dumas bulb experiments described by Kaya and Campbell' are recommended.
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'Kaya, J. J., and Campbell, J . A,, J. Chem, Educ.. 44,394 (1967). Alfred P. Sloan Research Fellow.
Vanderhilt University Nashville, Tennessee 37235
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D a n T-e. Wu William B. PeatmanZ
Volume 51. Number 3, March 1974
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