Harold W. Kohn Dickinson College Carlisle, Pennsylvania 17013
Student Flowmeters and an Air Pollution Experiment
A
qualitative or quantitative observation of hydrocarbon pollutants in the atmosphere can be made by passing a known amount of gas over a suitable filter. It has been observedlJ that porous glass acts as a very efficient scavenger for hydrocarbon pollutants in the atmosphere, especially for unsaturates. The water in porous glass, silica gel, and similar m a t e rials (molecular sieves for example), however, lends considerable complication to later observation of the pollutants. I have found thatglass wool will also act as an efficient absorbent for atmospheric contaminants and does not retain enough water to interfere with further experimentation. The glass wool should first be cleaned with acetone. A long plug about 20 cm long contained in a 10-mm i.d. hard glass tube will act as an efficient filter. After the filter has been dirtied as by pulling air through it with an aspirator, a second plug of glass wool may be inserted in the entry way of the pollution tube to prevent escape of the pollutants during the next step. Upon heating, as with a Meker burner, the discoloration due to hydrocarbon decomposition soon becomes evident. More sophisticated analysis of the trapped hydrocarbon or particulates can be envisioned, but in an elementary course (for nonscience majors) this treatment is adequate. Since one loses some hydrocarbon pollutant to oxidation during the heating step, a more elegant experiment can be done by sealing the tube off under vacuum. One would be well advised to try the experiment in advance. Local air can be cleaner than one would imagine and a rainstorm can clean the air considerably. Also the experiment works poorly on Mondays, after industries and commuter traffic have been shut down for the ~ e e k e n d . ~ In order to have an estimate of air quality some idea of the total flow must be obtained by measuring flow rates. In the absence of a rotometer (which must be calibrated) this can be done with any of the three devices sketched below. The plastic bag (Fig. 1) is a poor device, a near desperation method accurate only to *30y0. One simply measures the time required to empty a plastic bag of known volume. The balloon bottle4 is the most satisfactory of the devices shoxm (Fig. 2). A large balloon should be used so that the aspirator does not have to work against much elasticity.
or rubber hand ~ v b b e r rube
Figure 1.
~ l a s r i cbag
Simple plastic bag flowmeter.
One should be sure that the balloon is completely collapsed a t the start of the experiment. One can usually tell when the balloon has filled the bottle by a change in the tone of the aspirator. The glass w ~ t test meter-like device (Fig. 3) works very well hut only for slow flows such as a few hundred cubic centimeters per minute. One begins with the water level as shown in the diagram, and measures the time required to fill the test tube with air. The valving devices shown may be omitted for elementary n-ork. The experiment and the flommeters can serve to introduce the general subject of flow, flowmeters, and the principles by which they work.
Figure 2.
u
Flowmeter condruded from balloon inside of bottle.
YATaS, D. J. C., - A N D LUCCHES~ P. J., J. Amer. Chem. Soe., 86, 42.58 (1964). YATES,U. J. C., private communication. 'SMITH,RALPH G., SZAJNOR, JOANNE,A N D H ~ C K I ILAWR, RENCE, Ewimn. Sei. Technol., 4, 333 (1970). I have not found the balloon bottle described in the literature. It is also handy for blowing g h s when toxic substances are involved dthough better pressure transducers do exist.
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Volume 49, Number 9, September 1972
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