A DEMONSTRATION: FIXATION OF ATMOSPHERIC NITROGEN* HARRY L. OLSSON,YORKCOMMUNITY HIGHSCHOOL, ELMHURST, ILLINOIS Time need not be a factor in gaining educational objectives. The demonstration of nitrogen fixation by use of the electric arc does not require long-drawn-out projects with elaborate set-ups. High-school students can acquire an intelligent understanding of the process itself, as well as the reactions that take place, by observing a very simple set-up requiring only a minimum amount of time and apparatus. The arc process lends itself best t o demonstration since i t is the simplest of the nitrogen fixation processes and was the first to be commercially developed. Some demonstrations of this process require specially blown glass bulbs' and furnaces2 which are not usually a t hand in a high-school laboratory. For a number of years the demonstration here described has been used a t York, and i t never fails to catch the fancy and interest of the students in connection with the study of the oxides of nitrogen. The fact that such important chemical compounds as nitric oxide, nitrogen dioxide, and nitric acid may be made from mere air and water is of significance to a highschool student. The only apparatus needed is an ordinary induction spark coil, copper wire electrodes, Florence flask (Pyrex), test tube, aspirator, and six new dry cells. Combustion Chamber.-The use of a 500-cc. Florence flask (Pyrex) permits visibility. The flask may be easily converted into a combustion bulb by the use of a common blowpipe. Impinge the flame on a spot on the side of the flask. When soft, blow through 9011th of flask. The glass will give away a t the heated spot and a small opening will be made which can be enlarged by bending out edges, while soft, with a carbon rod or round file. Make the hole large enough to receive a No. 1 cork having a diameter of five-sixteenths of an inch a t the small end. Blow another hole diametrically opposite in the same manner. Into the mouth of the flask fit a two-holed stopper with two glass tubes, one with a right-angle bend leading to the test tube, and one with the end curved as in the figure. The tube with curved end should be so adjusted that the curve opens downward between the two electrodes. The Electrodes.-If platinum wire is not available use copper wire. Make a small loop on the inner end to help spread the arc. With a sharp knife, cut half way through the cork lengthwise, and insert the electrodes in the incisions. Adjust the electrodes in the flask so that there is between them a gap of a little more than one-half inch. 'Presented before the Chemical Education Group of the Chicago Section of the American Chemical Society, Chicago, Illinois, February 20, 1931. 1 D ~ S M O R''Laboratory E, Manual of Chemistry." Laurel1 Book Co., New York City, 1925, p. 81. 2 WILLIAMS. "Fertilizer from Air." J. CHEM.EDUC., 8, 463 (Mar., 1931). 1829
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Spark Coil.-An ordinary induction spark coil may be used. A Ford coil will serve the purpose though i t presents some difficulty in attaching terminals. As a source of current use six new dry cells in series. It is important that each cell be new and give its maximum amperage. Adjust the coil and electrodes until a continuous spark with a reddish flame follows through the blue spark across the gap. A thin, intermittent, or even a yellow, continuous spark, does not work well regardless of the capacity of the coil. The intense heat of the arc causes the nitrogen and oxygen of the air in the flask to combine forming NO. The nitric oxide thus formed reacts with the oxygen of the air forming brown fumes of NOz. The current should be turned on and off intermittently to allow the gases to cool down. These intervals should be of one minute duration. Continue thus for
three or four minutes. When the flask bas filled with brown fumes, further study of the process may be made by drawing the fumes into the water in the test tube by means of the aspirator. As the NO2is drawn from the flask more air enters through the curved glass tube and passes through the arc. The reaction of the NO2 with the water in the tube will soon give an acid test with litmus, and if the action is continued long enough there will be enough nitrate ions formed to give the brown ring test, using fresh FeS04 and concentrated H2S04. The reactions that probably take place are the common and well-known ones usually given in most elementary texts:a
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N B 01+2NO On-+2N01 2NO HIO +2HNOs NO. 3NOp D m s ~ o n e ,"Chemistry for Secondary Schools." L a u d Book %o.. New York City, 1925, p. 240.
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