LECTURE. DEMONSTRATION Of the ARC PROCESS PAUL ARTHUR
AND
CECIL V. KING
Washington Square College, New York University. New York City
0
NE of the most important commercial methods for the fixation of atmospheric nitrogen is the arc process. Air is circulated through an arc between carbon electrodes and a part of the nitrogen and oxygen combine to form nitric oxide.
By passing air through and around an arc between iron electrodes we have been able to improve the demonstration to a point where the formation of nitrogen teboxide, or nitric and nitrous acids, is plainly visible even to a large class in a few seconds after the arc is struck. The apparatus (shown in the diagram) is NI 0% energy c;2NO inexpensive, is very easy to make and is highly satisThe nitric oxide, when cool, unites with more oxygen factory. TWOsmall iron rods, inserted through holes in cork stoppers, are mounted in a glass tube 2 inches 2NO Oe ti2N02 (FtNzOl) (2) in diameter and 8 inches long; one rod.sbould slide and the nitrogen tetroxide dissolves in water to form easily and is fitted with a cork or wooden handle, various products according to the temperature of the wires are attached to the rods by means of spring water. clamps and to the 110-volt D.C. line through a 10- to 202NOz f HBO--+HNOi + HNOl ohm resistance (an electric heater coil serves admirably). HnO +2HNOa + NO 3NO. (3) The arc is struck o2+ 4NOn H1O +4HNOs by touching the The lecture-table demonstration for this reaction as e l e c trodes and usually described involves passing a discharge from an ID-zo ohm quickly drawing the induction coil across ends about ' / r inch /lo volt. D.C. 'a spark gap between apart; then air is two wire electrodes in blown or drawn a flask containing air; through the chamthe reaction which has ber, the bent glass occurred is detected tubes serving to by noting the brown pass it through and fumes which appear around the flame. (NOe) or the change If the gases are in color of moistened passed into a large blue litmus paper hung in the flask.' flask with a white background, the brown fumes can be We have found these methods unsatisfactory be- seen plainly even from some distance. ~h~ gases cause of the time required to a ~ c ~ m u l a enough te re- be bubbled through a very weak solution of alkali action products to be seen in a large classroom, unless with or through a starchan exceptionauy large induction coil capable of giving iodide solution. the latter case iodine is liberated a long, .hot spark is available. A hotter and bigger by the nitrous acid formed as the ~ 0 dissolves % in h m e is desirable, because reaction (1) is slow below the cold solution. For small classes the gases can .20oo0C. and produces little nitric oxide even a t this be bubbled through a little waterfor a few temperature. The following tabulation' shows the and the y,rom testfor nitrates demonstrated, equilibrium data for the reaction: Carbon electrodes, although used commerciallv, cannot be used in this apparitus because too much 2827 2927 3927 1721 2227 IOC. 1227 3.2 4.2. 10.0 of the available oxygen is converted into carbon di0.6 1.8 0.1 oxide. The oxide should be cleaned from the t i* ~ of s' H . F. DAVISON. "Collection of chemical lecture experiments," the iron electrodes after each run to enable easy striking Chemical Catalog Co., New York City, 1926, p. 108. KUEY, J. CHEM.EDUC.,7, 2167 (1930); SIMERrn BROCK, of the arc for the next run. If desired, the tube containing the electrodes may be ibid., 7, 2169 (1930); WILLIAMS, ibid., 8, 462 (1931); DOANE, i6id., 9, 1113 (1932); OLSSON,;bid.! 9,1829 (1932); TAIESSEN, mounted in spring brass clamps on a small board or iM.,10,497 (1933); HAZLEXURST, rhd., 10, 639 (1933). AND HILDEBRAND, book in inorganic suitable stand. It should, however, be easily removable chemistry," The Macmillan Co., New York City, 1929, p. 162. for cleaning.
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