OXIDATION of AMMONIA to NITRIC ACIDA DEMONSTRATION ARTHUR HAUT Grover Cleveland High School, New York City
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oxygen is passed through a length of brass or copper pipe or tubing heated strongly to a red heat near the outlet end, the catalyst being supplied by surface oxidation of the pipe. The length of the brass pipe is not imin portant. Pipes 1' to 2'12' in length and 1/2" to $",/ diimeter were used. The thickness of the walls should be about I mm.
in their textbooks and hence the logical thing from their viewpoint is to expect to see nitrogen peroxide at some point in the demonstration. 2. Elimination of electrical heating. The author believes that the introduction of electrical heating unnecessarily complicates the apparatus. The pupil's attention is withdrawn from the important chemical fact and centered on something he usually does not understand a t this time anyway. 3 . A good test for a nitrate. The product must give a test for a nitrate which the pupil knows. The brown ring test is the one with which such pupils are familiar, and any other advanced tests such as the diphenylamine or nitron test would not be good pedagogy. With these points in mind it was found that a highly satisfactory oxidation of ammonia to nitrogen peroxide could be made using copper oxide as the catalyst in the apparatus shown in Figure 1. A mixture of ammonia and
The important thing is to heat the pipe very strongly. This can be done with a good burner shielded from drafts. The reaction was found to proceed best when both the inside and outside of the pipe glowed from the heat. The oxygen was produced by merely heating a mixture of potassium chlorate and manganese dioxide, while the ammonia was generated by dropping concentrated ammonium hydroxide very slowly on a few sticks of caustic soda. In this manner a steady stream of ammonia gas is formed, demanding no attention on the part of the demonstrator once it has started. In a very short time the flask which should be of a t least 500-cc. capacity will show the brown color of nitrogen peroxide. Add about 25 cc. of water and shake to dissolve the gas. A piece of blue litmus paper will show the presence of an acid. To obtain a good test for a nitrate, the nitrite ion should first be eliminated. This may be done by oxidizing it with a dilute solution of potassium permanganate or some hydrogen peroxide. The brown ring test is then obtained very easily. Should the instructor object to oxidizing the nitrite ion, an alternate method is to destroy this ion by boiling the solution of nitrogen peroxide with dilute sulfuric acid, cooling, and then applying the brown ring test. Similar results may be obtained by substituting a glass tube for the metal tube and inserting a six-inch strip of copper gauze into the tube. The copper gauze should be folded so that as much surface as possible is
A method of transforming ammonia to nitrogen peroxide and nitric acid with copper oxide as the catalyst is described below.
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N THE October, 1933, issue of the JOURNAL OF CHEMICALEDUCATION Hazlehurst described an apparatus to demonstrate the catalytic oxidation of ammonia, using platinum as the catalyst. Hazlehurst's apparatus has the advantage of very nearly duplicating the actual commercial process. In teaching ammonia oxidation to a class of secondary-school pupils, especially where these happen to be as low as the second year, the author of the present paper set about to iind a means of demonstrating the oxidation of ammonia to nitric acid, satisfvine the followinn . points: 1 . The formation of Gsible b r m nitrogen peroxide. This does not seem to show when platinum is the catalyst because the oxidation in the presence of platinum proceeds largely to the direct formation of nitric acid. In teaching commercial processes for manufacturing nitric acid the arc process involving the oxides of nitrogen has usually already been shown. The pupils are therefore familiar with nitrogen peroxide as the forerunner of nitric acid. They see the equations :
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4NHa 502 +4 N 0 6H2O -+2NO2 2 N 0 0%
exposed to the mixed gases. The glass tube is heated a t the point where the gauze is placed, until the gauze starts glowing. When this is done the copper is, of course, oxidized to copper oxide which is really the catalyst. Better results seemed to be obtained with the metal tube, probably because a higher temperature is more easily reached. Explosions in the tube are rare though not unknown. They are harmless because the tube is open. When a platinum wire spiral is substituted for the
copper gauze it glows when the gases pass over it even after the flame is removed. Explosions with the platinum are much more frequent than with the copper oxide but they are also harmless. A good nitrate test may be obtained but again nitrites must be eliminated. A new piece of platinum wire does not give good results immediately. It will do so only after having been exposed to the mixed gases for several minutes. In this manner it becomes activated and changes in appearance to a dull gray with what looks like a pitted surface.
