DEMONSTRATING THE FIXATION OF NITROGEN SAVER10 ZUFFANTI Northeastern University. Boston, Massachusetts
I
N THE teaching of the fixation of nitrogen it is very desirable to employ an effective demonstration of one of the methods used in this industry. A simple experiment will motivate the student and give him a better understanding of the principles involved. The arc process lends itself very readily to classroom work, the fundamental equations leading to the formation of nitrogen peroxide and its absorption in water to form nitric acid being easily explained as follows:
+ +
-
NS OP 2N0 O2 2NOn 2NO 2HNOs NO 3NOr HSO
+
+
If a dilute sodium hydroxide solution is used in place of the water, for the purpose of absorbing the oxide of nitrogen, the formation of the nitric acid can be made more evident. A drop of phenolphthalein imparts to the caustic soda solution a red color, which will shift to colorless when enough nitric acid has been formed to neutralize the alkali present: NaOH
+ HNOl-
NaNOs
+ HsO
This experiment. which is efficient and easily constructed, and which requires no special apparatus or glass-blowing, may also be used in connection with the study of acids, bases, and neutralization.
APPARATUS
The apparatus is set up as illustrated in the accompanying drawing. The absorption chamber consists of a ten-inch test-tube fitted with a rubber stopper and two bent pieces of glass tubing; the reaction chamber consists of a Y-tube and two stout pieces of copper wire. Four to eight dry cells, connected in series, are attached to an old Ford coil, which serves as an induction coil and boosts the voltage to several thousand volts. The two wires from the secondary are introduced into the two arms of the Y-tube and act as the electrodes, supplying a good I/," arc. The advantage of using a glass Y-tube is that it makes the arc visible to the class. Best results are obtained by sucking a rapid stream of air through the apparatus. OPERATION
When the apparatus has been arranged, a solution of sodium hydroxide, made up by diluting 5 ml. of a 0.01 N sodium hydroxide solution to 50 ml., is colored with phenolphthalein and introduced into the test-tube and the suction is applied. When performed without the use of an arc, the experiment produces no color change and serves to prove that the following changes are not due to the carbon dioxide or other gases in the atmosphere. The air is sucked through the solution for a period of ten minutes. The experiment is repeated with a second test-tube containing an equal portion of sodium hydroxide solution of the same strength. The arc being used this time, the red alkaline solution turns colorless in approximately ten minutes, demonstrating effectivelythat nitric acid has been formed from the oxygen and the nitrogen of the air. The entire demonstration can be performed in a half hour. The instructor should perform this experiment several times before attempting to demonstrate i t t o the class. The size and intensity of the arc, slight variations in the concentration of the caustic solution, and differences in the rate of flow of the air will affect the time required for the color change. I have, however, consistently obtained the above results under the conditions that I describe.
