JOURNAL OF CHEMICAL EDUCATION
442
THE EFFECT OF NITROUS ACID ON THE OXIDIZING POWER OF NITRIC ACID
0
A Lecture Demonstration
ROBERT C. BRASTED University of Minnesota, Minneapolis, Minnesota
T o MOST students in general chemistry nitriclacid typifies the oxidizing agent. The following lecture experiments will dispel this idea and demonstrate, as Feigll has shown, that the oxidizing action of nitric acid may he considerably enhanced by small quantities of nitrous acid which are usually present. PROCEDURE
Deeolorizing HNO,. Place portions of concentrated HN03 in each of two beakers (beakers 1and 2). Point out that the yellowish color is due to traces of NO1 in the acid, which also contains NO and HN02. Some reactions are heat, sunlight 4HNOa ------+ 280 On 4N01 cold 2NO9 H1O -+HNOs HNOs hot 3 N 0 ~ H1O +2HNOs NO 2HN0, +HnO NOn NO
+ +
+ +
+
+
+
+
Add about 0.5 g. of sulfate-free sulfamic acid to the acid in heaker 2; in half a minute the HN03 becomes
-
colorless as the nitrous acid is removedz by the reaction HNO,
+ HSOsNH,
N,
+ H,S04 + H,O
Effect of HNOa on dilute iodides. Stirring constantly, run in a few ml. of the yellowish acid from heaker 1 into a starch-iodide solution in beaker 3. Immediately run an equal volume of the colorless liquid from beaker 2 into a similar starch-iodide solution in heaker 4. The instantaneous appearance of starch-iodine blue in heaker 3, in contrast to its slow formation in beaker 4, shows the catalytic effect of the HNOz which mas in beaker 1. This idea is substantiated by adding a crystal of NaN02 to the mixture in beaker 4, resulting in immediate blue coloration. Oxidation with concentrated H N 0 3 . Finally, drop crystals of X I into heakers 1 and 2. Vigorous reaction in both cases, with liberation of iodine, demonstrates that concentrated acid, even in the absence of HN02in heaker 2, will oxidize the iodide ion to free iodine. Other methods of decolorizing the HNOa are: (a) Boil the acid and coal; this is slower than the sulfamic acid methad. ( b ) Add urea-but the urea $80 interacts with the HNOa. Sulfamic acid will not interact with the HNOannlessit is hat or its coneentratian is above 73 per cent. (c) Add sodium aaide; but take care, since the aaides are poisonous and certain of them me explosive.
