Question

Anil is a laboratory assistant in a college. He prepared nitric acid but stored it in a transparent container exposed to sunlight. After a few days, brown fumes were observed in the container and the acid turned slightly yellowish in colour. 

(a) What was the error made by Anil while storing the acid? 
(b) Why were brown fumes observed in the container? 
(c) How can the yellowish tinge in the acid be removed? 
 

Hint:

Nitric acid is a powerful oxidizing agent. Its yellow color is purely due to dissolved NO_2 impurity; pure nitric acid is actually colorless!

Answer 1

(a) Storage Error: Anil should have stored the nitric acid in an amber-colored (dark) bottle and kept it in a cool, dark place away from sunlight.

(b) Thermal Decomposition: Nitric acid is unstable in the presence of heat or sunlight. It decomposes to give nitrogen dioxide (NO_2), which appears as brown fumes and dissolves in the remaining acid to give it a yellow tint. [ 4HNO_3 xrightarrowtextsunlight 4NO_2 uparrow + O_2 uparrow + 2H_2O ] 

(c) Removal of Yellow Tinge: The yellow color can be removed by bubbling dry air or carbon dioxide through the acid (to drive out the dissolved NO_2) or by adding a small amount of water.