Question

Concentrated nitric acid, upon long standing, turns yellow brown due to the formation

Answer 1

HELLO THERE!

Let's recall the preparation of Nitric acid in atmosphere:

4NO₂ + O₂ + 2H₂O ----> 4HNO₃

Now, nitrogen oxide gas is reddish brown in colour.

If a Nitric acid bottle is kept un-stoppered, it is in direct contact with air. As a result, the above reaction takes place in reverse, forming nitrogen dioxide gas back again.

This NO₂ gets dissolved in the nitric acid, and since by itself, it is reddish brown, the solution of nitric acid seems yellowish brown.

Method to remove this NO₂ and make the acid colourless:

The NO₂ can be removed by two methods:

=> Bubbling of Carbon dioxide gas through the acid drives out the dissolved NO₂.

=> When excess water is added, the NO₂ present gets dissolved in the water, leaving behind the acid only.

HOPE MY ANSWER IS SATISFACTORY...

Thanks!