How is the yellow colour of nitric acid removed?

Question

Nitric acid obtained in laboratory is slightly yellow due to the dissolution of nitrogen dioxide which is produced due to thermal decomposition of a portion of $\ce{HNO3}$.

$$ \ce {4HNO3 -> 2H2O + 4NO2 + O2}$$

Its says here that if $\ce{CO2}$ is bubbled through the acid, it turns colourless- because it drives out $\ce{NO2}$ from warm acid which is further oxidized to nitric acid.

What does it mean by drives out ? How can carbon dioxide drive out nitrogen dioxide that's dissolved in the acid? And also, the oxidation of 'what' is being talked about here?

Answer 1

I think you can explain this using the equilibrium:

$\ce{NO_2(g) <=> NO_2(dissolved)}$

When you pass $CO_2$ through the warm acid, the partial pressure of $CO_2$ ($P_{CO_2}$) above the gas increases (Since partial pressure generally represents the amount of gas above the solution).

Since

1. total pressure $P = P_{NO_2} + P_{CO_2}$ (Dalton's Law)

2. And $P$ remains constant at atmospheric pressure (assuming that the system is not closed).

$P_{NO_2}$ will decrease.

According to Le Chatelier's Principle, the equilibrium proceeds backward to compensate the decrease in $P_{NO_2}$ and thus more $NO_2$ gas is released. The gas flows quickly along with the bubbled gas and this prevents the forward reaction.

This seems like quite a roundabout method but it works. I'm sure there could be a much simpler explanation though.

The oxidation mentioned is most probably the disproportion reaction of $NO_2$ when it comes in contact with water:

$\ce{3NO_2 + H_2O -> 2HNO_3 + NO}$