Laboratory preparation of dinitrogen
The collection of nitrogen $(\ce{N2})$ by downward displacement of water works since nitrogen is poorly soluble in water (at room temperature and standard pressure) and does not chemically react with water.

For the same reason, some impurities (aerosols, water-soluble gases and vapours – for example $\ce{HNO3}$) can be removed by passing the stream of nitrogen through a washing bottle that is filled with water.

Nitric oxide $(\ce{NO})$ is only poorly soluble in water and therefore cannot be removed by this method. However, $\ce{NO}$ can be oxidized. Strong oxidizing agents can convert $\ce{NO}$ into $\ce{NO3-}$:

$$\ce{NO + 2H2O &lt;=&gt; NO3- + 4H+ + 3e-}\quad E^\circ = +0.959\ \mathrm V$$

A suitable strong oxidizing agent is acidic dichromate $(\ce{Cr2O7^2-})$:

$$\ce{2Cr^3+ + 7H2O &lt;=&gt; Cr2O7^2- +14H+ + 6e-}\quad E^\circ = +1.38\ \mathrm V$$

The balanced overall reaction is:

$$\ce{2NO + Cr2O7^2- +6H+ &lt;=&gt; 2NO3- + 2Cr^3+ + 3H2O}$$