Question

Name the following:
A compound which is oxidized by nitric acid in the presence of sulphuric acid and changes its colour from pale green to orange yellow.

Answer 1

• The brown ring test can be used to detect nitrate ions in a solution. The presence of nitrate ions is detected by the production of a brown ring at the junction of two layers of the solution, as the name suggests.
• An aqueous solution of dilute nitric acid $\left({\mathrm{HNO}}_3\right)$ is acidified with conc. ${\mathrm{H}}_2{\mathrm{SO}}_4$ after being treated with an equal volume of freshly made saturated ferrous sulphate solution. Pure concentrated ${\mathrm{H}}_2{\mathrm{SO}}_4$ is poured along the test tube's walls to generate a distinct layer at the bottom. For one minute, the test tube must stay undisturbed. The reduction of nitric acid into nitrogen monoxide $\left(\mathrm{NO}\right)$ by ferrous sulphate $\left({\mathrm{FeSO}}_4\right)$ results in the formation of a dark brown ring at the junction, which then dissolves in the rest of the ferrous sulphate solution, forming a brown ring.
• In brown ring test, ferrous sulphate (greenish in color) is oxidized to ferric sulphate $\left({\mathrm{Fe}}_2{\left({\mathrm{SO}}_4\right)}_3\right)$ (orange-yellow in color).
  $6{\mathrm{FeSO}}_4\left(\mathrm{aq}\right)+3{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)+2{\mathrm{HNO}}_3\left(\mathrm{aq}\right)\to 3{\mathrm{Fe}}_2{\left({\mathrm{SO}}_4\right)}_3\left(\mathrm{aq}\right)+4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+2\mathrm{NO}\left(\mathrm{g}\right)$
• Thus, ferrous sulphate is the compound which is oxidized by nitric acid in the presence of sulphuric acid and changes its colour from pale green to orange yellow.