Question

Which of the following ions is responsible for the brown colour in the ring test for nitrate?

• A. $[\mathrm{Fe}{\left({\mathrm{NO}}_2\right)}_6{]}^{4-}$
• B. ${\left[\mathrm{Fe}{\left({\mathrm{H}}_2\right)}_5{\mathrm{NO}}_2\right]}^{+}$
• C. ${\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_5{\mathrm{NO}}_2\right]}^{-}$
• D. $[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_5\mathrm{NO}{]}^{2+}$

Answer 1

The correct option is D

$[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_5\mathrm{NO}{]}^{2+}$

Explanation for the correct option:

(D) $[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_5\mathrm{NO}{]}^{2+}$

Brown ring test:

• A brown ring test is performed to detect the nitrate ion $\left({\mathrm{NO}}_3^{-}\right)$ in the solution.
• The procedure of the test:
• Add iron (II) sulfate $\left({\mathrm{FeSO}}_4\right)$ to the nitrate solution followed by slow addition of concentrated sulfuric acid$\left({\mathrm{H}}_2{\mathrm{SO}}_4\right)$, it produces Ferric sulfate$\left({\mathrm{Fe}}_2{\left({\mathrm{SO}}_4\right)}_3\right)$, Nitric oxide$\left(\mathrm{NO}\right)$, and water. The Hexa aqua iron sulfate (II) $\left(\left[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]{\mathrm{SO}}_4\right)$ reacts with nitric oxide to produce a nitrosyl complex of iron.
• Two layers are formed and a brown ring at the junction of the two layers is formed if a nitrate ion is present in the solution.
• The chemical reactions can be depicted as:

$2{\mathrm{HNO}}_3\left(\mathrm{aq}\right)+6{\mathrm{FeSO}}_4\left(\mathrm{aq}\right)+3{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)\to 3{\mathrm{Fe}}_2{\left({\mathrm{SO}}_4\right)}_3\left(\mathrm{aq}\right)+2\mathrm{NO}\left(\mathrm{aq}\right)+4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

$\left[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]{\mathrm{SO}}_4\left(\mathrm{aq}\right)+\mathrm{NO}\left(\mathrm{aq}\right)\to \left[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_5\mathrm{NO}\right]{\mathrm{SO}}_4\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

• Hence, $[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_5\mathrm{NO}{]}^{2+}$of nitrosyl complex is responsible for the brown color in the ring test for nitrate.

Explanation for the incorrect options:

The brown ring is formed due to the formation of $[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_5\mathrm{NO}{]}^{2+}$ complex. Thus, options A, B, and C are incorrect.

Hence, the correct option is D i.e. $[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_5\mathrm{NO}{]}^{2+}$.