Question

Describe the brown ring test for nitric acid.

Answer 1

• The brown ring test also known as nitrate test is a chemical procedure for determining the presence of nitrate ions(${\mathrm{NO}}_3-$) in a sample.
• It's a standard qualitative test for confirming the presence of nitrate ions in any solution that results in the creation of a brown-colored ring.
• The addition of ferrous sulphate(${\mathrm{FeSO}}_4$) to the supplied solution, which contains nitrate ions, results in the formation of a brown ring, which is further acidified by adding strong sulphuric acid(${\mathrm{H}}_2{\mathrm{SO}}_4$) to the entire mixture.
• The brown ring usually appears at the intersection of sulphuric acid and ferrous sulphate.
• This test is a reduction reaction as Ferrous (II) reduces the nitrate to nitric oxide($\mathrm{NO}$), which is then oxidized to ferrous oxide (III).
• The nitrosyl complex($\left[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_5\mathrm{NO}\right]{\mathrm{SO}}_4$) is generated when the nitric oxide formed reacts with the ferrous (II) left behind.
• Chemical equations involved in brown ring test are as follows:

$\underset{\mathrm{Nitric}\mathrm{Acid}}{2{\mathrm{HNO}}_3}+\underset{\mathrm{Sulphuric}\mathrm{Acid}}{3{\mathrm{H}}_2{\mathrm{SO}}_4}+\underset{\mathrm{Iron}\mathrm{II}\mathrm{Sulphate}}{6{\mathrm{FeSO}}_4}\to \underset{\mathrm{Iron}\mathrm{III}\mathrm{Sulphate}}{3{\mathrm{Fe}}_2{\left({\mathrm{SO}}_4\right)}_3}+\underset{\mathrm{Nitric}\mathrm{Oxide}}{2\mathrm{NO}}+\underset{\mathrm{Water}}{4{\mathrm{H}}_2\mathrm{O}}$

$\underset{\mathrm{Hexa}\mathrm{Aqua}\mathrm{Iron}\mathrm{II}\mathrm{Sulphate}}{\left[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]{\mathrm{SO}}_4}+\underset{\mathrm{Nitric}\mathrm{Oxide}}{\mathrm{NO}}\to \underset{\mathrm{Nitrosyl}\mathrm{Complex}}{\left[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_5\mathrm{NO}\right]{\mathrm{SO}}_4}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}}$