Question

Give the chemical name and formula of the substance formed as a brown ring in the test for nitrate radical.

Answer 1

Brown ring in the test for nitrate radical:

• The presence of Nitrate can be detected from the Brown ring test.
• To check the presence of Nitrate, take a sample in which you suspect Nitrate is present in a test tube and mix freshly prepared Ferrous sulphate solution in the sample then add concentrated Sulphuric acid solution along the side to the bottom of the test tube.
• If nitrate is present in that sample, then a brown ring will form at the junction of two layers (one layer of a mixture of Ferrous sulphate solution + sample solution and the second layer of concentrated Sulphuric acid solution).

$$\begin{gathered} \underset{\mathrm{Nitric}\mathrm{acid}}{2\mathrm{H}{\mathrm{NO}}_3\left(\mathrm{aq}\right)}+\underset{\mathrm{Sulphuric}\mathrm{acid}}{3{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)}+\underset{\mathrm{Ferrous}\mathrm{sulphate}}{{\mathrm{FeSO}}_4\left(\mathrm{aq}\right)}\to \underset{\mathrm{Ferric}\mathrm{sulphate}}{3{\mathrm{Fe}}_2{\left({\mathrm{SO}}_4\right)}_3\left(\mathrm{aq}\right)}+\underset{\mathrm{Nitric}\mathrm{oxide}}{\mathrm{NO}\left(\mathrm{g}\right)}+\underset{\mathrm{Water}}{4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)} \\ \underset{\mathrm{Hexa}\mathrm{aqua}\mathrm{Iron}\mathrm{sulphate}\left(\mathrm{II}\right)}{\left[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]{\mathrm{SO}}_4\left(\mathrm{aq}\right)}+\underset{\mathrm{Nitric}\mathrm{oxide}}{\mathrm{NO}\left(\mathrm{g}\right)}\to \underset{\mathrm{Penta}\mathrm{aquanitrosylium}\mathrm{iron}\left(\mathrm{I}\right)\mathrm{sulphate}(\mathrm{Brown}\mathrm{ring})}{\left[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_5\mathrm{NO}\right]{\mathrm{SO}}_4\left(\mathrm{aq}\right)}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)} \end{gathered}$$

• The Penta aqua nitrosyl iron sulphate (Brown ring) confirms the presence of Nitrate in the sample.

Therefore, the chemical name and formula of the substance that formed a brown ring in the test for nitrate radical is Penta aqua nitrosyl iron sulphate and the chemical formula is $\left[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_5\mathrm{NO}\right]S{O}_{\mathit{4}}$.