What is the brown ring in a brown ring test?

Brown ring test

• The presence of nitrate ions in a sample is detected using the brown ring test.
• The test is commonly performed by mixing a dilute ferrous sulfate solution with an aqueous solution containing nitrate ion, then carefully pouring strong sulphuric acid along the sides of the test tube.
• The presence of nitrate ions in the solution is shown by a brown ring at the interface between the solution and the sulphuric acid layers.

Reaction for this test

• The reaction is as follows:-

$$\begin{gathered} {\mathrm{NO}}_3^{-}+3{\mathrm{Fe}}^{2+}+4{\mathrm{H}}^{+}\to \mathrm{NO}+3{\mathrm{Fe}}^{3+}+2{\mathrm{H}}_2\mathrm{O} \\ {\left[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}+\mathrm{NO}\to {\left[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_5\mathrm{NO}\right]}^{2+}+{\mathrm{H}}_2\mathrm{O} \\ \mathrm{brown} \end{gathered}$$