Question

The brown ring test is performed for the qualitative detection of ______.
(a) bromides
(b) iodides
(c) nitrates
(d) phosphates.

Answer 1

Hint: One of the reagents in the brown ring test is iron(II) sulphate solution and sulphuric acid. If the test is positive, we get a brown ring in the solution.

Complete step by step solution:
> Brown ring test is a qualitative test. In this test when ferrous sulphate and concentrated sulphuric acid is added a brown ring is formed. This is not shown by bromides, iodides and phosphates. It is a test to confirm the presence of nitrate groups. This test is used for qualitative analysis of nitrates.
> Most of the nitrates dissolve in water and therefore its presence is determined by wet chemistry. This is not as easy compared to qualitative analysis of other anions. Many tests of nitrate depend on its oxidising behaviours.
The procedures for brown ring test are:
-To the nitrate solution, ferrous sulphate is added.
-Now slowly to the above solution, concentrated sulphuric acid is added through the sides of the test tube. This will lead to the formation of an acid layer below which there is an aqueous layer.
In between these layers or at the junction of these two layers a brown colour ring is formed. This is why the test is called the brown ring test. His indicates the presence of nitrate.
The reactions are as follows:
\[NO_{3}^{-}+3F{{e}^{2+}}+4{{H}^{+}}\to NO+3F{{e}^{3+}}+2{{H}_{2}}O\]
\[{{[Fe({{H}_{2}}O)]}^{2+}}+NO\to \underset{brownring}{\mathop{{{[Fe{{({{H}_{2}}O)}_{5}}NO]}^{2+}}}}\,+{{H}_{2}}O\]
When ferrous sulphate solution is added, nitrate ion oxidised iron(II) to iron(III). It forms a complex when sulphuric acid is added. The brown ring is the complex \[{{[Fe{{({{H}_{2}}O)}_{5}}NO]}^{2+}}\].
Thus, the correct option is C.

Note: Only concentrated sulphuric acid should be added in brown ring test. This is because it is denser than the nitrate and iron(II) sulphate solution. So, would help in the formation of two layers and the brown ring in at the junction of the two layers.