Question

In brown ring test, the brown colour of the ring is due to:
(A) a mixture to NO and $\text{N}{{\text{O}}_{\text{2}}}$
(B) nitroso ferrous sulphate
(C) ferrous nitrate
(D) ferric nitrate

Answer 1

Hint: Think about inorganic qualitative analysis of anions. Identify which anion gives brown ring test. Determine the reagents used in the brown ring test and refer the method used to perform this test. Write the reaction to get the answer.


Complete step by step solution:
- Inorganic qualitative analysis is the method used to identify the cations and anions given in an inorganic salt mixture. It is classified into two categories namely, tests for cations and tests for anions.
- Tests for cations are further classified into dry tests and wet tests. Similarly, tests for anions are also classified into dry tests and wet tests.
- The brown ring test is a wet test, in tests for anions, used to determine nitrate, $NO_{3}^{-}$ ion.
- In brown ring test, first a saturated solution of ferrous sulphate or ferrous ammonium sulphate is prepared in a test-tube. In another test-tube, an aqueous solution of nitrate ions that is, a solution of salt containing nitrate, is prepared. Now, equal amounts of both the solutions are mixed to form one homogenous solution. This solution is boiled and cooled. Once it is cooled completely, using a dropper or Pasteur pipette, concentrated sulphuric acid is added along the sides of the test-tube very carefully without disturbing the solution.
- A brown ring is obtained at the junction of the layers of the solution and sulphuric acid which is known as a brown ring which confirms the presence of nitrate ions in the solution.
- The reaction is given as follows:
$NO_{3}^{-}+3F{{e}^{2+}}+4{{H}^{+}}\to NO+3F{{e}^{3+}}+2{{H}_{2}}O$
\[\left[ Fe{{\left( {{H}_{2}}O \right)}_{6}} \right]S{{O}_{4}}+NO\to \underset{\text{Brown ring complex}}{\mathop{\left[ Fe{{\left( {{H}_{2}}O \right)}_{5}}NO \right]S{{O}_{4}}}}\,+{{H}_{2}}O\]
- Therefore, the brown ring is due to the formation of Pentaaqua Nitroso Iron(II) sulphate complex. It is also known as ferrous nitroso sulphate or nitroso ferrous sulphate complex in short.


- Therefore, answer is option (B).


Note: Brown ring test is one of the most important reactions in inorganic qualitative analysis of anions. Brown ring test confirms the presence of nitrate ion in the given salt mixture. Remember the brown colour of the ring is due to nitroso ferrous sulphate complex.