Question

With respect to the brown ring test for nitrates, explain:

1. Freshly prepared ferrous sulphate is used.
2. Brown ring disappears if the test tube is disturbed.
3. Lead nitrate does not respond properly to the brown ring test. 
   Hint: Salt solutions forming insoluble sulphates interfere with the result of the Brown ring test.
4. What is the name and formula of the brown ring formed?

Answer 1

(a) Ferrous sulphate solution is freshly prepared because Fe²⁺ ions are easily oxidised to Fe³⁺ ions on standing. The brown ring test requires Fe²⁺ ions to form the characteristic nitrosyl complex.

(b) The brown ring forms specifically at the junction of the ferrous sulphate solution and concentrated sulphuric acid. If the test tube is shaken or disturbed, the layers mix and the distinct brown ring disappears.

(c) Lead nitrate forms insoluble lead sulphate (PbSO₄) with concentrated sulphuric acid. This precipitate interferes with the formation and observation of the brown ring.

\[\ce{Pb(NO3)2 + H2SO4 -> PbSO4 + 2HNO3}\]

(d) The brown ring is nitrosyl ferrous sulphate.

[Fe(H₂O)₅(NO)]SO₄