Question

Why nitric acid is added in the Lassaigne’s test for halogens?

Answer 1

1. if N (or) S is present in the organic compound along with the halogen, we might obtain Na₂S and NaCN in the solution which interfere with the detection of the halogen in the AgNO₃ test.

2. Therefore we boil the Lassiagne’s extract with HNO₃ which decomposes NaCN and Na₂S as follows:

NaCN + HNO₃ \(\overset{\Delta}{\rightarrow}\) NaNO₃ + HCN

Na₂S + 2HNO₃ \(\rightarrow\) 2NaNO₃ + H₂S↑