Which of the following compound is added to the sodium extract before addition of silver nitrate for testing of halogens?
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Remember the correct acid for Lassaigne's test. You can't use HCl because it contains Cl⁻ ions, which would give a false positive test for chlorine. You can't use H₂SO₄ because it might cause precipitation of silver sulfate. Only HNO₃ is suitable as it doesn't add interfering ions.
In the Lassaigne's test for halogens, the organic compound is fused with sodium metal to convert covalently bonded halogens into ionic sodium halides (NaX).
The sodium extract (Lassaigne's extract) may also contain sodium cyanide (NaCN) and sodium sulfide (Na$_2$S) if the original organic compound contained nitrogen and sulfur, respectively.
Before adding silver nitrate (AgNO$_3$) to test for halide ions, it is essential to decompose any NaCN and Na$_2$S present.
If not removed, these ions would react with AgNO$_3$ to form precipitates of AgCN (white) and Ag$_2$S (black), which would interfere with the identification of the halide precipitate (AgCl, AgBr, AgI).
Concentrated nitric acid (HNO$_3$) is added for this purpose. It acidifies the solution and decomposes NaCN and Na$_2$S into gases.
Na$_2$S + 2HNO$_3$ $\rightarrow$ 2NaNO$_3$ + H$_2$S(g)
NaCN + HNO$_3$ $\rightarrow$ NaNO$_3$ + HCN(g)
After boiling off these gases, the solution is free from interfering ions, and AgNO$_3$ can be added to test specifically for halogens.
