Which of the following compound is added to the sodium extract before addition of silver nitrate for testing of…

Step 1: Understand the Purpose of the Pre-Treatment

In Lassaigne's test, after sodium fusion, the extract may contain:

• $\ce{NaCN}$ (if N present)
• $\ce{Na2S}$ (if S present)
• $\ce{NaX}$ (if halogen present, X = Cl, Br, I)

To test for halogens, $\ce{AgNO3}$ is added. The problem is that $\ce{CN^-}$ and $\ce{S^{2-}}$ ions would give interfering precipitates:

• $\ce{Ag^+ + CN^- -> AgCN (white ppt)}$ — looks identical to $\ce{AgCl}$
• $\ce{2Ag^+ + S^{2-} -> Ag2S (black ppt)}$ — masks any halide precipitate

Step 2: Why Acidify with Nitric Acid?

Adding dilute $\ce{HNO3}$ (and boiling) decomposes $\ce{NaCN}$ and $\ce{Na2S}$: $\ce{NaCN + HNO3 -> HCN(g) + NaNO3}$ $\ce{Na2S + 2HNO3 -> H2S(g) + 2NaNO3}$

Both $\ce{HCN}$ and $\ce{H2S}$ escape as gases. Only $\ce{NaX}$ remains, and: $\ce{NaX + AgNO3 -> AgX (ppt) + NaNO3}$

Step 3: Evaluate Each Option

| Option | Assessment | |---|---| | (a) $\ce{NaOH}$ | Would make solution alkaline — $\ce{AgOH}$ brown ppt would form, not halide test | | (b) $\ce{HCl}$ | Would introduce $\ce{Cl^-}$ → $\ce{AgCl}$ precipitate regardless of halogen test → Wrong choice | | (c) $\ce{HNO3}$ ✅ | Decomposes $\ce{CN^-}$ and $\ce{S^{2-}}$; doesn't interfere with $\ce{AgNO3}$ halide test | | (d) $\ce{NH3}$ | Would dissolve $\ce{AgCl}$ and $\ce{AgBr}$ — would destroy the precipitate |

Answer: (c) Nitric acid

Key Points to Remember:

• Never add HCl before AgNO₃ — it introduces Cl⁻ and invalidates the test!
• HNO₃ is the ONLY acid that works here: it decomposes interfering ions AND doesn't introduce new interfering ions
• Boil with HNO₃ to completely expel HCN and H₂S gases
• $\ce{NH3}$ is added AFTER the precipitate forms (to distinguish AgCl, AgBr, AgI)