Identify the test being discussed

This is the Lassaigne's test for halogens in organic compounds. After preparing the sodium extract, the test for halogens is done by adding silver nitrate.

Why is something added before silver nitrate?

The sodium extract may contain interfering ions like cyanide and sulfide if nitrogen or sulfur are present in the organic compound.

These ions also give precipitates with silver nitrate:

• $\text{AgCN}$
• $\text{Ag}_2\text{S}$

Hence, they must be removed before adding $\text{AgNO}_3$.

Role of nitric acid

The sodium extract is first boiled with nitric acid.

Nitric acid decomposes/removes the interfering ions such as $\text{CN}^-$ and $\text{S}^{2-}$, so that only halide ions react with silver nitrate.

Then silver nitrate is added

After treatment with nitric acid, addition of $\text{AgNO}_3$ gives:

• White ppt for $\text{Cl}^-$
• Pale yellow ppt for $\text{Br}^-$
• Yellow ppt for $\text{I}^-$

Evaluate the options

• A: Nitric acid — Correct
• B: Sodium hydroxide — Incorrect, not used before silver nitrate in halogen test
• C: Ammonia — Incorrect, ammonia is used later to distinguish silver halides
• D: Hydrochloric acid — Incorrect, it would itself introduce $\text{Cl}^-$ ions and interfere

Final answer

The compound added before silver nitrate is nitric acid.