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

Q: Why do we even need to make a sodium extract?

Organic compounds have covalent bonds. We cannot test for halogens directly because they don't ionize in water. Fusing with sodium converts them into ionic sodium salts (like $NaX$), which can be easily tested.

Q: What if the compound only has halogens and no nitrogen or sulphur? Do we still add $HNO_3$?

Yes, it's a standard procedure. We usually don't know beforehand if N or S are present. Adding $HNO_3$ is a precautionary step to ensure no false positives occur.

Q: Why does $AgCN$ or $Ag_2S$ interfere with the halogen test?

The halogen test relies on observing the color of the silver halide precipitate ($AgCl$ is white, $AgBr$ is pale yellow, $AgI$ is yellow). $AgCN$ is also white, which can be confused with $AgCl$. $Ag_2S$ is black, which can completely mask the colors of the silver halides.

Q: Can we use $H_2SO_4$ instead of $HNO_3$?

While $H_2SO_4$ can also expel $HCN$ and $H_2S$, concentrated $H_2SO_4$ might oxidize some halide ions (like $Br^-$ or $I^-$) to their respective halogen gases ($Br_2$, $I_2$), ruining the test. Also, $Ag_2SO_4$ is sparingly soluble and might precipitate. $HNO_3$ is the safest choice.

Q: Why do we boil the solution after adding $HNO_3$?

Boiling decreases the solubility of gases in liquids. It ensures that all the formed $HCN$ and $H_2S$ gases are completely expelled from the solution, leaving zero interference for the $AgNO_3$ test.

Question

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

Options:

(A) Hydrochloric acid

(B) Sodium hydroxide

(C) Nitric acid

(D) Ammonia

Sigma Percentile · Accepted Answer

### The Magic of Lassaigne's Test

When we want to detect the presence of extra elements like nitrogen, sulphur, or halogens in an organic compound, we can't just dissolve the compound in water and test it. Why? Because organic compounds are covalent! The elements are tightly bound and won't ionize.

To solve this, we perform **Lassaigne's test**. We fuse the organic compound with sodium metal. This violent, fiery reaction converts the covalent bonds into ionic sodium salts. Nitrogen becomes sodium cyanide ($NaCN$), sulphur becomes sodium sulphide ($Na_2S$), and halogens become sodium halides ($NaX$). We extract these salts in water, creating what we call the **Sodium Extract**.

### The Interference Problem

Now, imagine you want to test specifically for halogens. The standard test is to add silver nitrate ($AgNO_3$). If halogens are present, they form beautiful precipitates: white for $AgCl$, pale yellow for $AgBr$, and yellow for $AgI$.

But there is a massive catch! What if your organic compound also contained nitrogen and sulphur? Your sodium extract will now contain $CN^-$ and $S^{2-}$ ions alongside the halide ions.

If you directly add $AgNO_3$, the cyanide ions will react to form a white precipitate of silver cyanide ($AgCN$). The sulphide ions will react to form a thick black precipitate of silver sulphide ($Ag_2S$). These unwanted precipitates will completely mask the colors of your silver halides, ruining the test.

### The Nitric Acid Solution

To clear the path for our halogen test, we must eliminate the cyanide and sulphide ions. We do this by adding **concentrated Nitric Acid ($HNO_3$)** and boiling the solution.

Nitric acid is a strong acid that attacks these interfering salts, converting them into volatile gases:

$$NaCN + HNO_3 \rightarrow NaNO_3 + HCN \uparrow$$

$$Na_2S + 2HNO_3 \rightarrow 2NaNO_3 + H_2S \uparrow$$

Because we are boiling the mixture, the hydrogen cyanide ($HCN$) and hydrogen sulphide ($H_2S$) gases escape into the atmosphere. The interfering ions are literally boiled away!

### Why Not Other Acids?

You might wonder, why specifically nitric acid? Why not hydrochloric acid ($HCl$)?

Think about it. If you add $HCl$ to remove the impurities, you are intentionally dumping chloride ($Cl^-$) ions into your test tube! When you subsequently add $AgNO_3$, it will react with the $Cl^-$ you just added, giving a massive white precipitate of $AgCl$. You would get a **false positive** for chlorine every single time.

Therefore, nitric acid is the perfect, non-interfering choice to prepare the sodium extract for a flawless halogen test.

Animated Solution for Chemistry - Organic Chemistry: Which of the following compound is added to the sodium extract before addition of silver nitrate for testing of halogens?

Select Answer:

Visualized Solution

$Lassaigne’s Extract$

$The Interference Problem$

$Adding Nitric Acid$

$Decomposition Reactions$

$Safe Halogen Testing$

$Why not HCl or H_{2} S O_{4} ?$

The Sigma Insight: Nomenclature and Characterisation

The Magic of Lassaigne's Test

The Interference Problem

The Nitric Acid Solution

Why Not Other Acids?

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