In the Lassaigne's test for nitrogen in an organic compound, the Prussian blue colour is obtained due to the f

Exercises · 8.37

Q. In the Lassaigne's test for nitrogen in an organic compound, the Prussian blue colour is obtained due to the formation of:

(a) Na4[Fe(CN)6]

(b) Fe4[Fe(CN)6]3

(d) Fe3[Fe(CN)6]4.

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Concept understanding — Lassaigne Test Chemistry

The Lassaigne Test: Why We Burn the Sample First

Imagine you have an organic compound — say, a drug, a pesticide, or a dye — and you need to know if it contains nitrogen, sulfur, or a halogen (chlorine, bromine, iodine). You can't just test the compound directly because these atoms are covalently bonded inside the molecule. They won't simply fall off and react with a reagent.

The core problem: covalent bonds are stubborn. You need to break the molecule apart and convert those atoms into simple, water-soluble ions that you can detect with standard inorganic tests. That's exactly what the Lassaigne test does.

Note

The Lassaigne test is also called the sodium fusion test. It was developed by the French chemist J.L. Lassaigne in the 19th century.

The Intuition: Fusion with Sodium

The trick is to heat the organic compound with a piece of metallic sodium. Sodium is a powerful reducing agent. When you fuse them together (heat strongly in a fusion tube), the sodium rips the molecule apart. Here's what happens to the key elements:

Nitrogen → gets converted to sodium cyanide ( $\text{NaCN}$ )
Sulfur → gets converted to sodium sulfide ( $\text{Na}_2\text{S}$ )
Halogens (Cl, Br, I) → get converted to sodium halides ( $\text{NaX}$ , where X = Cl, Br, I)

The product of this fusion is a dark, charred mass. You then extract it with distilled water, boil, and filter. The clear filtrate is called the Lassaigne extract (or sodium fusion extract). This extract now contains the ions you can test for.

Watch out

Sodium metal is extremely reactive with water and moisture. It must be handled with dry apparatus and stored under kerosene. Never let it come in contact with water directly — the fusion tube is heated, then dropped into water after cooling.

The Precise Statement

Lassaigne test: A qualitative analysis method in which an organic compound is fused with metallic sodium to convert covalently bonded nitrogen, sulfur, and halogens into their respective water-soluble inorganic sodium salts (NaCN, Na₂S, NaX). These ions are then detected in the aqueous extract using specific chemical tests.

How to Detect Each Element in the Extract

1. Detection of Nitrogen

Test: Add a few drops of freshly prepared ferrous sulfate ( $\text{FeSO}_4$ ) solution to the extract. Boil, then cool. Add dilute sulfuric acid and a drop of ferric chloride ( $\text{FeCl}_3$ ).

What happens: The cyanide ion ( $\text{CN}^-$ ) reacts with ferrous ions to form ferrous cyanide, which then reacts with ferric ions to form Prussian blue — a deep blue precipitate of $\text{Fe}_4[\text{Fe(CN)}_6]_3$ .

6\text{NaCN} + \text{FeSO}_4 \rightarrow \text{Na}_4[\text{Fe(CN)}_6] + \text{Na}_2\text{SO}_4

Result: A blue colour or precipitate confirms nitrogen.

2. Detection of Sulfur

Test: Add a few drops of sodium nitroprusside ( $\text{Na}_2[\text{Fe(CN)}_5\text{NO}]$ ) solution to the extract.

What happens: Sulfide ions ( $\text{S}^{2-}$ ) react with sodium nitroprusside to form a violet colour complex.

\text{Na}_2\text{S} + \text{Na}_2[\text{Fe(CN)}_5\text{NO}] \rightarrow \text{Na}_4[\text{Fe(CN)}_5\text{NOS}] \text{ (violet)}

Result: A violet colour confirms sulfur.

3. Detection of Halogens

Test: Acidify the extract with dilute nitric acid ( $\text{HNO}_3$ ), then add silver nitrate ( $\text{AgNO}_3$ ) solution.

What happens: Halide ions ( $\text{Cl}^-$ , $\text{Br}^-$ , $\text{I}^-$ ) form precipitates with silver ions.

Halogen	Precipitate	Colour	Solubility in $\text{NH}_3$
Chlorine	$\text{AgCl}$	White	Soluble
Bromine	$\text{AgBr}$	Pale yellow	Partially soluble
Iodine	$\text{AgI}$	Yellow	Insoluble

Watch out

If nitrogen or sulfur is present, you must remove them before testing for halogens. Why? Because $\text{NaCN}$ and $\text{Na}_2\text{S}$ also react with $\text{AgNO}_3$ to form precipitates ( $\text{AgCN}$ and $\text{Ag}_2\text{S}$ ), giving false positives. To remove them, boil the extract with dilute $\text{HNO}_3$ — this converts $\text{CN}^-$ to $\text{HCN}$ gas and $\text{S}^{2-}$ to $\text{H}_2\text{S}$ gas, both of which escape.

Common Mistakes Students Make …

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The Prussian blue colour in Lassaigne’s test comes from the formation of ferric ferrocyanide, $\text{Fe}_4[\text{Fe}(\text{CN})_6]_3$ , which is option (b). The test works by converting nitrogen in the organic compound to cyanide ion, then to ferrocyanide, and finally reacting with ferric ion to give the deep blue complex.

Why Lassaigne’s test works — the chemistry behind the colour

When you fuse an organic compound with sodium metal, any nitrogen present is converted into sodium cyanide (NaCN). This is the key first step — the sodium breaks the organic molecule and “captures” the nitrogen as cyanide ion.

But cyanide ion alone doesn’t give a coloured product with iron salts in a useful way. The test relies on a clever sequence: you first convert the cyanide into ferrocyanide ion, $[\text{Fe}(\text{CN})_6]^{4-}$ , by adding ferrous sulphate. Then, when you add ferric chloride, the ferric ion ( $\text{Fe}^{3+}$ ) reacts with ferrocyanide to form an intensely blue precipitate — Prussian blue.

The exact composition of Prussian blue has been debated historically, but the formula accepted in standard textbooks (and in exam contexts) is $\text{Fe}_4[\text{Fe}(\text{CN})_6]_3$ . Let’s see why.

Step-by-step reasoning

Fusion with sodium The organic compound (containing N) is heated with Na metal.

\text{Organic-N} + \text{Na} \xrightarrow{\Delta} \text{NaCN}

This sodium cyanide dissolves in water when the fused mass is extracted.

Formation of sodium ferrocyanide Freshly prepared ferrous sulphate is added to the extract. The $\text{Fe}^{2+}$ ions react with CN⁻ to first form $\text{Fe}(\text{CN})_2$ , which then reacts with excess CN⁻ to give the ferrocyanide complex:

\text{Fe}^{2+} + 6\text{CN}^- \rightarrow [\text{Fe}(\text{CN})_6]^{4-}

The sodium salt, $\text{Na}_4[\text{Fe}(\text{CN})_6]$ , is formed in solution.

Boiling to oxidise iron

The solution is boiled. This step is crucial — it oxidises any leftover $\text{Fe}^{2+}$ to $\text{Fe}^{3+}$ , and also ensures that the ferrocyanide is fully formed. If you skip boiling, you might get a greenish colour instead of blue.
Addition of ferric chloride

Now you add $\text{FeCl}_3$ . The $\text{Fe}^{3+}$ ions react with the ferrocyanide ion:

4\text{Fe}^{3+} + 3[\text{Fe}(\text{CN})_6]^{4-} \rightarrow \text{Fe}_4[\text{Fe}(\text{CN})_6]_3

This is Prussian blue — an insoluble, deep blue complex. …

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Q.In the Lassaigne's test for nitrogen in an organic compound, the Prussian blue colour is obtained due to the formation of:

Why Lassaigne’s test works — the chemistry behind the colour

Step-by-step reasoning

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