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Qualitative Analysis of Organic Compounds | chemca

Practical Organic Chemistry

Qualitative Analysis of Elements

Detection of Nitrogen, Sulfur, Halogens, and Phosphorus using Lassaigne's Test.

By chemca Team • Updated Jan 2026

Organic compounds mainly consist of Carbon and Hydrogen. However, they often contain "heteroatoms" like Nitrogen, Sulfur, Halogens, and Phosphorus. To detect these, we convert the covalent organic bonds into ionic bonds using the Lassaigne's Test (Sodium Fusion Test).

1. Preparation of Sodium Fusion Extract (SFE)

The organic compound is fused with metallic Sodium ($Na$). The elements present ($N, S, X$) are converted into water-soluble sodium salts.

$$ Na + C + N \xrightarrow{\Delta} NaCN \quad \text{(Sodium Cyanide)} $$ $$ 2Na + S \xrightarrow{\Delta} Na_2S \quad \text{(Sodium Sulfide)} $$ $$ Na + X \xrightarrow{\Delta} NaX \quad \text{(Sodium Halide)} $$

If both N and S are present: $Na + C + N + S \rightarrow NaSCN$ (Sodium Thiocyanate).

2. Detection of Nitrogen

Test for Cyanide ($CN^-$)

The extract is boiled with Iron(II) Sulfate ($FeSO_4$) and acidified with concentrated Sulfuric Acid ($H_2SO_4$).

Observation: Appearance of a Prussian Blue color confirms Nitrogen.

Reactions:
1. $Fe^{2+} + 6CN^- \rightarrow [Fe(CN)_6]^{4-}$ (Ferrocyanide)
2. $Fe^{2+}$ is oxidized to $Fe^{3+}$ during boiling.
3. $4Fe^{3+} + 3[Fe(CN)_6]^{4-} \xrightarrow{xH_2O} Fe_4[Fe(CN)_6]_3 \cdot xH_2O$ (Prussian Blue)

Chemical Name: Iron(III) hexacyanidoferrate(II)

3. Detection of Sulfur

Test for Sulfide ($S^{2-}$)

A. Lead Acetate Test:
Acidify SFE with acetic acid and add lead acetate.
Observation: Black ppt ($PbS$)

B. Sodium Nitroprusside Test:
Add sodium nitroprusside to SFE.
Observation: Violet Color

$$ S^{2-} + [Fe(CN)_5NO]^{2-} \rightarrow [Fe(CN)_5NOS]^{4-} $$ (Violet Complex: Sodium Thionitroprusside)

4. Detection of Halogens

Silver Nitrate Test

Important: SFE is first boiled with conc. $HNO_3$ to decompose any $NaCN$ or $Na_2S$, which would interfere.

Then, Silver Nitrate ($AgNO_3$) solution is added.

Halogen	Precipitate	Solubility in $NH_4OH$
Chlorine ($Cl$)	White ppt ($AgCl$)	Soluble
Bromine ($Br$)	Pale Yellow ppt ($AgBr$)	Sparingly Soluble
Iodine ($I$)	Yellow ppt ($AgI$)	Insoluble

5. Detection of Phosphorus

Ammonium Molybdate Test

The compound is heated with an oxidizing agent ($Na_2O_2$) to convert Phosphorus to Phosphate ($PO_4^{3-}$). The solution is boiled with $HNO_3$ and then Ammonium Molybdate is added.

Observation: A Canary Yellow precipitate or coloration indicates Phosphorus.

$$ Na_3PO_4 + 12(NH_4)_2MoO_4 + 21HNO_3 \rightarrow $$ $$ (NH_4)_3PO_4 \cdot 12MoO_3 \text{ (Yellow ppt)} + ... $$

ppt is Ammonium Phosphomolybdate

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