Lassaigne’s Test - Test for Nitrogen, Sulphur, Halogens

What is the Lassaigne’s Test?

Nitrogenn, Sulphur, and halogens present in organic compounds are detected by Lassaigne’s test. Here, a small piece of Na metal is heated in a fusion tube with the organic compound. The principle is that, in doing so, Na converts all the elements present into ionic form.

  • Na + C + N → NaCN
  • 2Na + S → Na2S
  • Na + X → NaX ( X= Cl, Br, or I)

The formed ionic salts are extracted from the fused mass by boiling it with distilled water. This is called sodium fusion extract.

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Test for Nitrogen

The extract is boiled with FeSO4 and acidified with concentrated H2SO4. The appearance on Prussian blue colour indicates the presence of nitrogen.

The following reactions occur,

  1. Fe2+ + 6CN → [Fe(CN)6]4-
  2. Fe2+ + H+→ Fe3+ + e
  3. [Fe(CN)6]4- + 4Fe3+ → Fe4[Fe(CN)6].H2O

The acid helps in the oxidation of ferrous ions to ferric ions. The formation of ferriferous cyanide indicates the presence of nitrogen.

Note: This test is not given by compounds containing N but not C atoms. For example, NH2NH2 does not answer this test despite having an N atom. This is because both C and N are required to form CN ion.

This test is not given by diazonium salts as they decompose to give nitrogen gas on heating.

Test for Sulphur

The extract is treated with sodium nitroprusside. The appearance of violet colour indicates the presence of sulphur.

The following reaction occurs:

S2- + [Fe(CN)5NO]2- → [Fe(CN)5NOS]4-

Test for Halogens

The extract is acidified with HNO3 and then treated with AgNO3. A white precipitate soluble in NH4OH indicates the presence of Cl, a yellowish precipitate sparingly soluble in NH4OH indicates the presence of Br, and a yellow precipitate insoluble in NH4OH indicates the presence of I.

AgNO3 + NaX → AgX ↓ + NaNO3

 


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