Lassaigne's Test — Master Reference — Notes

Wikimedia Diagram:

Cue Column | Note Column

What is the principle? | Organic compound fused with Na metal (high temp) → non-metallic elements converted to water-soluble ionic salts. Extract tested with specific reagents.

Ionic forms produced: |

C + N → NaCN
S → Na2S
Halogen (X) → NaX
N + S (together) → NaSCN (NOT NaCN + Na2S separately)

Test for N (S absent): | NaCN + FeSO4 + NaOH (boil, cool) + H2SO4 → Fe4[Fe(CN)6]3 — Prussian blue precipitate

Test for S: |

Na2S + Na2[Fe(CN)5NO] → purple/violet (sodium nitroprusside test)
Na2S + Pb(CH3COO)2 → PbS black precipitate

Critical trap — N + S together: | NaSCN formed → FeCl3 + NaSCN → Fe(SCN)3 — blood-red color, NOT Prussian blue

Test for halogens: | NaX + HNO3 (boil first if N or S present) + AgNO3 → AgX precipitate

Why boil with HNO3 first? | To decompose NaCN (→ AgCN interference) and Na2S (→ Ag2S black interference) before AgNO3 test

Summary

The single most-tested concept in Lassaigne's test is: when both N and S are present, blood-red color (not Prussian blue) is observed with FeCl3, because NaSCN forms instead of NaCN.