What is Lassaigne's test

The Detection of Elements in an Organic Compound Lassaigne Sodium Fusion Test



Chemicals: sodium; urea (or organic compound contain N); chlorobenzene (or organic compound
containing halogen); sodium benzenesulphonate (or organic compound containing S); iron (II)
sulphate; iron (III) chloride; dilute hydrochloric acid; silver nitrate solution; dilute nitric acid;
sodium nitroprusside solution.

Apparatus: small test tube; breast burner; wire gauze; evaporating basin; safety goggle; filter funnel;
filter paper; test tubes; crucible tong.

Organic compounds contain carbon, and usually contain hydrogen. They may contain one or
more of the elements oxygen, bromine, iodine, nitrogen, sulfur, phosphorus; a metal may also be
present. The detection of these elements is based on the following considerations.
Normally it is only necessary to test for nitrogen, sulfur, the halogens and phosphorous. This
may be done by carrying out a Lassaigne sodium fusion test. The presence of carbon and hydrogen
is assumed since very few organic compounds do not contain hydrogen. Oxygen is usually detected,
if present, in later tests for functional groups.


1. A little of the substance (about 50mg or 2 drops) is added to a small pellet of sodium in an
ignition tube and then heated, gently at first but then more strongly, to dull redness.
2. The tube, while still hot, is plunged into about 10-15 cm3 of cold distilled water in an evaporating
basin. The mixture is boiled for a short time, stirring with the broken portion of the tube and
then filtered.
3. The filtrate, which should be both clear and colorless, is ready to be examined for the presence
of nitrogen, sulfur and halogens.

** Safety precautions are essential as, in addition to the hazard of shattering glass, the reaction with
certain compounds, particularly those containing chlorine, may be explosive. **


The Lassaigne test converts any nitrogen present in the compound to sodium cyanide.
1. About 0.5 cm3 of a cold saturated solution of iron (II) sulphate, that has been boiled momentarily
to ensure the presence of some iron (III) salt, is added to a portion (4 cm3) of the filtrate.
2. The mixture is boiled for about one-half to one minute, cooled rapidly, and then acidified by
adding concentrated hydrochloric acid (or 3 M sulfuric acid) drop-wise.
3. The formation of a Prussian blue (bluish green) precipitate indicated the presence of nitrogen.
Detection of N, S, X (Cl, Br, I)
(Sodium fusion test or Lassaign’s test)

Compound + Na (s) --- > NaCN (aq) + Na2S (aq) + NaX (aq)

(red ignition tube + contents + beaker of distilled water)

--- > bulk alkaline solution ---- > fulter --- > test for (a) CN- (b) S2- (c) X-

B. Test for CN-;

1. Add excess FeSO4 crystal / solution.

Fe2 + + CN- --- > [Fe2 + (CN) 6] 4- iron (II) cyanide

2. Boil the mixture;

Fe2 + ------- > Fe3 + brown ppt of Fe (OH) 3

Oxidized by air (because solution still alkaline)

3. Acidify the resulting solution with H2SO4 to dissolve Fe (OH) 3

Fe3 + + [Fe (CN) 6] 4- ----- > [Fe2 + (CN) 6] - iron (II, III) cyanide (Prussian blue)

C. Test for S2-

The Lassaigne test converts any sulfur present in the compound to sodium sulphide.
1. Add a few drops of a cold, freshly prepared, dilute solution of sodium
pentacyanonitrosylferrate (II) (nitroprusside) to a portion of the filtrate.
2. The presence of sulfur is indicated by the production of a rich purple color. (This test is
very sensitive)

filtrate + sodium nitroprusside solution --- > violet coloration if S is present (but composition

is uncertain)

D. Test for X-
1. The presence of halogens is tested for the usual way by acidifying a portion of the filtrate with
dilute nitric acid and adding silver (I) nitrate solution. Halogens are indicated by the formation of
a white or yellow precipitate.
2. A further portion of the alkaline filtrate is acidified with 2M hydrochloric acid before adding
about 1 cm3 of tetrachloromethane (or other organic solvent) and a few drops of chlorine water.
The color of the tetrachloromethane layer, after shaking, enables the halogen to be identified.
If the tetrachloromethane layer is colorless, chlorine is indicated; brown, bromine is indicated;
or violet, iodine is indicated.

Filtrates + AgNO3 acidified with dil. HNO3 ──── > AgX (s)

AgCl - White ppt. Soluble in excess ammonia solution

AgBr - Creamy yellow ppt. Sparingly soluble in ammonia solution
AgI - Yellow ppt. Insoluble in ammonia solution

E. Test for P:
1. Add conc. HNO3 to convert all P compounds into phosphate (V) ions.
2. Add ammonium molybdate (NH4) 6Mo7O24.4H2O) solution. On gently warming to no more
than 50oC, yellow ppt. Of ammonium phosphomolybdate (NH4) 3PO4.12MoO3 appears.
Name: ___________________ Class: __________ Date: ______________